Elements,

Aids, Techniques, Methods, and Procedures of Airphoto

Interpretation

Definitions
Photo Interpretation: The act of examining aerial photographs/images for the
purpose of identifying objects and judging their significance.
Photography: The art or process of producing images on a sensitized surface by the
action of light or other radiant energy.
Image: A reproduction or imitation of the form of a person or thing. The optical
counterpart of an object produced by a lens or mirror or other optical system.
Photogrammetry: The science or art of obtaining reliable measurements by means of
photography.

Activities of Airphoto/Image Interpretation
Detection/Identification - This is primarily a stimulus and response activity. The
stimuli are the elements of image interpretation. The interpreter conveys his or her
response to these stimuli with descriptions and labels that are expressed in
qualitative terms e.g. likely, possible, or probable. Very rarely do interpreter use
definite statements with regard to describing features identified on aerial
photography.
Measurement - As opposed to detection and identification, the making of
measurements is primarily quantitative. Techniques used by air photo interpreters
typically are not as precise as those employed by photogrammetrists who use
sophisticated instruments in making their measurements. Measurements made by
photo interpreters will get you close; given high quality, high resolution, large-scale
aerial photographs and appropriate interpretation tools and equipment, you can
expect to be within feet; whereas with photogrammetry if you employ the same type
of photography and the appropriate equipment you could expect to be within
inches.
Problem Solving - Interpreters are often required to identify objects from a study of
associated objects that they can identify; or to identify object complexes from an
analysis of their component objects. Analysts may also be asked to examine an
image, which depicts the effects of some process, and suggest a possible or probable
cause. A solution may not always consist of a positive identification. The answer
may be expressed as a number of likely scenarios with statements of probability of
correctness attached by the interpreter.
Air photo interpretation is to photogrammetry as statistics is to mathematics; one
deals with precision the other with probability.

Elements of Airphoto/Image Interpretation

Tone/Color Tone can be defined as each distinguishable variation from white to
black. Color may be defined as each distinguishable variation on an
image produced by a multitude of combinations of hue, value and
chroma. Many factors influence the tone or color of objects or
features recorded on photographic emulsions.
But, if there is not sufficient contrast between an object and it's
background to permit, at least, detection there can be no
identification. While a human interpreter may only be able to
distinguish between ten and twenty shades of gray; interpreters can
distinguish many more colors. Some authors state that interpreters
can distinguish at least 100 times more variations of color on color
photography than shades of gray on black and white photography.
Resolution Resolution can be defined as the ability of the entire photographic
system, including lens, exposure, processing, and other factors, to
render a sharply defined image. An object or feature must be
 resolved to be detected and/or identified. Resolution is one of the
most difficult concepts to address in image analysis because it can be
described for systems in terms of modulation transfer (or point
spread) functions, or it can be discussed for camera lenses in terms
of being able to resolve so many line pairs per millimeter. There are
resolution targets that help to determine this when testing camera
lenses for metric quality.
Photo interpreters often talk about resolution in terms of ground
resolved distance, which is the smallest normal contrast object that
can be identified and measured.


Basic - 1st Order Elements
Tone/Color
Tone can be defined as each distinguishable variation from white to black. Color
may be defined as each distinguishable variation on an image produced by a
multitude of combinations of hue, value and chroma. Many factors influence the
tone or color of objects or features recorded on photographic emulsions. But, if
there is not sufficient contrast between an object and it's background to permit, at
least, detection there can be no identification. While a human interpreter may only
be able to distinguish between ten and twenty shades of gray; interpreters can
distinguish many more colors. Some authors state that interpreters can distinguish
at least 100 times more variations of color on color photography than shades of gray
on black and white photography.

Resolution
Resolution can be defined as the ability of the entire photographic system, including
lens, exposure, processing, and other factors, to render a sharply defined image. An
object or feature must be resolved to be detected and/or identified. Resolution is
one of the most difficult concepts to address in image analysis because it can be
described for systems in terms of modulation transfer (or point spread) functions,
or it can be discussed for camera lenses in terms of being able to resolve so many
line pairs per millimeter. There are resolution targets that help to determine this
when testing camera lenses for metric quality. Photo interpreters often talk about
resolution in terms of ground resolved distance which is the smallest normal
contrast object that can be identified and measured.




2nd Order - Geometric Arrangements of Tone/Color
Size
Size can be important in discriminating objects and features (cars vs. trucks or
buses, single family vs. multifamily residences, brush vs. trees, etc. ). In the use of
size as a diagnostic characteristic both the relative and absolute sizes of objects can
be important. Size can also be used in judging the significance of objects and
features. The size of the crowns of trees can be related to board feet that may be cut
for specific species in managed forests. The size of agricultural fields can be related
to water use in arid areas, or the amount of fertilizers used. The size of runways
gives an indication of the types of aircraft that can be accommodated.

