FGDC-STD-008-1999

Content Standards for Digital Orthoimagery

Subcommittee on Base Cartographic Data

Federal Geographic Data Committee

February 1999

Federal Geographic Data Committee

Department of Agriculture •Department of Commerce •Department of Defense •Department of Energy
Department of Housing and Urban Development •Department of the Interior •Department of State
Department of Transportation •Environmental Protection Agency
Federal Emergency Management Agency •Library of Congress
National Aeronautics and Space Administration •National Archives and Records Administration
Tennessee Valley Authority
 Federal Geographic Data Committee

Established by Office of Management and Budget Circular A-16, the Federal Geographic Data Committee
(FGDC) promotes the coordinated development, use, sharing, and dissemination of geographic data.

The FGDC is composed of representatives from the Departments of Agriculture, Commerce, Defense,
Energy, Housing and Urban Development, the Interior, State, and Transportation; the Environmental
Protection Agency; the Federal Emergency Management Agency; the Library of Congress; the National
Aeronautics and Space Administration; the National Archives and Records Administration; and the
Tennessee Valley Authority. Additional Federal agencies participate on FGDC subcommittees and
working groups. The Department of the Interior chairs the committee.

FGDC subcommittees work on issues related to data categories coordinated under the circular.
Subcommittees establish and implement standards for data content, quality, and transfer; encourage the
exchange of information and the transfer of data; and organize the collection of geographic data to reduce
duplication of effort. Working groups are established for issues that transcend data categories.

For more information about the committee, or to be added to the committee's newsletter mailing list,
please contact:

Federal Geographic Data Committee Secretariat

c/o U.S. Geological Survey
590 National Center
Reston, Virginia 20192

Telephone: (703) 648-5514

Facsimile: (703) 648-5755
Internet (electronic mail): fgdc@www.fgdc.gov
Anonymous FTP: ftp://www.fgdc.gov/pub
World Wide Web: http://www.fgdc.gov/fgdc.html

CONTENTS

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Page

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.1 Objective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.2 Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.3 Applicability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.4 Relationship to existing standards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.5 Standards Development Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.6 Maintenance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Data Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Digital Orthoimagery Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3.1 Image Radiometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Digital Transfer Formats . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
4.1 Non-image Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
5. Sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
5.1 Seasonal and Time-of-day Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
5.2 Aerial Photography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
5.2.1 Scanned images from aerial photography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
5.3 Electro-optical Images . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
5.4 Elevation Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
5.5 Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
5.6 Calibration Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
6. Areal Extent . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
7. Georeferencing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
8. Resolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
8.1 Pixel Ground Resolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
8.2 Radiometric Resolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
9. Accuracy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
10. Data Quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
10.1 Geometric Corrections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
10.1.1 Image smears . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
10.1.2 Other DEM-related geometric distortions . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

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10.2 Radiometric Correction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

11. Data Completeness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
11.1 Cloud Coverage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
12. Image Mosaicking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
13. Metadata . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

APPENDICES

Appendix A: Example of an FGDC Compliant Metadata File (informative) . . . . . . . . . . . . . . . . . . . . 18

Appendix B: Definitions (informative) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

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LIST OF PAGES

A complete and current copy of the “Content Standards for Digital Orthoimagery” consists of the pages
(and most recent creation or revision dates) listed below.

Page Date
ii thru
v 02/99
1 thru
37 02/99

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1. INTRODUCTION

1.1 Objective
The objective of this standard is to define the orthoimagery theme of the digital geospatial
data framework as envisioned by the FGDC. It is the intent of this standard to set a common
baseline that will ensure the widest utility of digital orthoimagery for the user and producer
communities through enhanced data sharing and the reduction of redundant data production.
The framework will provide a base on which to collect, register, and integrate digital
geospatial information accurately. Digital orthoimagery is a part of this basic set of data
described as framework data.

This standard is intended to facilitate the interchange and use of digital orthoimagery data
under the framework concept. Because of rapidly changing technologies in the geospatial
sciences, this standard for digital orthoimagery covers a range of specification issues, many
in general terms. This document stresses complete and accurate reporting of information
relating to quality control and standards employed in testing orthoimagery data.

1.2 Scope
This standard describes processing, accuracy, reporting, and applications considerations for
NSDI Framework digital orthoimagery, and may be applicable to other data sets which
employ the FGDC Framework concepts. This standard is classified as a Data Content
Standard by the Federal Geographic Data Committee Standards Reference Model. Data
content standards provide semantic definitions of a set of objects, such as those described
above.

1.3 Applicability
This standard applies to NSDI Framework digital orthoimagery produced, or disseminated
by or for the Federal Government. According to Executive Order 12906, Coordinating
Geographic Data Acquisition and Access: the National Spatial Data Infrastructure (Clinton,
1994, Sec. 4., Data Standards Activities), Federal agencies collecting or producing geospatial
data, either directly or indirectly (e.g. through grants, partnerships, or contracts with other
entities), shall ensure, prior to obligating funds for such activities, that data will be collected

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in a manner that meets all relevant standards adopted through the FGDC process.

1.4 Relationship to Existing Standards

Throughout this text there are numerous references to metadata and the FGDC's "Content
Standard for Digital Geospatial Metadata" (FGDC, 1994). Whenever a comment about
metadata appears, the location of the data element description in that standard, placed in
parentheses ( ), will follow, or passages will be pointed to from the metadata example in
Appendix A. This document will also reference the Spatial Data Transfer Standards (Dept.
of Commerce, 1992), the National Map Accuracy Standard (U.S. Bureau of the Budget,
1947), and the FGDC National Standard for Spatial Data Accuracy (FGCS,1996).

1.5 Standards Development Procedures

The draft Content Standards for Digital Orthoimagery have been developed by the
Subcommittee on Base Cartographic Data of the FGDC. The development of this standard is
guided by the FGDC Standards Reference Model. The Standards Reference Model,
developed by the Standards Working Group of the FGDC, provides guidance to FGDC
subcommittees for the standards development process. The model also defines the
expectations of FGDC standards, describes different types of geospatial standards, and
documents the FGDC standards process.

1.6 Maintenance
The U.S. Department of the Interior, United States Geological Survey (USGS), National
Mapping Division, maintains the Content Standards for Digital Orthoimagery for the
Federal Geographic Data Committee. Address questions concerning this standard to: Chief,
National Mapping Division, USGS, 516 National Center, Reston, VA 20192.

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2. DATA DESCRIPTION
A digital orthoimage is a georeferenced image prepared from a perspective photograph or
other remotely-sensed data in which displacement of objects due to sensor orientation and
terrain relief have been removed. It has the geometric characteristics of a map and the
image qualities of a photograph. Digital orthoimages are composed of an array of
georeferenced pixels that encode ground reflectance as a discrete value. Digital
orthoimagery comes from various sources and in a number of formats, spatial resolutions,
and areas of coverage. Many geographic features, including some in other framework data
themes, can be interpreted and compiled from an orthoimage. Accurately positioned, high
resolution data are considered the most useful to support the compilation of framework
features.

3. DIGITAL ORTHOIMAGERY STRUCTURE

Framework digital orthoimagery shall consist of two-dimensional, rectangular arrays of
pixels, which correspond to ground areas called ground resolution cells. The pixels shall be
arranged in horizontal rows (lines) and vertical columns (samples). The order of the rows
shall be from top to bottom; the order of columns shall be from left to right. The uppermost
left-hand pixel shall be designated pixel (0,0). Each line of image pixels represents a
physical record in the file with the total set of records constituting a single file. Images
describing more than 1 band of electromagnetic radiation (true color, color-infrared, multi-
band) shall be stored in one of three formats: band-interleaved by line (BIL), band
interleaved by pixel (BIP), or band sequential (BSQ).

