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.
Overlaps:
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.
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