Planning of Aerial Photography-Overlaps, Crab, Drift, Scale, Image Movement, Height accuracy, Camera and others

Before discussing the many aspects, which enter into consideration in planning an aerial photographic mission, it will be helpful to define the following terms for better understanding. Planning of aerial photogrammetry includes Overlaps, Crab, Drift, Scale, Image Movement, Height accuracy, Camera and others important factors.
Planning of Aerial Photography

Vertical aerial photographic coverage of an area is normally taken as a series of overlapping flight strips. As illustrated in below pictures, the end lap is the overlapping of successive photos along a flight strip and the side lap is the overlap of adjacent flight srips.

If stereoscopic coverage of an area is required, the absolute min end lap is 50%. However, to prevent gaps from occurring in the stereoscopic coverage due to crab, tilt, fllying height variations, and terrain variations, end laps > 50% are used. Also, if the photos are to be used for Photogrammetry control extension, images of some points must appear in three successive photographs, a condition requiring > 50% end lap. For these reasons aerial photography for mapping purpose is normally taken with about 60% end lap + 0r – about 5%.

Side lap is required in aerial photography to prevent gaps from occurring between flight strips as a result of drift, crab, tilt, flying height variations and terrain variations. Mapping photography is normally taken with a side lap of about 30%. An advantage realized from using this large a percentage is elimination of the need to use the extreme edges of the photography, where the imagery is of poorer quality. Having distortions of image due to tilt and relief.

Crab :
Crab is a disparity in the orientation of the camera in the aircraft with respect to the aircraft’s actual travel direction. It is usually the result of side winds, which cause the aircraft’s direction of heading to deviate form its actual travel direction as showed in below picture.

Drift :
Drift is the term applied to a failure of the pilot to fly along planned flight lines. It is often caused by strong winds. Excessive drifts are the most common cause for gaps in photo coverage when this occurs, reflights are necessary.

Scale of photography :
The mean scale of photography depends on the focal length of the camera and flying height as evident from the following relationship.

Scale of photography = focal length of the camera / flying altitude

The above expression can be taken as a good approximation of the scale of photography. Though in general the aerial photography is referred to as having certain specific scale like 1/5000, 1/10,000, it is only an approx scale. As, it is not practicable to have the same flying height in a given area. Strictly speaking, the scale of photography not only differs from photo to photo bot also from point to point on the same photo. This is due to

The deviation of the camera axis from the vertical at the time of exposure (tilt)
Variations in the flying height because of the changes in the elevations of the ground.
Variations in the aircraft’s flying height from exposure to exposure. For want of uniform scale, it is not possible to obtain reliable measurements directly form the photographs.
If the terrain is more or less flat, it is possible to “rectify” the photographs and bring them all to a uniform scale by eliminating the effect of the tilt of camera axis. If the terrain is undulating and hilly, simple rectification is not possible.

Image Movement
Image movements occur as a result of forward and other vibratory movements of the aircraft at the time of camera exposure. Its magnitude depends also on the flying height, speed of aircraft and shutter speed of the camera.
Image movement = velocity of aircraft x scale of photography x exposure time
Image motion compensation can be achieved by making the film move in the focal plane of the camera with the same velocity as the image.

Height accuracy or contour interval :
This is a very important consideration, usually applicable in the case of photography for Photogrammetry. The height accuracy expected can be in the order of 0.12 to 0.18 per 1000 of flying height. Thus, if we know the accuracy of heights required, we can fix the flying height, which means the scale of photography.

Camera :
Choice of camera is related to the scale of photography, type of terrain, object of photography etc. For built up areas camera with long focal length is preferable because there will be less of dead ground covered by tall building. If it is  a desert area, a camera with short focal length i.e. SWA camera can be used which will be economical.

Seasons and Time :
Season of photography is very important consideration because the information capacity of photographs depends on it. For example : a forester wants photography  when trees are full of leaves, where as a soil scientist wants photography when trees do not have leaves, so that the soil can be studied better. Thus, the requirements will be clashing depending upon the object of photography. Season, therefore, has to be carefully chosen taking to consideration the local conditions and the type of information expected from the photography.

Time :

Time of photography is also equally important. Shadows are use full for photo interpretation. As a general guide photography should be taken when the suns elevation is between the 30 degrees and 60 degrees.  This is to avoid long shadows. But in flat areas sun’s elevation can be even 20 degrees. In other words, photography should be preferably restricted to 1.5 hr to 3 hr after sunrise and before sunset. In mountainous terrain it is better to take photography during noontime.