The file shall have equal record lengths, resulting in a rectangular or squared image. This
may be accomplished by padding with over edge image or non-image pixels, with digital
number (DN) equal to zero (black), to an edge defined by the extremes of the image. The
bounding coordinates of the image must be documented in accordance with the FGDC
"Content Standard for Digital Geospatial Metadata.” For images that contain over edge
imagery or are padded with non-image pixels, descriptions of both the specific area of
interest and any over edge imagery must be documented by the metadata standard. For
instance, some digital orthoimagery quadrangles include over edge imagery beyond the

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boundaries of the area of interest. Therefore, the producer is obliged to describe the image
quadrangle in metadata. Both the image area of interest proper, and the over-edge, shall be
documented in the metadata field: (Spatial_Domain/Bounding_Coordinates and
Data_Quality_Information/ Attribute_Accuracy/Completeness_Report).

3.1 Image Radiometry

Relative radiance of ground resolution cells are described by numerical representations (DNs
or brightness values) of reflected radiance amplitudes. The cell value is recorded as a series
of binary digits or bits, with the number of bits per cell determining the radiometric
resolution of the image. Brightness values are commonly represented as 8-bit binary
numbers with a range of values from zero, (black) to 255 (white).

4. DATA TRANSFER FORMATS

Data transfer formats for digital orthoimagery will not be specified in this standard.
However, data producers are encouraged to use the Raster Profile (draft) of the Spatial Data
Transfer Standard (SDTS) as the model for formatting their digital orthoimagery. Other
data transfer formats are permitted, however data producers are encouraged to employ the
more widely used and accepted raster image formats, listed in the "Content Standards for
Digital Geospatial Metadata". In all cases, producers shall provide detailed descriptions of
the format. Copies of the "Spatial Data Transfer Standard" (Department of Commerce,
1992) are available from:

National Technical Information Service

U.S. Department of Commerce
Springfield, VA 22161

or are available on the World Wide Web at:

ftp://sdts.er.usgs.gov/pub/sdts/standard/

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4.1 Non-image data

Image files may contain non-image data in the form of header or trailer records which are
physically attached to the image data. These records offer information used to identify,
georeference, and impart other information about the data. They are generally in a different
format than the image data. Producers of imagery shall document pertinent information
about these records: e.g., their location, byte counts, etc., in the metadata. See Section 13.
METADATA.

5. SOURCES
Source imagery for digital orthoimagery is collected by a variety of remote sensors and
processed in a number of ways. All sources employed in the construction of digital
orthoimagery shall be documented in the metadata field: (Data_Quality
Information/Lineage/Source_Information)

In general, the data needed to create orthoimagery are:

! an unrectified raster image file, from scanned aerial photographs or other remote
sensing instruments
! digital elevation data that covers the same area as the image
! ground control
! calibration information about the sensor

These four inputs are used collectively to register the raw image file mathematically to the
scanner or to the sensor platform, to determine the orientation and location of the sensor
platform with respect to the ground, and to remove the relief displacement from the image
file.

Remote sensing systems can be divided into two general categories: imaging and non-
imaging. This standard focuses on imaging systems. Commonly used types of imaging
systems include: photo-optical, electro-optical, passive microwave, RADAR, LIDAR,
IFSAR, SONAR.

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5.1 Seasonal and Time-of-Day Considerations

The season of the year and the time of day when images are acquired can be significant
factors to the utility of the imagery. Users engaged in the mapping of terrain features
generally prefer the spring and fall “leaf off” seasons with little or no snow cover, while
users engaged in vegetation analysis prefer imagery gathered during the growing or “leaf on”
season. Similar considerations are true with respect to the time of day imagery is acquired,
as for enhanced shadow requirements. Recognizing the variability of user needs, this
standard will not specify the times or seasons the source imagery shall be acquired. The
date that the imagery was acquired, and the time of day, if it is an important consideration
for acquisition, shall be documented in the metadata field:
(Lineage:Source_Information/Source_Time_Period_of_Content/Calendar_Date)
For example, if the contract specifications for source photography require enhanced shadow
effect, that would be an important consideration outside the usual aerial photography
specifications.

5.2 Aerial Photography

Aerial photography is the primary image source currently used to produce digital
orthoimagery. Film types for orthoimagery compliant with the standard shall be confined to
black and white (panchromatic), color infrared (CIR), and natural (true) color. Black and
white orthoimagery may be generated from CIR and natural color source. For aerial photo
identification, the type of film, manufacturer or agency identification, and roll and exposure
number shall be documented in the metadata field:
(Lineage:Source_Information/Source_Citation)

5.2.1 Scanned images from aerial photography

The combination of the Instantaneous Field Of View (IFOV) of the scanner and the scale of
the source imagery shall determine the pixel ground resolution which can be attained for the
digital orthoimagery (Pratt, 1978). Resampling to a pixel ground resolution greater (coarser)
than that of the original scan is acceptable and, in many cases desirable, to create smaller file
sizes. Excessive subsampling to attain a pixel ground resolution value less (finer) than that
of the source imagery is discouraged. (See Section 8. Resolution: Pixel Ground Resolution)

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5.3 Electro-optical Images

Electro-optical imaging instruments are non-film detectors which typically use two-
dimensional detector arrays of charge-couple devices (CCDs). Each detector in the array is
the equivalent of one pixel in the image. At the present, because of the relatively small size
of the arrays, electro-optical instruments such as digital cameras are more suited for
capturing large scale images with ground sample distances measuring in the sub-meters.
Appropriate information about the device, type, array size, pixel resolution, and flight
height, will be cited in the image metadata.
(Data_Quality_ Information /Lineage/Process_Step/Process_Description)

5.4 Elevation Data

Elevation data used to correct displacement shall be sufficiently accurate to ensure the image
meets user defined accuracy requirements for the intended scale. Producers of digital
orthoimagery shall use elevation data with the appropriate ground sample distances and
areal coverage to reliably describe the terrain and meet the accuracy requirements of the
image. A detailed description of the source Elevation Model shall be documented in the
metadata field: (Lineage:Source_Information/Source_Citation)
For more information on elevation data refer to the FGDC "Content Standards for Digital
Gridded Land Elevation Data".

5.5 Control
Ground control from surveyed ground targets and control points established in
aerotriangulation (AT) shall be sufficient to meet the accuracy requirements of the intended
resolution of the digital orthoimage. Control acquired from maps or other similarly
inaccurate methods is not recommended for large-scale digital orthoimagery. A description
of the methods used to establish control shall be documented in the metadata field:
(Data_Quality_Information/Positional_Accuracy/Horizontal_Positional_Accuracy/Horizontal
_Positional_Accuracy_Report)

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5.6 Calibration Data

While camera or imaging instrument calibration parameters are required for production
purposes, specifications for that data will not be covered by this standard. Information on
camera calibration can be found in the USGS publication "USGS Aerial Camera
Specifications" (10/93).

6. AREAL EXTENT
This standard places no constraints on the geographic extent of orthoimagery. Areal extent
of quadrilateral orthoimagery may be adjusted as appropriate for the type of sensor and
sensor platform, height, requirements of the user, etc. However, it is recommended that
producers of digital orthoimagery data utilize a widely used or familiar partitioning scheme.
Numerous established schemes exist for partitioning the Earth’s surface. The USGS 7.5-
minute topographic map series utilizes one such method. Schemes based upon subsets of the
7.5-minute topographic map could be used for large-scale image partitioning schemes.
Other examples include tiles based on the Public Land Survey System (PLSS) or other
cadastral systems based on county boundaries, tax plats, etc.

The spatial domain of an image shall be documented in the metadata field:

(Identification_Information/Spatial_Domain).

7. GEOREFERENCING
A common method for referencing coordinate positions on the Earth is essential for
integrating framework data. While it is desirable that framework data be described by
longitude and latitude coordinates, orthoimagery is more appropriately represented in a
grid coordinate system, such as Universal Transverse Mercator (UTM) or State Plane
Coordinate Systems (SPCS). In any case, the horizontal coordinate system of the image
shall be documented in the metadata field:
(Spatial_Reference_ Information/Horizontal_Coordinate_System_Definition).