Shape
The shape of objects/features can provide diagnostic clues that aid identification.
The Pentagon building in Washington is a diagnostic shape. Man-made features have
straight edges, while natural features tend not to. Roads can have right angle (90)
turns, railroads can't. Other examples include freeway interchanges, old
fortifications (European cities), military installations (surface to air missile sites).


2nd. Order - Spatial Arrangement of Tone/Color
Texture
Texture is the frequency of change and arrangement of tones. This is a micro image
characteristic. The visual impression of smoothness or roughness of an area can
often be a valuable clue in image interpretation. Still water bodies are typically fine
textured, grass medium, brush rough. There are always exceptions though and scale
can and does play a role; grass could be smooth, brush medium and forest rough on
higher altitude aerial photograph of the same area.

Pattern
Pattern is the spatial arrangement of objects. Patterns can be either man-made or
natural. Pattern is a macro image characteristic. It is the regular arrangement of
objects that can be diagnostic of features on the landscape. An orchard has a
particular pattern. Pattern can also be important in geologic or geomorphologic
analysis; drainage pattern can reveal a great deal about the lithology and geologic
structural patterns of the underlying strata. Dendridic drainage patterns develop on
flat-bedded sediments, radial on domes, linear or trellis in areas with faults etc. It
must be noted here that pattern is highly scale dependent.

3rd. Order - Locational or Positional Elements
Site
Site refers to how objects are arranged with respect to one another, or with respect
to terrain features. Aspect, topography, geology, soil, vegetation and cultural
features on the landscape are distinctive factors that the interpreter should be
aware of when examining a site. The relative importance of each of these factors will
vary with local conditions, but all are important. Just as some vegetation grows in
swamps others grow on sandy ridges or on the sunny side vs. the shaded sides of
hills. Crop types may prefer certain conditions (e.g. orchards on hillsides). Man
made features may also be found on rivers (e.g. power plant) or on hilltops (e.g.
observatory or radar facility).
Association
Some objects are so commonly associated with one another that identification of
one tends to indicate or confirm the existence of another. Smoke stacks, cooling
ponds, transformer yards, coal piles, railroad tracks = coal fired power plant. Arid
terrain, basin bottom location, highly reflective surface, sparse vegetation = playa,
which typically have halophytic vegetation e.g. saltbush. Association is one of the
most helpful interpretation clues in identifying man made installations. Aluminum
manufacture requires large amounts of electrical energy. Schools of different grade
levels typically have characteristic playing fields, parking lots and clusters of
buildings. Nuclear power plants are associated with a source of cooling water,
weather patterns can be associated with pollution sources etc.






3rd. Order - Interpreted from lower order elements


Height For some types of analysis e.g. land forms, forestry and some
 intelligence applications, some interpreters believe that after
tone/color height is the most important element for
identification. This is a point of debate, but height can add
significant information in many types of interpretation tasks,
particularly those that deal with the analysis of man-made
features and vegetation. How tall a tree is can tell something
about the expected amount of board feet. How deep an
excavation is can tell something about the amount of material
that was removed (in some mining operations excavators are
paid on the basis of material removed as determined by
photogrammetric measurement of volume).
Shadow Geologists like low sun angle photography because of the
features that shadow patterns can help identify (e.g. fault
lines and fracture patterns). Church steeples and smokestacks
can cast shadows that can facilitate their identification. Tree
identification can be aided by an examination of the shadows
thrown. Shadows can also inhibit interpretation. On infrared
aerial photography shadows are typically very black and can
render targets in shadows uninterpretable.




