Photogrammetry is the science of obtaining information about physical objects through process of recording, measuring, and interpreting of photographs of the area.
Aerial photogrammetry is the branch of photogrammetry in which photographs of the area are taken with a camera fixed on or near the ground. It is also called as Ground Photogrammetry.
In terrestrial photogrammetry is limited to the plotting of special features e.g., vertical cliff, mountainous terrain etc. However, aerial photograph is used for topographical surveys, preliminary route surveys, i.e., highways, railways pipelines, etc., forest and agriculture surveys.
The main advantages of aerial photogrammetry are the speed with which an area is covered, the case with which topography of inaccessible areas can be detailed, there is no possibility of omitting any field data, and the tremendous amount of details shown.
Types of Photographs
Aerial photographs are classified into two types:
- Vertical Photographs
- Oblique Photographs
Vertical Photographs are taken when the camera axis is vertical i.e. It coincides with line of the gravity of camera.
When the camera axis is perfectly vertical, the photo plane is parallel to the datum plane and the resulting photograph is truly vertical photograph. When the camera axis is tilled slightly from vertical, the resulting photographs is known as tilled photographs. The tilt is generally less than 1st and rarely exceeds 3rd. This tilt is unintentional.
Oblique aerial photographs
Oblique aerial photographs are taken with a camera axis considerably inclined to the vertical. The camera axis is intentionally kept oblique from the vertical.
A low oblique photograph does not include the horizon. Whereas a high oblique photograph includes the horizon.
Aerial Camera Lens
In aerial camera, thick lenses are commonly used, the figure below shows a thick combination lens commonly used in aerial cameras. Point N and N’ are the nodal points that lie on the optical axis. The point N is known as the incident or front nodal point and the point N’ is known as the emergent or rear nodal point.
A ray of light directed towards the incident nodal point N also passes through the emergent nodal point N’ and finally emerges on the other side of the lens in a direction parallel to the direction of the original incident ray.
In other words, we can say that the ray AN is parallel to the ray N’a and the ray BN is parallel to the ray N’b.
Focal plane and fiducial marks
- The focal plane is a plane in an aerial camera on which all incident rays from the object are brought to focus.
- As we all know that objects are very large distance in comparison to the image distances, all the aerial cameras have their focus fixed for infinite object distances.
- The focal plane is set exactly at a distance equal to the focal length behind the nodal point N and the upper surface of the focal plane frame defines the focal plane.
- Fiducial marks are located in a camera image plane to provide reference lines for the measurement of image distances. Generally, 4 or 8 fiducial marks are provided.
- Lines joining the objective fiducial marks are known as the fiducial lines. These lines intersect at a point called the center of collimation.
- In aerial cameras, the center of collimation is at or very close to the principal point.
- The principal point is one of the most important reference points in the focal plane where a line from the rear nodal point N’ of the lens and perpendicular to the focal plane cuts the focal plane.
- In other words, we can say that the principal point is the foot of the perpendicular from the rear nodal point to the focal plane.
- Fiducial lines provide a rectangular coordinate system for the measurement of positions of the images on the photographs.
Map v/s Aerial Photograph
- The map is an orthographic projection, whereas an aerial photograph is a central projection, i.e. perspective projection.
- The map has a single constant scale, whereas it varies from point to point depending upon their elevations in an aerial photograph. In the case of the photograph, the terrain area which has higher elevation is closer to the camera and therefore appears larger than the terrains area laying at a lower elevation.
- The number of details on a map are selective whereas in a serial photograph there is a large number of details.
- Due to the symbolic representation, the clarity of details is more on the maps than on photo.
The above figure shows the geometry of a vertical photograph.
- The lens L is at the exposure station.
- The negative is placed at a distance = focal length (f), behind the rear nodal point (N’) of the camera.
- The object space ABCD is projected as a’b’c’d’ on the negative of the photograph.
- Point O’ is the principal point of the photograph.
- The distance LO’ from the lens to the photograph is equal to the focal length f of the camera.
- As the printing process produces a reversal of the tone and geometry of the negative, the tone and geometry of the positive are exactly the same as that of the object space ABCD on the ground.
- The plane of a contact-printed positive is situated at a distance OL equal to the focal length measured from the front nodal point N of the camera lens.
- The geometry of the positive abcd is the same as that of the object space ABCD on the ground.
The scale of a Vertical Photograph
Scale of photograph (s)= Photo distance/Ground distance
The elevation of the lens above the datum called the Flying Height (H). The elevations of A and B are h above the datum. The datum is usually taken at Mean Sea Level (M.S.L).
Scale of photograph, S = ab/AB
S = f/(H-h)
For a camera of a fixed focal length, the scale of the photograph varies with variation in the flying height H’. Because the height H of the aircraft above the datum is assumed as constant, the scale of the photograph varies with elevation (h) of the terrain above the datum. Obviously, when the elevation is higher, the scale is larger.
Variable elevation terrain
In case of variable elevation terrain scale of a point at a higher elevation will be larger than those at lower elevation
H = Flying height above datum
ha = Elevation of point ‘a’ above datum
Datum Scale (Sd)
The datum scale of a vertical photograph is the scale that would be effective over the entire photograph if all the ground points were projected vertically downwards on the datum (M.S.L.) before being photographed.
Sd = aO/A0P = LO/LP = f/H
Average Scale (Sav)
The average scale is the scale that would be effective over the entire photograph if all the ground points were projected vertically downwards or upwards on a plane of the average elevation of the terrain before being photographed.
In the figure, above the average elevation of the terrain between two points A and B is h. Therefore, the average scale for the terrain between A and B is
The average elevation of the entire terrain between points C and D is also indicated by hav. Therefore, the average scale for the entire terrain between points C and D is
Sav = f/(H-hav)
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