This standard recommends that the North American Datum of 1983 (NAD83) be used as the
horizontal datum for digital orthoimagery. In recognition of significant application of other
widely accepted datums throughout the digital geospatial community, other datums may be

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referenced. In each instance the horizontal datum shall be documented in the metadata field:
(Spatial_Reference_Information/Horizontal_Coordinate_System_Definition
/Geodetic_Model)

Georegistration of the image is also essential to complete georeferencing of the image.

Georegistration will be described by a 4-tuple in the metadata which will establish the
position of the first pixel in the first row of the image [pixel (0,0)]. The metadata will reflect
the row # = 0, column # = 0, and georeference values in X and Y for the documented datum
and horizontal coordinate system. Under this standard, georegistration (spatial coordinates)
refers to the center of the pixel. This establishes the georegistration at one point in the
orthoimage. Since row and column offsets are both constant and known, (XY_pixel
resolution), all other points can be georegistered. Additional 4-tuples may be provided for
additional georegistration. Georegistration of pixel (0,0) shall be documented in the
metadata field:
(Spatial_Reference_Information/Horizontal_Coordinate_System_Definition/Planar_Coordin
ate_Information/Local_Planar_Georeference_Information)

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8. RESOLUTION
Two separate resolution measurements are important for image data: pixel ground
resolution, which is sometimes referred to as horizontal ground resolution or ground sample
distance, and radiometric resolution. For this standard, pixel ground resolution defines the
area of the ground represented in each pixel in x and y components, while radiometric
resolution defines the sensitivity of a detector to differences in wavelength as it records
radiant flux reflected or emitted from the ground.

8.1 Pixel Ground Resolution

Images may be resampled to create coarser resolution images than the original raster data.
Subsampling of images may be applied only within the limits defined by the Nyquist theorem
(Pratt, 1978). The Nyquist frequency limits subsampling to a maximum two times (2X) to
avoid undesirable aliasing.
The pixel ground resolution shall be documented in the metadata field:
(Spatial_Reference_Information/Horizontal_Coordinate_System_Definition/Planar/Planar_C
oordinate_Information).

8.2 Radiometric Resolution

This standard recommends that black and white image data be represented as 8-bit binary
data, and color images be represented as 24-bit, 3 byte data. For 8-bit and 24-bit image
data, digital numbers, or image brightness values shall be represented by 256 gray levels and
represented by a number in a range of zero-255. A value of zero shall represent the color
black and a value of 255, the color white. All intermediate values are shades of gray
varying uniformly from black to white. Areas where the image is incomplete shall be
represented with a numeric value of zero. Radiometric resolution shall be documented in
the metadata field:
Spatial_Data_Organization_Information:Direct_Spatial_Reference_Method/
Raster_Object_Information)

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9. ACCURACY
Framework digital orthoimagery accuracy shall employ the National Standard for Spatial
Data Accuracy (NSSDA), which implements a statistical and testing methodology for
estimating the positional accuracy of points in digital geospatial data, with respect to
georeferenced ground positions of higher accuracy. This reporting methodology provides a
common language for reporting positional accuracy so that users can evaluate data sets for
fitness of use for their applications. The NSSDA uses root-mean-square error (RMSE) to
estimate positional accuracy. Accuracy is reported in ground distances at the 95%
confidence level. Accuracy reported at the 95% confidence level means that 95% of the
positions in the data set will have an error with respect to true ground position that is equal
to or smaller than the reported accuracy value. The reported accuracy value reflects all
uncertainties, including those introduced by geodetic control coordinates, compilation, and
final computation of ground coordinate values in the product.

The NSSDA does not define threshold accuracy values. Users are encouraged to establish
thresholds for their product specifications and applications and for contracting purposes.
Data producers may elect to use accuracy thresholds in standards such as the National Map
Accuracy Standards of 1947 (U.S. Bureau of the Budget, 1947) or Accuracy Standards for
Large-Scale Maps [American Society for Photogrammetry and Remote Sensing (ASPRS)
Specifications and Standards Committee, 1990] if they decide that these values are
applicable to their digital geospatial data accuracy requirements. However, accuracy of new
or revised data products will be reported according to the NSSDA. Data producers shall
ensure that all critical components have known accuracies suitable for the construction of
orthoimagery, and that those accuracies are reported in the metadata.

Producers of digital orthoimagery must report the horizontal positional accuracy of data.
The horizontal positional accuracy report shall be documented in the metadata field: (Data_
Quality_Information/Positional_Accuracy/Horizontal_Positional_Accuracy). The FGDC
"Content Standards for Digital Geospatial Metadata" establishes a mandatory if applicable
requirement for horizontal positional accuracy data. This should not be misconstrued as an
optional data element. By definition, orthoimagery exhibits geometric qualities which
distinguish it from unrectified imagery, hence accurate measurements can be made from

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Federal Geographic Data Committee FGDC-STD-008-1999
Content Standards for Digital Orthoimagery, February 1999

digital orthoimagery and features on orthoimagery will be correctly geopositioned. The

accuracy characteristics of digital orthoimagery are tested during production or post-
production and recorded in a report on the positional quality and the assessment process.
Recommendations on information to be reported and tests to be performed are found in
Chapter 3 of Part 1, Spatial Data Quality, of the Department of Commerce, 1992, "Spatial
Data Transfer Standard " (Federal Information Processing Standard 173): Washington,
Department of Commerce, National Institute of Standards and Technology.)

10. DATA QUALITY

Different orthoimagery production systems have unique characteristics, however all accept
raw (or unprocessed) imagery which contain some degree of error in geometry (geometric
distortion) and in the measured brightness values of the pixels (radiometric distortion).
Image rectification and restoration are processes for correcting distortions and degradations
which result from image acquisition. This standard requires specification of rectification or
restoration procedures only in context of geometric and radiometric corrections.

Detailed descriptions of the processes used to correct distortions in an image shall be

documented in the metadata field: (Data_Quality_ Information
/Lineage/Process_Step/Process_Description).

10.1 Geometric Correction

All systematic and random errors shall be removed to the extent required to meet map
accuracy requirements as defined by the intended user. Geometric corrections are performed
to match raw image data to map geometry. Distortions can be classified as either systematic
(predictable errors that follow some definite mathematical or physical law or pattern
associated with particular processes and instruments) or random (errors that are wholly due
to chance and do not recur). Most of the distortions associated with orthoimagery are
random. Terrain relief, platform position, and faulty elevation data are the sources of
nonsystematic distortion, or random errors. These random errors can be detected by
comparing identifiable points on an image to their known ground coordinates.

Nearest neighbor, bilinear interpolation, and cubic convolution resampling algorithms are

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Federal Geographic Data Committee FGDC-STD-008-1999
Content Standards for Digital Orthoimagery, February 1999

common methods used to transform image values to fit map geolocation values. Nearest
neighbor resampling is not recommended for the large-scale framework because of the
disjointed appearance in the output due to spatial offsets as great as one-half pixel. Images
transformed using bilinear interpolation are generally acceptable. A precise resampling
method such as cubic convolution is recommended. Most importantly, the resampling
process utilized in the production of the image must be documented in the metadata
(Data_Quality_Information /Lineage/Process_Step/Process_Description).

10.1.1 Image smear

Occasionally, because of spikes in the elevation data or excessive topographic relief, an
anomaly or artifact best described as an "image smear" may appear on a rectified image.
Basically, the steepness of the terrain is such that some ground image is effectively hidden
from view (e.g. on the backside of the mountain or the sides of a steep cliff). This can be
especially prominent near the edge of images from large-scale aerial photography (incidence
of the anomaly decreases as the altitude of the sensor platform increases). When that portion
of the scanned raster image is adjusted to its conjugate area on the elevation model, the void
in the image is assigned brightness values via an interpolation algorithm which uses the
visible image surrounding the void. This sometimes results in a "smeared" or "stretched"
area on the image.

