UNIT 5 : AERIAL PHOTOGRAPHY

Photogrammetry:
Photogrammetry is the science and technology of obtaining reliable information about physical
objects and the environment through the process of recording, measuring and interpreting
photographic images and patterns of electromagnetic radiant imagery and other phenomena.

Aerial photography:
Aerial photography is the photographs of terrain in an area taken by a precision photogrammetric
camera mounted in an air craft flying over an area.

Objects of Aerial photography:

1. To construct or prepare the planimetric and topographies maps.

2. To make reconnaissance survey.
3. To find geological and soil data details.
4. To make survey of buildings.
5. To make survey of mountains and hilly areas.
6. To make survey for inaccessible regions and unhealthy regions.

Uses of Aerial photography:

7. Geological Investigation
8. Soil Survey
9. Land Survey
10. Tax Mapping
11. Reconnaissance and military intelligence
12. Urban and Regional Development
13. Transportation system investigation
14. Quantity Estimation ( Mountains )
15. Shore Erosion

Aerial photography is used in cartography. (particularly in photogrammetric surveys, which are

often the basis for topographic maps), land-use planning, archaeology, movie
production, environmental studies, power line inspection, surveillance, commercial
advertising, conveyancing, and artistic projects
Difference between Maps and Arial Photographs:

MAPS ARIAL PHOTOGRAPS

1. Maps are plotted by actual taking the linear 1. A photograph in which a photograph of
and angular measurements of the ground and Terrain or ground at the time of exposure and
Terrine . showing all the details of the Terrine it’s called
as aerial photograph.
2. There is no distortion caused by tilt or relief 2. There is distortion of detail of Terrain ground
displacement because these maps are prepared cause by tilt and relief displacement.
after taking the necessary and required
measurements.
3. Contour maps which is plotted by carrying 3. Elevation of the different objects or points
actual measurements can be used to determine cannot be taken directly.
the elevation of desired object directly
4. Plotting of maps and its reading is common 4. The photos obtained by aerial photographs are
and more familiar to users. not familiar to the eye of User and its need to
Various objects like roads, ports, River, Valley special techniques to read the photo views.
Railway Crossing, rail lines etc. can be shown by
conventional signs and symbols.
5. Plotting of maps are economical. 5. Preparation of maps and aerial photographs
are uneconomical.

Types of Aerial Cameras:

1. Early Aerial Camera ( Pigeon Mounted )

2. Magazine Camera :
It holds the roll of unexposed film, advances the film between exposures, holds the film in
place and winds-up the exposed film. Aerial camera systems also have a mounting bracket,
power supply, vacuum lines, heating jackets, filters, forward motion compensation (FMC)
and an Interial Motion Unit (IMU). Also part of the system is a viewfinder for targeting the
camera, an intervalometer that determines the rate at which exposures are taken (the
amount of overlap) as the plane flies along the flight path, a navigation control system and
an exposure control system. Computers have assumed many of these tasks which were once
manual/mechanical. Global Positioning Systems (GPS) are now integrated into the camera
system to provide very precise in-flight positional control.
 3. Unmanned aerial vehicle :

An unmanned aerial vehicle (UAV) or uncrewed aerial vehicle, commonly known as a drone is
an aircraft without a human pilot on board and a type of unmanned vehicle. UAVs are a component
of an unmanned aircraft system (UAS); which include a UAV, a ground-based controller, and a
system of communications between the two. The flight of UAVs may operate with various degrees
of autonomy: either under remote control by a human operator or autonomously by on board
computers

Compared to crewed aircraft, UAVs were originally used for missions too "dull, dirty or
dangerous for humans. While they originated mostly in military applications, their use is rapidly
expanding to commercial, scientific, recreational, agricultural, and other applications, such as
policing and surveillance, product deliveries, aerial photography, infrastructure inspections,
smuggling, and drone racing. Civilian UAVs now vastly outnumber military UAVs, with estimates of
over a million sold by 2015.

Types of Photographs:
1. Vertical Photographs
2. Tilted Photograps
3. Oblique Photographs
4. Convergent Photographs
5. Trimtrogen photographs

1. Vertical Photographs:
A vertical photograph is one which has been taken with the camera axis directed toward the ground
as vertically as possible.

OR

The axis of the camera (and lense) is perpendicular to the ground.

  The lance axis is perpendicular to the surface of earth.
 It covers small area.
 The shape of the ground area covered on a single vertical photo closely approximate a
square of rectangle.

2. Tilted Photographs :
An aerial photograph made with axis of the camera Called as optical axis is tilted from the vertical
unintentionally by a small amount is called as tilted photograph.
camera axis or optical axis is in such case is tilted from the vertical usually less than 3° shows tilted
photography

Axis of camera < 3 Degree.

3. Oblique Photographs:
The axis of the camera/lense is tilted at an angle from 3 to 90 degrees to the ground. Scale is
constantly changing from the foreground to the background.

OR

Oblique photograph is one which has been taken with the camera axis directed at an inclination to
the ground.

Types of Oblique Photographs:

1. Low Oblique
2. High Oblique

1. Low Oblique:
In low oblique photograph the angle of tilt may be up to 30° with respect to vertical and image of
Horizon is not included.

2. High Oblique:
The tilt can be more than 30° but less than 90° with respect to vertical and the image of horizon is
also included.
  Ground area covered is trapezoidal.
 No scale is applicable to the entire photograph and distance cannot be measured.
 Parallel lines on ground are not parallel on photographs
 It does not shoe the horizon.

IMPORTANT DEFINATIONS:

Exposure station : The point in the atmosphere occupied by centre of camera lenses at instance of
photography.

Flying height : Vertical distance between exposure station and mean sea level.