Techniques of Photographic/Image Interpretation

Collateral Material
A review of all existing source material that pertains to a given area, process, type of
facility or object, can aid in the interpretation process. The use of collateral material
may also result in a better definition of the scope, objectives and problems
associated with a given project. Also called "ancillary data", collateral material may
come in the form of text, tables, maps, graphs, or image metadata. Census data, a
map or description of the flora of a given area, a land use map, meteorological
statistics, or agricultural crop reports can all be used in support of a given
interpretation. Basically, collateral material represents data/information that an
interpreter may use to aid in the interpretation process. Material contained within a
Geographic Information System (GIS) that is used to assist an interpreter in an
analysis task can be considered collateral data. Two classes of collateral materials
deserve special mention: interpretation keys and field verification.
Interpretation Keys
An interpretation key is a set of guidelines used to assist interpreters in rapidly
identifying features. Determination of the type of key and the method of
presentation to be employed will depend upon, a) The number of objects or
conditions to be identified; and, b) The variability typically encountered within each
class of features or objects within the key.
Some authors say that as a general rule, keys are more easily constructed and used
for the identification of man-made objects and features than for natural vegetation
and landforms. For analysis of natural features, training and field experience are
often essential to achieve consistent results. Basically, an interpretation key helps
the interpreter organize the information present in image form and guides him/her
to the correct identification of unknown objects. Keys can be used in conjunction
with any type of remotely sensed data. Such keys can differ from those employed in
other disciplines in that they can consist largely of illustrations, e.g. landforms,
industrial facilities, military installations. Many types of keys are already available, if
you can find or get your hands on them. This can often be very difficult and a reason
why people develop their own keys.
Depending upon the manner in which the diagnostic features are organized, two
types of keys are generally recognized. 1) Selective keys, and 2) Elimination keys.
Selective keys are arranged in such a way that an interpreter simply selects that
example that most closely corresponds to the object they are trying to identify, e.g.
industries, landforms etc. Elimination Keys are arranged so that the interpreter
follows a precise step-wise process that leads to the elimination of all items except
the one(s) that they are is trying to identify. Dichotomous keys are essentially a class
of elimination key. Most interpreters prefer to use elimination keys in their
analyses.
Field Verification
Field verification can be considered a form of collateral material because it is
typically conducted to assist in the analysis process. Essentially, this is the process
of familiarizing the interpreter with the area or type of feature. This type of
verification is done prior to the interpretation to develop a visual "signature" of how
the feature(s) of interest appear on the ground. After an interpretation is made field
verification can be conducted to verify accuracy. Fieldwork is sometimes calculated
as being three times as expensive as lab analysis. (This is why good interpreters can
be so valuable). The nature, amount, timing, method of acquisition, and data
integration procedures should be carefully thought out. Will you use windshield
surveys, point or transect sampling? Will the sampling be random or systematic?
The amount and type of field work required for a given project may vary greatly and
is generally dependent upon the,
a. Type of analysis involved.
b. Image quality, including scale resolution and information to be interpreted.
c. Accuracy requirements for both classification, and boundary delineation.
d. Experience of the interpreter and the knowledge of the sensor, area, and subject.
e. Terrain conditions, and the accessibility of the study area.
f. Personnel availability, access to ancillary material.
g. Cost considerations.

Handling of Imagery
Although a good deal of photo interpretation is still done using paper prints, the use
of diapositive transparencies is increasing. Transparencies can be used either as
single frames or as a roll. Care should be taken when handling transparencies so
that they are not marred. An orderly procedure for the handling of either prints or
transparencies should be developed and adhered to in any interpretation project.
Airphotos are typically numbered with flight name and/or frame number, and
should be kept in order in so far as practical. Different dates and flight lines should
be kept separate, etc. Anytime transparencies are used surfaces should be as clean
as possible and the interpreter should either wear cotton gloves or be sure not to
touch the emulsion surface as skin oils can cause image deterioration.
Stereo Viewing
Binocular vision is natural to all of us, but to the trained interpreter the ability to
perceive stereo is an incredibly valuable asset. Stereo viewing will be covered in
detail later, but suffice it to say that viewing high quality stereo aerial photography
though a mirror stereoscope is like seeing in another dimension. Although the
identification and interpretation of many landscapes can be accomplished with
mono, stereo is required for certain types of studies. The following are some tips for
using stereo effectively.
Basics for Stereo Viewing
1. Make certain that the photos are properly aligned, preferably with the shadows
falling toward the viewer.
2. Keep the eye base and the long axis of the stereoscope parallel to the flight line.
3. Maintain an even glare free illumination on the prints or transparencies.
4. Arrange for comfortable sitting and sufficient illumination.
5. Keep the lenses of the stereoscope clean, properly focused and separated to your
inter-pupillary distance.
6. The novice interpreter should not work with stereo more than 30 minutes out of
any hour period. You have not had a headache until you've had one the comes from
doing stereo interpretation for too long!
Trouble Shooting Stereo
1. Your eyes may be of unequal strength. If you normally wears glasses for reading
or close-up work, you should also wear glasses when using the stereoscope.
2. Poorly illumination, misaligned prints or uncomfortable viewing positions may
result in eye fatigue.
3. Illness or severe emotional distress may create sensations of dizziness in one
using a stereoscope.
4. Reversal of prints may cause psuedo-stereo. A similar problem may occur if prints
are aligned with the shadows falling away from rather than towards the interpreter.
5. Objects that change positions between exposures cannot be viewed in stereo.
6. In areas of steep topography, scale differences in adjacent photographs may make
it difficult to obtain a three dimensional image.
7. Dark shadows or clouds may prohibit stereo viewing of an area by obscuring an
object on one photo.