When image smears occur, all reasonable means to correct them shall be applied. The
elimination of elevation spike error can easily correct this defect. The potential value to be
added to the image when attempting to correct stretched or smeared artifacts caused by
extensive relief should be weighed against the amount of smearing, the time and effort
investment to correct the artifact and affected features, and the intended use of the image. It
may not be cost-effective or necessary to correct all image smear artifacts. Determining an
acceptable amount smearing in a image is subjective, depending on user requirements.
Until reliable methods to assess the location and amount of smearing are established,
determination of the acceptability of an image will be by visual inspection. Images may be
determined to be unacceptable when artifacts appear in areas where critical features are
evident, or if artifacts are of such an extent to render the image unusable.

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Content Standards for Digital Orthoimagery, February 1999

10.1.2 Other elevation-related geometric distortions.

Double or missing features in the image may be indications of a poor Elevation Model or
unsuitable control. Such distortions may render the image unusable.

10.2 Radiometric Correction

Image brightness values may deviate from the brightness values of the original imagery, due
to image value interpolation during the scanning, rectification, and post-processing
procedures. However, data producers are cautioned to minimize the amount of radiometric
correction applied to an image. It is common practice to perform some radiometric
enhancements and corrections (e.g., contrast stretching, analog dodging, noise filtering,
destriping, edge matching) to images prior to release of the data. Data producers shall use
processing techniques which minimize data loss from the time the information was captured
until its release to the users. Any image restoration or enhancement processes applied to an
image shall be documented in the metadata field:
(Data_Quality_Information/Lineage/Process_Step/Process_Description).

Radiometric accuracy can be verified by visual comparison of the digital orthoimage with the
original unrectified image to determine if the digital orthoimagery has the same or better
image quality as the original unrectified input image(s). Radiometric accuracy verification
process and results shall be documented in the metadata field:
(Data_Quality_Information:Attribute_Accuracy/Attribute_Accuracy_Report).

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Federal Geographic Data Committee FGDC-STD-008-1999
Content Standards for Digital Orthoimagery, February 1999

11. DATA COMPLETENESS

Visual verification shall be performed for image completeness, to ensure that, whenever
possible, no gaps exist in the image area. Areas of omission, in incomplete images, shall be
documented in the metadata field: (Data_Quality_Information/Completeness_Report).

11.1 Cloud Cover

Any cloud cover or cloud shadows which obscure image features may render the image
unusable. However, for some areas of an image (e.g. over broad bodies of water) cloud cover
obstruction may be deemed acceptable to some users. Therefore, some users may find
images containing varying percentages of cloud cover or cloud shadow to be acceptable. The
percentage of cloud cover obstruction shall be recorded in the in the metadata field:
(Data_Quality_Information/Cloud_Cover).

12. IMAGE MOSAICKING

Single orthoimages are commonly created through the mosaicking of multiple images.
Temporal and seasonal differences between source images should be minimized to avoid
incongruence across join lines. When a mosaic of two or more digital orthoimage chips is
made, the chip judged by visual inspection to have the best contrast shall be used as the
reference image. The brightness values of the other chips shall be adjusted to match that of
the reference chip. The join lines between the overlapping chips shall be chosen so as to
minimize tonal variations. Localized adjustment of the brightness values shall be performed
to minimize tonal differences between join areas. Identification of the multiple sources as
well as the extent of each chip of a mosaicked image shall be documented in the metadata
field:
(Data_Quality_Information/Lineage/Source_ Information/Source_Citation).

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Federal Geographic Data Committee FGDC-STD-008-1999
Content Standards for Digital Orthoimagery, February 1999

13. METADATA
The FGDC emphasizes the importance of good metadata, in order to provide quality
information about data which will allow users to match data to their needs. This standard
describes a general set of specifications, and as such, places most of the burden on the user to
assess quality and applicability of data. Appropriate metadata facilitates this process.
Certainly, for the user, data with documentation is more useful than data that has none. The
more high quality metadata there is for a product, the more it can support the user’s
determination of its reliability, quality, and accuracy. Metadata is intended to be of value to
the producer as well as to the user.

The FGDC’s "Content Standards for Digital Geospatial Metadata" will be the source for all
issues relating to terminology and definitions relating to metadata. Executive Order 12906
"Coordinating Geographic Data Acquisition and Access: The National Spatial Data
Infrastructure," requires all Federal agencies to use the standard to document data that they
produce beginning in 1995. For more information about the FGDC and the Content
Standard for Digital Geospatial Metadata, contact:

Federal Geographic Data Committee Secretariat

c/o U.S. Geological Survey
590 National Center
Reston, Virginia 20192

Telephone: (703) 648-5514

Facsimile (703) 648-5775
Internet(electronic mail): fgdc@www.fgdc.gov
Anonymous FTP: ftp://www.fgdc.gov
World Wide Web (WWW): http://www.fgdc.gov/fgdc.html

Appendix A contains an example of a metadata file for a specific orthoimage. The example
cited is compliant with the FGDC Content Standard for Geospatial Metadata.

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Federal Geographic Data Committee FGDC-STD-008-1999
Content Standards for Digital Orthoimagery, February 1999

References:

Clinton, William J., 1994. Executive Order 12906, Coordinating Geographic Data Acquisition and Access:
The National Spatial Data Infrastructure. Washington, D.C., Federal Register, Volume 59, Number 71,
pp. 17671-17674.

Department of Commerce, 1992, Spatial Data Transfer Standard. (SDTS) (Federal Information Processing
Standard 173). Washington, Department of Commerce, National Institute of Standards and Technology.

Federal Geographic Data Committee. 1998. Content Standards for Digital Geospatial Metadata, FGDC-STD-
001-1998. Federal Geographic Data Committee, Washington, D.C..

Federal Geographic Data Committee. 1996. FGDC Standards Reference Model. Federal Geographic Data
Committee, Reston, VA.

Federal Geographic Data Committee, 1998. Draft Content Standards for Digital Gridded Land Elevation
Data. Federal Geographic Data Committee, Washington, D.C..

Federal Geographic Data Committee. 1995. Development of a National Digital Geospatial Data Framework.
Federal Geographic Data Committee, Washington, D.C..

Federal Geographic Data Committee, 1998, Geospatial Positioning Accuracy Standards Part 3: National
Standard for Spatial Data Accuracy. FGDC-STD-007.3-1998. Federal Geographic Data Committee,
Washington, D.C.

Pratt, W.K., Copyright ©1978, Digital Image Processing. John Wiley & Sons Inc.

United States Bureau of the Budget, 1947, United States National Map Accuracy Standards, U.S. Bureau of
the Budget, Washington, D.C.

United States Geological Survey, 1998, Aerial Camera Specifications. United States Geological Survey,
Reston, VA.

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Content Standards for Digital Orthoimagery, February 1999

Appendix A
Example of an FGDC Compliant Metadata File
(informative)

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Federal Geographic Data Committee FGDC-STD-008-1999
Content Standards for Digital Orthoimagery, February 1999
Appendix A (informative): Example of an FGDC Compliant Metadata File

This appendix illustrates FGDC compliant metadata reporting, using a USGS 3.75-minute digital
orthophoto (Washington West SE) as an EXAMPLE. The following text illustrates a file specific level
implementation of the "Content Standards for Digital Geospatial Metadata". Numbers preceding element
names indicate the location of the element definition in the metadata standard, and are for reference
only. Reference line numbers should not be included in metadata produced for actual products.
Element names are in bold type.

