Relief Displacements in a vertical photograph
- Relief displacement is the lateral displacement or shift in the photographic position of the image of an object due to its relief. (The relief is the elevation of the object above or below the datum.)
- If the object is above the datum, the relief displacement is outwards. Whereas, if the object is below the datum, the relief displacement is inwards.
- As the datum is generally taken at the lowest level, the relief displacements are normally outwards.
- As shown in the above figure, the point A’ is the point vertically below A at the datum plane.
- The point A is plotted as ‘a’ & plots A’ plots ‘a’ on the photographs.
- Hence, this shift in the position of two points is equal to aa’ = relief displacement.
Relief displacement is radial from principal point O of photograph.
From the figure
r/R = f/(H-hA)
r= radial distance of a from O, on the photograph
R= radial distance of A from P, on the ground
where, d is the relief displacement of point A.
d= relief displacement
h= height of the object above datum
r= radial distance of the image of the top of the object from O.
- The relief displacement d increases with an increase in the elevation of the object above the datum. It decreases with an increase in the flying height above the datum.
- The relief displacement equation is extremely useful for the determination of the height of various objects such as buildings, poles, chimneys, etc. which appear on an aerial photograph.
In such cases, it is convenient to write the equation-
h = rh/H
In aerial survey, vertical photographs are taken at the proper interval to give the desired overlap of photographs in the strip. The photographs are exposed in such a way that say two consecutive photographs have some common coverage. This overlapping of the photographs along the flight strip is known as longitudinal overlap, end lap, forward overlap or simply overlap.
It is the general practice to have longitudinal overlap of 60%.
Two adjacent flight strips are so photographed that there is a side lap or lateral overlap. The side lap generally varies from 25-35%, with an average value of 30%.
Advantage of Overlaps
- The end lap gives a coverage of the entire strip from two viewpoints. Stereoscopic viewing and measurement require such a coverage.
- Because of the end lap, it is possible to discard the end portion and consider only central portion for construction of mosaics.
- Distortions of the photograph caused by tilt and relief are more pronounced in the edges of the photograph. Because of overlaps, such portions can be rejected and only central portion considered.
- If side lap is not provided, some gaps of uncovered portion would be left in case flights are not straight and parallel.
Selection of Flying height
The flying height of the aircraft above the ground is selected according to the required contour interval in topographic mapping.
Flying Height = (C – factor) * contour interval
The value of C-factor normally varies between 500-1500, depending upon the conditions of the map-compilation process.
Required Number of Photographs
The number of photographs required to cover given a ground area can be determined using the net area covered by a photograph.
Time Interval Between Exposures
Exposures of the photographs should be made at the required time interval (T). This time interval depends upon the ground speed (V) of the aircraft and the ground length (L) covered between exposures.
L= ground length covered by each photograph in the direction of flight (m)
V= Ground speed of the aircraft (m/s)
Inter velometer used to determine the time interval between exposure
Crab of a photograph is the angle formed between the flight line & edges of the photograph in the direction of flight.
Crab occurs when the aircraft is not oriented with the flight line; photo edges are not parallel to the flight line and it usually occurs when the pilot is trying to compensate for a cross wind and orients the plane into the wind to maintain the flight line.
Crabbing should be eliminated as far as possible because it reduces the effective coverage of the photograph.
To correct crab there is an arrangement to rotate the camera about the vertical axis of the mount.
Drift is the lateral shifting of the photograph. The photograph doesn’t stay on the predetermined flight line due to winds. If the aircraft is set on its course without considering the wind velocity, drift will occur.
Photomaps and Mosaics
Photomaps are the aerial photographs which are used as a substitute of maps.
The photomap may consist of one photograph, but generally photomaps are obtained by assembling two or more photographs to form a large map. These large photomaps are assembled from two or more photographs are known as Mosaics.
A mosaic is required if a single photo doesn’t cover the complete area. A mosaic is also required when the given photograph cannot be enlarged to the required scale.
Mosaics are classified as below-
- Strip Mosaic
- Index Mosaic
A strip mosaic is prepared by assembling a series of photographs taken along a single flight strip covering the project area such as roads, canals, railways, etc.
An index mosaic is prepared by assembling a number of photographs by matching the images. These photographs are pasted on a fibre board, and the whole assembly is photographed again.
Advantages of photomaps and mosaics over maps
- The cost of the preparation of photomaps and mosaics is generally less than the maps prepared from the photographs.
- The mosaics of a large area can be prepared in a shorter period than maps.
- The mosaics show all the objects and features in the project area, whereas maps show only a selected feature.
- A layman can easily understand a photomap or a mosaic.
Disadvantage of Photomaps and Mosaics
- Photomaps and mosaics are not the true planimetric representation of the area as they are the perspective projection of that area.
- Photomaps and mosaics cannot be used for quantitative analysis. The measured distances and angles are not the correct values. Therefore, photo maps and mosaics can be used for qualitative analysis.
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