The Multi Concept


Multi-Station The successive overlapping of images taken along a given flight
line as being flown by an aircraft or by a satellite along an orbit
path. Think of it like "multi-position". Not to be confused with
 multi-stage.
Multi-Band Multi-band indicates individual spectral bands within a given
region of the EM spectrum (e.g. the red green and blue bands of
the visible portion of the EM spectrum). Often seen to have an
overlapping meaning with the next term, multi-spectral.
Multi-Spectral The use of images from various regions of the EM spectrum
(e.g. ultra-violet, visible, infrared, thermal and microwave).
Multi-Date The use of multiple aerial photographs or remotely sensed
images taken over time of a given area.
Multi-Stage This typically means using ground based photos, oblique low
altitude photos and vertical photographs or remotely sensed
images from platforms flying at different altitudes. Multi-stage
has also been applied to sampling strategies; A multi-stage
sampling scheme as used in statistics is one where
progressively more information is obtained for progressively
smaller sub-samples of the area being studied.
Multi- There are times when more information can be obtained using
Direction viewing angles other than vertical.
Multi- Basically, no one interpreter can know everything about a
Disciplinary system in question. By using teams of interpreters and experts
with expertise in different disciplines more information may be
gained for a given application. In the legal system this is most
similar to the "convergence of evidence" idea; having different
viewpoints and different information sources to prove a point
adds validity.
Multi- Remote sensing images are one-time write, many times read.
Thematic Many different themes (e.g. hydrology, vegetation,
transportation, urban areas, etc.) can be extracted from a single
set of images.
Multi-Use Many potential users from environmental planners to resource
managers to public policy decision-makers can use outputs
derived from image analysis and interpretation.


Methods of Search
There are basically two techniques that people tend to follow when searching for
imagery. One is logical search and the other can be termed the "fishing expedition".
In the latter, the fishing expedition, the interpreter searches the imagery in a
random fashion attempting to find recognizable features or object that will lead to
whatever the interpretation goal happens to be. At some point even this type of
interpretation begins to logically converge. Patterns of anomalous vegetation may
lead to looking for water sources, which may lead to looking for transportation
systems, illegal growing etc. Logical search is a more systematic method of analysis
most often used by interpreters.
The logical search involve these things:
1. The interpreter should always keep in mind the basic qualities of the imagery
they are dealing with, e.g. film filter combination, the season and time of day of
acquisition, and the image scale, etc. In addition the interpreter should always
remember to examine all the titling information on an image.
2. Interpretation should begin with the general and proceed to the specific. After
gaining an overall impression of the photograph the interpreter should begin to
examine the physical features (e.g. water bodies, mountains, forests, etc.) and
cultural features (e.g. urban areas, farms, road networks etc.). The interpreter
should then move to more specific questions e.g. what type of trees make up the
forest? What types of roads are present?
3. Interpretation should be conducted logically one step at a time. Following from 2
above it is good to go from a detailed examination of landforms to vegetation, to
hydrology and so on. Then address cultural features in the same fashion. What types
of urban features are present single family residences, multi-family residences,
industries, retail districts and so on.

Convergence of Evidence
1. Image interpretation is basically a deductive process. Features that can be
detected and identified lead the interpreter to the location and identification of
other features. This is convergence, and for many applications of air photo
interpretation this involves the activities of one or two individuals synthesizing a
large amount of information.
2. Deductive interpretation requires either the conscious or unconscious
consideration of all of the elements of image interpretation. The completeness and
accuracy of an interpretation is in some measure proportional to the interpreters
understanding of the "how and the why" of the elements, techniques and methods of
interpretation.
* THE MORE IMAGES YOU INTERPRET THE BETTER INTERPRETER YOU BECOME.

References
Jensen, J.R. , 2000, Remote Sensing of the Environment: An Earth Resource
Perspective, Upper Saddle River, NJ: Prentice Hall, 544 pages. [Ch 5 pages119-135]
Avery T. E. and G. L Berlin, 1992, Fundamentals of remote Sensing and Airphoto
Interpretation, Fifth Edition, New York, Macmillan Publishing Company, 472 p.
[pages 51-67]
Estes, J.E. , E.J. Hajic, and L.R. Tinney (Author-editors), Fundamentals of Image
Analysis: Analysis of Visible and Thermal Infrared Data, Chapter 24, in Manual
of Remote Sensing, 2nd. ed. Falls Church, Va. American Society of Photogrammetry,
pp. 987-1124.
Paine, D. E, 1981, Aerial Photography and Image Interpretation for Resource
Management, New York, John Wiley and Sons, 571 p.