1. Identification_Information:
1.1 Citation:
8.1 Originator: WMC U.S. Geological Survey
8.2 Publication_Date: 19930608
8.4 Title: Washington West SE
8.6 Geospatial_Data_Presentation_Form: remote-sensing image
8.8 Publication_Information:
8.8.1 Publication_Place: Reston, VA
8.8.2 Publisher: U.S. Geological Survey
1.2 Description:
1.2.1 Abstract:
A digital orthophoto is a raster image of remotely sensed data in which displacement in the
image due to sensor orientation and terrain relief have been removed. Orthophotos combine the
image characteristics of a photograph with the geometric qualities of a map. The primary digital
orthophoto quad (DOQ) is a 1-meter ground resolution, quarter-quadrangle (3.75-minutes of
latitude by 3.75-minutes of longitude) image cast on the Universal Transverse Mercator
Projection (UTM) on the North American Datum of 1983 (NAD83). The geographic extent of
the DOQ is equivalent to a quarter-quad plus overedge. The overedge ranges a minimum of 50
meters to a maximum of 300 meters beyond the extremes of the primary and secondary corner
points. The overedge is included to facilitate tonal matching for mosaicking and for the
placement of the NAD83 and secondary datum corner ticks. The normal orientation of data is by
lines (rows) and samples (columns). Each line contains a series of pixels ordered from west to
east with the order of the lines from north to south. The standard, archived digital orthophoto is
formatted as four ASCII header records, followed by a series of 8-bit binary image data records.
The radiometric image brightness values are stored as 256 gray levels ranging from 0 to 255. The

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Federal Geographic Data Committee FGDC-STD-008-1999
Content Standards for Digital Orthoimagery, February 1999
Appendix A (informative): Example of an FGDC Compliant Metadata File

metadata provided in the digital orthophoto contain a wide range of descriptive information
including format source information, production instrumentation and dates, and data to assist
with displaying and georeferencing the image. The standard distribution format of DOQs will be
JPEG compressed images on CD-ROM by counties or special regions. The reconstituted image
from the CD-ROM will exhibit some radiometric differences when compared to its uncompressed
original but will retain the geometry of the uncompressed DOQ. Uncompressed DOQs are
distributed on tape.
1.2.2 Purpose:
DOQ's serve a variety of purposes, from interim maps to field references for earth science
investigations and analysis. The DOQ is useful as a layer of a geographic information system and
as a tool for revision of digital line graphs and topographic maps.
1.3 Time_Period_of_Content:
9.1 Single Time/Date:
9.1.1 Calendar Date: 19930514
1.3.1 Currentness_Reference: ground condition
1.4 Status:
1.4.1 Progress: Complete
1.4.2 Maintenance_and_Update_Frequency: Irregular
1.5 Spatial_Domain:
1.5.1 Bounding_Coordinates:
1.5.1.1 West_Bounding_Coordinate: -077.0625
1.5.1.2 East_Bounding_Coordinate: -077.00
1.5.1.3 North_Bounding_Coordinate: 38.9375
1.5.1.4 South_Bounding_Coordinate: 38.875
1.6 Keywords:
1.6.1 Theme:
1.6.1.1 Theme_Keyword_Thesaurus: None
1.6.1.2 Theme_Keyword: DOQ
1.6.1.2 Theme_Keyword: DOQQ
1.6.1.2 Theme_Keyword: digital orthophoto
1.6.1.2 Theme_Keyword: digital orthophoto quad

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Appendix A (informative): Example of an FGDC Compliant Metadata File

1.6.1.2 Theme_Keyword: digital image map

1.6.1.2 Theme_Keyword: aerial photograph
1.6.1.2 Theme_Keyword: rectified photograph
1.6.1.2 Theme_Keyword: rectified image
1.6.1.2 Theme_Keyword: orthophoto
1.6.1.2 Theme_Keyword: quarter-quadrangle orthophoto
1.6.1.2 Theme_Keyword: 1-meter orthophoto
1.6.1.2 Theme_Keyword: 2-meter orthophoto
1.6.1.2 Theme_Keyword: 3.75- x 3.75-minute orthophoto
1.6.1.2 Theme_Keyword: 7.5- x 7.5-minute orthophoto
1.6.2 Place:
1.6.2.1 Place_Keyword_Thesaurus:
U.S. Department of Commerce, 1977, Countries, dependencies, areas of special sovereignty,
and their principal administrative divisions (Federal Information Processing Standard
10-3):Washington, D.C., National Institute of Standards and Technology.
1.6.2.2 Place_Keyword: US
1.6.2.1 Place_Keyword_Thesaurus:
U.S. Department of Commerce, 1987, Codes for the identification of the States, the District
of Columbia and the outlying areas of The United States, and associated areas (Federal
Information Processing Standard 5-2): Washington, D. C., National Institute of Standards
and Technology.
1.6.2.2 Place_Keyword: DC
1.6.2.2 Place_Keyword: VA
1.6.2.1 Place_Keyword_Thesaurus:
U.S. Department of Commerce, 1990, Counties and equivalent entities of The United States,
its possessions, and associated areas (Federal Information Processing Standard 6-4):
Washington, D.C. National Institute of Standards and Technology.
1.6.2.2 Place_Keyword: 001
1.6.2.2 Place_Keyword: 013
1.7 Access_Constraints: None
1.8 Use_Constraints: None. Acknowledgment of the U.S. Geological Survey would be appreciated in

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Appendix A (informative): Example of an FGDC Compliant Metadata File

products derived from these data.

1.13 Native_Data_Set_Environment: DV1.2 03/94 OV1.1 04/93 bytes=47702272

2. Data_Quality_Information
2.1 Attribute_Accuracy:
2.1.1 Attribute_Accuracy_Report:
During photographic reproduction of the source photography, limited analog dodging is
performed to improve image quality. Analog dodging consists of holding back light from certain
areas of the sensitized photographic material to avoid overexposure. The diapositive is inspected
to insure clarity and radiometric uniformity. Diapositive image brightness values are collected
with a minimum of image quality manipulation. Image brightness values may deviate from
brightness values of the original imagery due to image value interpolation during the scanning
and rectification processes. Radiometry is verified by visual inspection of the digital orthophoto
quadrangle with the original unrectified image to determine if the digital orthophoto has the
same or better image quality as the original unrectified input image. Slight systematic
radiometric differences can be detected between adjacent DOQ files due primarily to differences
in source photography capture dates and sun angles of aerial photography along flight lines.
These differences can be observed in an image's general lightness or darkness when compared to
adjacent DOQ file coverages.
2.2 Logical_Consistency_Report:
All DOQ header data and image file sizes are validated by the Tape Validation System (TVS)
software prior to archiving in the National Digital Cartographic Data Base (NDCDB). This validation
procedure assures correct physical format and field values for header record elements. Logical
relationships between header record elements are tested.
2.3 Completeness_Report:
All DOQ imagery is visually inspected for completeness to ensure that no gaps, or image
misplacement exists in the 3.75' image area or in overedge coverage. DOQ images may be derived by
mosaicking multiple images, in order to insure complete coverage. All DOQ's are cloud free within
the 3.75' image area. Some clouds may, very infrequently, be encountered only in the overedge
coverage. Source photography is leaf-off in deciduous vegetation regions. Void areas having a
radiometric value of zero and appearing black may exist. These are areas for which no photographic
source is available or result from image transformation from other planimetric systems to the

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Appendix A (informative): Example of an FGDC Compliant Metadata File

Universal Transverse Mercator (UTM). In the latter case, the void sliver areas are on the outside
edges of the overedge area. The data set field content of each DOQ header record element is
validated to assure completeness prior to archiving in the NDCDB.

The area of coverage for a standard USGS digital orthophoto is either a quarter-quadrangle (3.75-
minutes of latitude by 3.75-minutes of longitude plus overedge) or quadrangle (7.5-minutes of latitude
by 7.5-minutes of longitude plus overedge).

USGS requires image overedge to provide overlap coverage between adjoining DOQ's to facilitate
edge matching and mosaicking. That overedge extent is 300 (±30) meters beyond the extremes of the
primary and secondary datum corner points for the standard digital orthophoto quad. However, some
Federal, State and local agencies, and private entities not associated with the National Digital
Orthophoto Program (NDOP) may provide DOQs to the USGS under cooperative agreement
programs.