Flight line: Line traced by exposure station in atmosphere. ( On which path line ; aircraft is moving
to take photographs )

Photo principal plane (k): It is the point photograph obtained by projecting camera axis to
intersect at a point on photograph known as photo principal point.

Ground principal point (K):

Camera axis extended up to
ground the point obtained on
ground is called as ground
principal point.

Photo Nadir point: it is a point

on photograph obtained by
dropping vertical line from
camera Centre. That plumb line
extended up to ground gives
ground Nadir point (N).

Horizon point (h) : It is point of

intersection of horizontal line
through centre of lenses and
principal line (np) on
photograph.

Principal plane: Plane defined

by exposure station(O), ground
Nadir point(N) and ground
principal point (P). i.e plane NOK
Principal line: Line of intersection of principal plane with photograph Plane.

Azimuth (A): Clockwise horizontal angle measured about ground nadir point from true north to the
principal plane of photograph.

Swing (S): Angle measured in plain photograph from +Y axis clockwise to photograph to photo
nadir point.

Isocenter (i): Point on photo where bisection of tilt falls on photograph.( Dotted line in sketch)

Axis of tilt: It is line in plain of photograph perpendicular to principal line at the isocenter.

Relief Displacement :

Definition : Illustration of deformation of an aerial image according to the distance from the nadir
point that is the centre of the image. A vertical object( such as building) will appear to be laying
along a line radial to the image nadir point this deformation is called as relief displacement.

 L is the perspective centre of the camera system.

 A is the point on ground at an elevation of h with respect to the datum.
 ‘a’ is the image of ground point on photograph.
 a’ is the location of projected point A’ on the datum.

Expression to provide magnitude of relief distortion:

Ground control points (GCPs)

Ground control points (or GCPs) are points on the ground with known coordinates. In an aerial
mapping survey, GCPs are points which the surveyor can precisely pinpoint: with a handful of
known coordinates, it's possible to accurately map large areas.

Or

GCP is any point whose positions are known in an object-space reference coordinate system and
whose images can be positively identified in the photographs.

Why are GCPs needed in photogrammetry

 The GCP in nature determines the position of its aerial photo mage in the coordinate
system.
 To calculate the coordinates for each point on the aerial photography, several ground
control points’ coordinates are used and photogrammetric procedures followed.
 GCPs are necessary for orientation and placement of aerial photographs in the spatial
coordinate system, which is a prerequisite for the production of georeferenced metric
and 3D models of the earth’s surface (point cloud, DSM, DTM, orthophoto plan).
 Namely, computer processing and analysis require spatial coordination models – from
point cloud to orthophoto mosaics.
 In practice , it is sufficient to use from 5 to 10 ground control points. Using a larger
number of points does not significantly contribute to a higher accuracy. The more the
terrain is naturally varied, the higher number of ground control points are necessary to
achieve the desired accuracy.
 Ground control is essential when you need High Global Accuracy (in other words
Absolute Accuracy)
 GCPS are needed when you need products to certain range of accuracy
 They are required to Verify the Accuracy.

Difference between connection of photos with GCP and without GCP

The accuracy of finished photogrammetric products can be no better than the ground control.
This are used for; Geo-referencing products and Accurate determination of processing parameters
(intrinsics & extrinsics)

GROUND CONTROL POINTS

Where do we need GCPs ( Applications )

• Mapping the terrain - Contour Map / DTM / DSM
• Civil Engineering Construction Project design
• Property boundary survey
• Land title surveys
• Insurance assessments
• Large volume measurements
• Volume measurements: When Contracted to provide measurements within a certain range of
accuracy
• General liability

OVERLAPS:
When photographs are taking by using satellite or any aircraft it is necessary to overlap the
number of photographs by 50-60% to generate a proper Maps or mosaics.
Types of Overlaps:
1. End lap
2. Side lap

End lap: Laps are considering in the same flight line (Approx. 60%)
Side lap: Laps are considering in different flight lines ( Approx. 25%)

Reasons for Overlaps

 Arrangement of mosaic
 Remove errors due to distortion, displacement, and tilt.
 For view in stereoscope- 3D view.
 Avoid repetition of aerial survey

Number of photographs required to cover the area :

The size of each photograph = l * w (Net area of photograph )

The size of each photograph= (1- Pl ) l * ( 1- Pw )w

Ground area covered by each photograph = s (1- Pl )l * s (1-Pw ) w

Where,
A= Total area to be photographed W = Net ground area covered by each photo
L = Length of photo in direction of flight A = net ground area covered by each photo
w = width of photo normal to direction Pl = % overlap in direction of flight (length)
s = scale of photograph (f/H) Pw = % overlap in side (width)
L = Net ground distance corresponding to l
Crab and Drift:

CRAB:

Angle formed between flight line and edges of photo in direction of flight and caused by not
having focal plane square with direction of flight at time of exposure.

DRIFT:
When aircraft is swayed away from its pre-planned flight line then it is known as drift.

FLIGHT PLANNING:

A flight planning consists of a flight (navigation) map which shows where the aerial
photographs are to be taken and parameters (specifications) which outlines the specific
requirements such as aerial camera and film requirements, scale, flying height, end lap, side lap,
tilt and swing round (yaw) tolerances, etc.

Procedure for Aerial survey (Flight planning)

Parameters are as follows :
Altitude of flight (H)
Focal length of camera (f)
Size of photograph
Size of area or land to be photographed
Alignment of flight line and parallel flight line
Lateral overlap
No of photo in each flight line and overlap
Scale of flight map.
Ground speed of aeroplane
Time interval of successive photographs
Photomaps and Mosaics :
Photomaps: Photomap is single photograph taken by satellite or any aircraft.
Mosaics: Two or more photograph combined is known as mosaic.
Cost and time saves in mosaics preparing. Mosaics can be used by non-technical person.