In order to meet the requirements of the NDOP program and include other sources of DOQs, the
geographic extent for DOQs shall be:
o For DOQs produced under National Digital Orthophoto Program funding agreements: 300
(±30) meters minimum beyond the extremes of the primary and secondary datum corner
points.
o For DOQs produced under other cooperative agreements: a minimum of 50 meters beyond
the primary and secondary horizontal datum corner point extremes.
The resulting digital orthophoto is a rectangle whose size may vary in relation to adjoining digital
orthophotos.
2.4 Positional_Accuracy:
2.4.1 Horizontal_Positional_Accuracy:
2.4.1.1 Horizontal_Positional_Accuracy_Report:
The DOQ horizontal positional accuracy and the assurance of that accuracy depend, in part,
on the accuracy of the data inputs to the rectification process. These inputs consist of the
digital elevation model (DEM),aerotriangulation control and methods, the photo source
camera calibration, scanner calibration, and aerial photographs that meet National Aerial
Photography Program (NAPP) standards. The vertical accuracy of the verified USGS format

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Appendix A (informative): Example of an FGDC Compliant Metadata File

Elevation Model is equivalent to or better than a USGS level 1 or 2 DEM, with a root mean
square error (RMSE) of no greater than 7.0 meters. Field control is acquired by third order
class 1 or better survey methods sufficiently spaced to meet National Map Accuracy
Standards (NMAS) for 1:12,000-scale products. Aerial cameras have current certification
from the USGS, National Mapping Division, Optical Science Laboratory. Test calibration
scans are performed on all source photography scanners. Horizontal positional accuracy is
determined by the Orthophoto Accuracy (ORACC) software program for DOQ data produced
by the National Mapping Division. The program determines the accuracy by finding the line
and sample coordinates of the passpoints in the DOQ and fitting these to their ground
coordinates to develop a root mean square error (RMSE). Four to nine points are checked.
As a further accuracy test, the image line and sample coordinates of the DEM corners are
transformed and compared with the actual X,Y DEM corner values to determine if they are
within the RMSE. Additional information on this testing procedure can be found in U.S.
Department of the Interior, U.S. Geological Survey, 1993, Technical Instructions, ORACC
Users Manual (draft): Reston, VA. Adjacent DOQ's, when displayed together in a common
planimetric coordinate system, may exhibit slight positional discrepancies across common
DOQ boundaries. Linear features, such as streets, may not be continuous. These edge
mismatches, however, still conform to positional horizontal accuracy within the NMAS.
Field investigations to validate DOQ positional accuracy reliability are periodically
conducted by the USGS, National Mapping Division, Geometronics Standards Section.
DOQ's produced by cooperators and contractors use similarly approved RMSE test
procedures.
2.4.1.2 Quantitative_Horizontal_Positional_Accuracy_Assessment:
2.4.1.2.1 Horizontal_Positional_Accuracy_Value: 0.8
2.4.1.2.2 Horizontal_Positional_Accuracy_Explanation:
U.S.Bureau of the Budget, 1947, United States National Map Accuracy Standard.
2.5 Lineage:
2.5.1 Source_Information:
2.5.1.1 Source_Citation:
8.1 Originator: U.S. Geological Survey
8.2 Publication_Date: unknown
8.4 Title: digital elevation model

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Appendix A (informative): Example of an FGDC Compliant Metadata File

8.8 Publication_Information:
8.8.1 Publication_Place: Reston, VA
8.8.2 Publisher: U.S. Geological Survey
2.5.1.3 Type_of_Source_Media: cartridge tape
2.5.1.4 Source_Time_Period_of_Content:
9.1 Single_Date/Time:
9.1.1 Calendar_Date: 1968
2.5.1.4.1 Source_Currentness_Reference: ground condition
2.5.1.5 Source_Citation_Abbreviation: DEM1
2.5.1.6 Source_Contribution:
Elevation data in the form of an ortho-DEM regridded to user-specified intervals and
bounds.
2.5.1 Source_Information:
2.5.1.1 Source_Citation:
8.1 Originator: U.S. Geological Survey
8.2 Publication_Date: Unknown
8.4 Title: NAPP 4-179
8.6 Geospatial_Data_Presentation_Form: remote-sensing image
8.8 Publication_Information:
8.8.1 Publication_Place: Reston, VA
8.8.2 Publisher: U.S. Geological Survey
2.5.1.2 Source_Scale_Denominator: 40000
2.5.1.3 Type_of_Source_Media: cartridge tape
2.5.1.4 Source_Time_Period_of_Content:
9.1 Single_Date/Time:
9.1.1 Calendar_Date: 19880405
2.5.1.4.1 Source_Currentness_Reference: ground condition
2.5.1.5 Source_Citation_Abbreviation: PHOTO1
2.5.1.6 Source_Contribution: Panchromatic Black and White NAPP
2.5.1 Source_Information:
2.5.1.1 Source_Citation:

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Appendix A (informative): Example of an FGDC Compliant Metadata File

8.1 Originator: U.S. Geological Survey

8.2 Publication_Date: Unpublished material
8.4 Title: project ground and photo control
8.8 Publication_Information:
8.8.1 Publication_Place: Reston, VA
8.8.2 Publisher: U.S. Geological Survey
2.5.1.3 Type_of_Source_Media: various media
2.5.1.4 Source_Time_Period_of_Content:
9.3 Range_of_Dates/Times:
9.3.1 Beginning_Date: various
9.3.2 Ending_Date: various
2.5.1.4.1 Source_Currentness_Reference: ground condition
2.5.1.5 Source_Citation_Abbreviation: CONTROL_INPUT
2.5.1.6 Source_Contribution:
Horizontal and vertical control used to establish positions and elevations for reference and
correlation purposes.
2.5.1 Source_Information:
2.5.1.1 Source_Citation:
8.1 Originator: U.S. Geological Survey
8.2 Publication_Date: Unpublished material
8.4 Title: report of calibration
8.8 Publication_Information:
8.8.1 Publication_Place: Reston, VA
8.8.2 Publisher: U.S. Geological Survey
2.5.1.3 Type_of_Source_Media: disc, paper
2.5.1.4 Source_Time_Period_of_Content:
9.3 Range_of_Dates/Times:
9.3.1 Beginning_Date: various
9.3.2 Ending_Date: various
2.5.1.4.1 Source_Currentness_Reference:
Date of the camera calibration associated with the source photography

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Appendix A (informative): Example of an FGDC Compliant Metadata File

2.5.1.5 Source_Citation_Abbreviation: CAMERA_INPUT

2.5.1.6 Source_Contribution: camera calibration parameters
2.5.2 Process_Step:
2.5.2.1 Process_Description:
The production procedures, instrumentation, hardware and software used in the collection of
standard USGS DOQ's vary depending on systems used at the contract, cooperator or USGS
production sites. The majority of DOQ data sets are acquired through government contract.
The process step describes, in general, the process used in the production of standard USGS
DOQ data sets.
The rectification process requires a user parameter file as input to control the rectification
process, a digital elevation model (DEM1) gridded to user specified bounds, projection, zone,
datum and X-Y units, a scanned digital image file (PHOTO1) covering the same area as the
DEM, ground X-Y-Z point values (CONTROL_INPUT) and their conjugate photo
coordinates in the camera coordinate system, and measurements of the fiducial marks
(CAMERA_INPUT) in the digitized image.
The camera calibration report (CAMERA_INPUT) provides the focal length of the camera
and the distances in millimeters from the camera's optical center to the camera's 8 fiducial
marks. These marks define the frame of reference for spatial measurements made from the
photograph. Ground control points (CONTROL_INPUT) acquired from ground surveys or
other sources are third order class 1 or better and meet National Map Accuracy Standards
(NMAS) for 1:12,000-scale. Ground control points are in the Universal Transverse
Mercator or the State Plane Coordinate System on NAD83. Horizontal and vertical residuals
of aerotriangulated tie-points are equal to or less than 2.5 meters. Standard
aerotriangulation passpoint configuration consists of 9 ground control points, one near each
corner, one at the center near each side and 1 near the center of the photograph, are used.
The conjugate positions of the ground control points on the photograph are measured and
recorded in camera coordinates.
The raster image file (PHOTO_1) is created by scanning an aerial photograph film
diapositive with a precision image scanner. An aperture of approximately 25 to 32 microns
is used, with an aperture no greater than 32 microns permitted. Using 1:40,000-scale
photographs, a 25-micron scan aperture equates to a ground resolution of 1-meter. The
scanner converts the photographic image densities to gray scale values ranging from 0 to 255

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Federal Geographic Data Committee FGDC-STD-008-1999
Content Standards for Digital Orthoimagery, February 1999
Appendix A (informative): Example of an FGDC Compliant Metadata File

for black and white photographs. Scan files with ground resolution less than 1 meter or
greater than 1 meter but less than 1.28 meters are resampled to 1 meter.
The principal elevation data source (DEM1) are standard DEM data sets from the National
Digital Cartographic Data Base (NDCDB). DEM's that meet USGS standards are also
produced by contractors to fulfill DOQ production requirements and are subsequently
archived in the NDCDB. All DEM data is equivalent to or better than USGS DEM standard
level 1. The DEM used in the production of DOQ's generally has a 30-meter grid post
spacing and possesses a vertical RMSE of 7-meters or less. A DEM covering the extent of
the photograph is used for the rectification. The DEM is traversed from user-selected
minimum to maximum X-Y values and the DEM X-Y-Z values are used to find pixel
coordinates in the digitized photograph using transformations mentioned above. For each
raster image cell subdivision, a brightness or gray-scale value is obtained using nearest
neighbor, bilinear, or cubic convolution resampling of the scanned image. The pixel
processing algorithm is indicated in the header file. An inverse transformation relates the
image coordinates referenced to the fiducial coordinate space back to scanner coordinate
space. For those areas for which a 7.5-minute DEM is unavailable and relief differences are
less than 150 feet, a planar-DEM (slope-plane substitute grid) may be used.
Rectification Process: The photo control points and focal length are iteratively fitted to their
conjugate ground control points using a single photo space resection equation. From this
mathematical fit a rotation matrix of constants about the three axes of the camera is
obtained. This rotation matrix can then be used to find the photograph or camera coordinates
of any other ground X-Y-Z point. Next a two dimensional fit is made between the measured
fiducial marks on the digitized photograph and their conjugate camera coordinates.
Transformation constants are developed from the fit and the camera or photo coordinates are
used in reverse to find their conjugate pixel coordinates on the digitized photograph.
Quality Control: All data is inspected according to a quality control plan. DOQ contractors
must meet DOQ standards for attribute accuracy, logical consistency, data completeness and
horizontal positional accuracy. During the initial production phase, all rectification inputs
and DOQ data sets are inspected for conformance to standards. After a production source
demonstrates high quality, inspections will be made to 10% of delivery lots 40 DOQs per
lot). All DOQ's are visually inspected for gross positional errors and tested for physical
format standards.

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Federal Geographic Data Committee FGDC-STD-008-1999
Content Standards for Digital Orthoimagery, February 1999
Appendix A (informative): Example of an FGDC Compliant Metadata File

2.5.2.2 Source_Used_Citation_Abbreviation: DEM1, PHOTO1, CONTROL_INPUT,

CAMERA_INPUT
2.5.2.3 Process_Date: 19930514

3. Spatial_Data_Organization_Information:
3.2 Direct_Spatial_Reference_Method: raster
3.4 Raster_Object_Information:
3.4.1 Raster_Object_Type: Pixel
3.4.2 Row_Count: 7680
3.4.3 Column_Count: 6208

4. Spatial_Reference_Information:
4.1 Horizontal_Coordinate_System_Definition:
4.1.2 Planar:
4.1.2.2 Grid_Coordinate_System:
4.1.2.2.1 Grid_Coordinate_System_Name: Universal Transverse Mercator
4.1.2.2.2 Universal_Transverse_Mercator:
4.1.2.2.2.1 UTM_Zone_Number: 18
4.1.2.1.2 Transverse_Mercator:
4.1.2.1.2.17 Scale_Factor_at_Central_Meridian: 0.9996
4.1.2.1.2.2 Longitude_of_Central_Meridian: -75.0
4.1.2.1.2.3 Latitude_of_Projection_Origin: 0.0
4.1.2.1.2.4 False_Easting: 500000.
4.1.2.1.2.5 False_Northing: 0.0
4.1.2.4 Planar_Coordinate_Information:
4.1.2.4.1 Planar_Coordinate_Encoding_Method: row and column
4.1.2.4.2 Coordinate_Representation:
4.1.2.4.2.1 Abscissa_Resolution: 1
4.1.2.4.2.2 Ordinate_Resolution: 1
4.1.2.4.4 Planar_Distance_Units: meters
4.1.4 Geodetic_Model:

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Federal Geographic Data Committee FGDC-STD-008-1999
Content Standards for Digital Orthoimagery, February 1999
Appendix A (informative): Example of an FGDC Compliant Metadata File

4.1.4.1 Horizontal_Datum_Name: North American Datum 1983

4.1.4.2 Ellipsoid_Name: Geodetic Reference System 80
4.1.4.3 Semi-major_Axis: 6378137
4.1.4.4 Denominator_of_Flattening_Ratio: 298.257

5. Entity_and_Attribute_Information:
5.2 Overview_Description:
5.2.1 Entity_and_Attribute_Overview:
For DOQ's from panchromatic source, each pixel contains an 8-bit gray-scale value between
0-255. Zero represents black, while 255 represents white. All values between zero and 255
represent a shade of gray varying from black to white. For color-infrared and natural color
DOQs', a digital number from zero to 255 will also be assigned to each pixel but that number will
refer to a color look-up table which will contain the RGB red, blue and green (RGB) values, each
from zero to 255, for that digital number. Areas where the rectification process is incomplete due
to incomplete data (i.e., lack of elevation data, gaps), are represented with the numeric value of
zero.
5.2.2 Entity_and_Attribute_Detail_Citation:
U.S. Department of the Interior, U.S. Geological Survey, 1992, Standards for Digital
Orthophotos: Reston, VA.

A hypertext version is available at:

http://www-nmd.usgs.gov/www/ti/DOQ/standards_doq.html

Softcopy in ASCII format is available at:

ftp://www-nmd.usgs.gov/pub/ti/DOQ/doqstnds/stdoqpt1.txt
ftp://www-nmd.usgs.gov/pub/ti/DOQ/doqstnds/stdoqpt2.txt
Softcopy in WordPerfect format is available at:
ftp//www-nmd.usgs.gov/pub/ti/DOQ/doqstnds/stdoqpt1.wp5
ftp://www-nmd.usgs.gov/pub/ti/DOQ/doqstnds/stdoqpt2.wp5
Softcopy in PostScript format is available at:
ftp://www-nmd.usgs.gov/pub/ti/DOQ/doqstnds/stdoqpt1.ps
ftp://www-nmd.usgs.gov/pub/ti/DOQ/doqstnds/stdoqpt2.ps

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Federal Geographic Data Committee FGDC-STD-008-1999
Content Standards for Digital Orthoimagery, February 1999
Appendix A (informative): Example of an FGDC Compliant Metadata File

6. Distribution_Information:
6.1 Distributor:
10.2 Contact_Organization_Primary:
10.1.2 Contact_Organization: Earth Science Information Center, U.S. Geological Survey
10.4 Contact_Address:
10.4.1 Address_Type: mailing address
10.4.2 Address: 507 National Center
10.4.3 City: Reston
10.4.4 State_or_Province: VA
10.4.5 Postal_Code: 20192
10.5 Contact_Voice_Telephone: 1 800 USA MAPS
10.9 Hours_of_Service: 0800-1600
10.10 Contact_Instructions:
In addition to the address above there are other ESIC offices throughout the country. A full list
of these offices is at:
http://www-nmd.usgs.gov/esic/esic_index.html
6.2 Resource_Description: Digital Orthophoto quad
6.2 Resource_Description: DOQ
6.2 Resource_Description: DOQQ
6.3 Distribution_Liability:
Although these data have been processed successfully on a computer system at the U.S. Geological
Survey, no warranty, expressed or implied, is made by the USGS regarding the utility of the data on
any other system, nor shall the act of distribution constitute any such warranty. The USGS will
warrant the delivery of this product in computer-readable format and will offer appropriate
adjustment of credit when the product is determined unreadable by correctly adjusted computer input
peripherals, or when the physical medium is delivered in damaged condition. Requests for
adjustments of credit must be made within 90 days from the date of this shipment from the ordering
site.
6.4 Standard_Order_Process:
6.4.2 Digital_Form:
6.4.2.1 Digital_Transfer_Information:

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Federal Geographic Data Committee FGDC-STD-008-1999
Content Standards for Digital Orthoimagery, February 1999
Appendix A (informative): Example of an FGDC Compliant Metadata File

6.4.2.1.1 Format_Name: DOQ

6.4.2.1.5 Format_Information_Content:
USGS uncompressed DOQ: The uncompressed USGS DOQ is a raw binary image file
preceded by a metadata header record which consists of four 400-byte ASCII records,
each blank padded to equal the length of a single line of image data.
6.4.2.2 Digital_Transfer_Option:
6.4.2.2.2 Offline_Option:
6.4.2.2.2.1 Offline_Media: 8-mm helical-scan cartridge tape
6.4.2.2.2.3 Recording_Format:
Unlabeled, uncompressed Unix DD archive format. Standard block size: 30,270,
but can be provided at 2,048 or multiples of 2,048.
6.4.2.2.2 Offline_Option:
6.4.2.2.2.1 Offline_Media: 9-track tape
6.4.2.2.2.3 Recording_Format:
Unlabeled, uncompressed Unix DD archive format. Blocksize = 6250.
6.4.2.2.2 Offline_Option:
6.4.2.2.2.1 Offline_Media: 3480 cartridge tape
6.4.2.2.2.3 Recording_Format:
Unlabelled, uncompressed Unix DD archive format. Blocksize = 6250.
6.4.3 Fees:
The online copy of the data set (when available electronically) may be accessed without charge.
For 8-mm cartridge and 9-track tapes the costs are:
1 digital product = $40
2 digital products = $60
3 digital products = $80
4 digital products = $100
5 digital products = $120
6 or more = $90 plus $7 per each product over six
6.4 Standard_Order_Process:
6.4.2 Digital_Form:
6.4.2.1 Digital_Transfer_Information:

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Federal Geographic Data Committee FGDC-STD-008-1999
Content Standards for Digital Orthoimagery, February 1999
Appendix A (informative): Example of an FGDC Compliant Metadata File

6.4.2.1.1 Format_Name: JPEG

6.4.2.1.5 Format_Information_Content:
The USGS compressed DOQ is an IJG JPEG-compressed file. JPEG is a lossy
compression technique. Unlike uncompressed DOQ's the compressed DOQ does not
contain an attached header record as data compression corrupts ASCII text. A separate
metadata file accompanies the compressed image file. The compressed data are
distributed on CD-ROM, generally by county. However, some CD's may contain regions
or partial counties and some counties may require multiple CD-ROM's. The presence of
a DOQ in the NDCDB does not necessarily indicate the file is available on a
compressed, county based CD-ROM.
6.4.2.1.6 File_Decompression_Technique:
The algorithm employed by USGS for compressing DOQs is IJG JPEG, Version 4.0.
This is a lossy compression using a standard Q or quality factor of 30.
6.4.2.1.7 Transfer_Size: 4.5
6.4.2.2 Digital_Transfer_Option:
6.4.2.2.1 Offline_Option:
6.4.2.2.2.1 Offline_Media: CD-ROM
6.4.2.2.2.3 Recording_Format: ISO 9660
6.4.2.2.2.4 Compatibility_Information:
This CD-ROM can be used with all computer operating systems that support
CD-ROM as a logical storage device. All text files on this disc are in ASCII
format. Data files are in ASCII or binary format.
6.4.3 Fees: The charge is $32 per CD-ROM.

7. Metadata_Reference_Information:
7.1 Metadata_Date: 19950627
7.4 Metadata_Contact:
10.2 Contact_Organization_Primary:
10.1.2 Contact_Organization: U.S. Geological Survey
10.4 Contact_Address:
10.4.2 Address: 590 National Center

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Federal Geographic Data Committee FGDC-STD-008-1999
Content Standards for Digital Orthoimagery, February 1999
Appendix A (informative): Example of an FGDC Compliant Metadata File

10.4.3 City: Reston

10.4.4 State_or_Province: VA
10.4.5 Postal_Code: 20192
10.5 Contact_Voice_Telephone: 703 648 5514
10.7 Contact_Facsimile_Telephone: 703 648 5755
10.8 Contact_Electronic_Mail_Address: fgdc@www.fgdc.gov
7.5 Metadata_Standard_Name: Content Standards for Digital Geospatial Metadata
7.6 Metadata_Standard_Version: 19940608

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Federal Geographic Data Committee FGDC-STD-008-1999
Draft Content Standards for Digital Orthoimagery, February 1999
Appendix B (informative): Definitions

Appendix B - Definitions
(informative)

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Federal Geographic Data Committee FGDC-STD-008-1999
Draft Content Standards for Digital Orthoimagery, February 1999
Appendix B (informative): Definitions

DEFINITIONS:
Band - a range of wavelengths of electromagnetic radiation specified to produce a single response
to a sensing device.

Band Interleaved - the ordered mixing of lines (band interleaved by line) or pixels (band
interleaved by pixels) of one or more bands with corresponding lines or pixels of other bands, for
the purpose of forming a single image file.

Band Sequential (BSQ)- a sequence of one image band followed by another image band. A band
sequential file may be formed by appending bands in sequence within a single file.

Bilinear interpolation - the mathematical computation for an unknown value based on the linear
interpolation along two axes. The axes are derived using a coordinate transformation algorithm
to locate the quadrilateral of the four nearest profile points surrounding the unknown point. The
interpolation computes the unknown value based on the average, by use of weights and distances,
of the four nearest known values.

Brightness value (Digital Number) - a number representing a discrete gray level in an image.

Cubic Convolution - a mathematical computation for the interpolation of an unknown value

based on a third degree polynomial equation using surrounding known values.

Digital Orthoimage - a georeferenced digital image prepared from a perspective photograph, or

other remotely-sensed data, in which displacement of objects in the image, due to sensor
orientation and terrain relief, have been removed.

Framework - collection of basic geospatial data upon which users may collect, register or
integrate geospatial information. Thematic categories comprising the framework include:
geodetic control, digital orthoimagery, elevation, transportation, hydrography, governmental
units, and cadastre (FGDC, 1995) .

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Federal Geographic Data Committee FGDC-STD-008-1999
Draft Content Standards for Digital Orthoimagery, February 1999
Appendix B (informative): Definitions

Metadata - Data about data. Textual information describing the content, quality, condition, and
other characteristics of data.

Micron (F) - The unit of length defined to be 0.000001 meter.

Nearest Neighbor - The mathematical computation for an unknown value based solely on the
value of the nearest known value.

Overedge - Refers to data extending beyond the defined primary area of interest. This may be
image data, or fill data required to “square” the image to achieve fixed record lengths.

Panchromatic (photography) - a term applied to photographic materials possessing sensitivity to

all visible spectral colors, including red.

Resample - the use of mathematical values on one cell-based structure based on values originally
given on another structure. Methods include interpolation and extrapolation. See nearest
neighbor, bilinear interpolation, and cubic convolution.

Resection, photogrammetric - determination of the location or height of a camera or of the

photograph taken by that camera with respect to a coordinate system external to the camera.

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