| Base lining
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- The flight planning is the process of making all relevant preparations and taking certain decisions for taking photographs to satisfy certain application requirements. This may include the following:
- deciding about flying height above datum
- spacing between successive exposures
- separation between flight lines
- After careful decision about these elements, the flight lines are carefully laid on the map of the study area to be photographed. This map is called the flight map.
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| Computation of flight plan
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- The following information is required for effective flight planning (Tiwari and Badjatia, 1985; Sadasivam, 1988):
- Camera focal length
- Photographic scale
- Flying height above datum
- Permissible scale variation
- Relief displacement
- Tilt of photographs
- Size of photograph
- Area to be photographed or ground coverage
- Air base
- Base-height ratio
- Flight line spacing
- Number of flight lines
- Forward or longitudinal overlap
- Side or lateral overlap
- Ground speed of the aircraft
- Number of photographs per flight line
- Drift angle
- Exposure interval, maximum exposure time
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| Focal length (f)
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- It is the most important parameter for flight planning and must be determined by proper calibration.
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| Photographic scale (S)
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- It is specified to ensure that the user can resolve the
smallest objects to be identified. Choice of scale is a
function of project at hand and the experience. This is
a function of the focal length (f) and altitude of aircraft
(H) at the exposure time. Scales vary from large to small.
Very large scales are used for cadastral surveys and small
scale for topographic mapping. The larger the scale, the
better and more accurate the interpretation and plotting.
But this will result in too many photographs and hence
more time and money for covering the same area.
- It is often more convenient to use an average scale corresponding to the average elevation in the study area
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| Flying height (H)
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- This is the height of the camera exposure station and recorded by altimeter (aneroid barometer) and sometimes with a statoscope. It is recorded on the picture itself for easy reference.
- For a given focal length, average scale, and focal length, the flying height H is fixed. Several interrelated factors affect choice of flying height. For example, scale, relief displacement, and tilt.
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| Permissible scale variation
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- Scale variation is caused by combination of change in ground elevation or flying height. Scale variation affect ground coverage. Large scale variation also affect the capability to view images in stereo mode.
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| Relief displacement
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- Large relief displacement create difficulty in forming continuous interrupted picture. Relief displacement decrease with height although increase in height reduces scale. Hence, these two effects have to be balanced.
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| Tilt of photograph
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- The tilt in a photograph can be resolved into two components: x-tilt and y-tilt, along x and y directions respectively. In a photo with y-tilt, the forward overlap will be higher on one side and lower on opposite side. The x-tilt causes the side lap to decrease on one side and to increase on another. Large x-tilt affects flight line spacing.
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| Crab and Drift
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- Crab is the angle formed between flight line and the edges of the photographs in the direction of flight. It reduces the effective width of photographic coverage. This can be rectified by rotating the camera about the vertical axis of camera mount.
- Drift is caused by failure of the aircraft to stay on predetermined flight line. It leads to serious gaps between adjacent flight lines.
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| Ground coverage
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- After choosing scale and camera format, the ground coverage with a single photograph can be calculated. If the longitudinal and lateral overlaps are known, the ground coverage by a stereomodel can be calculated. This coverage is important since it provides approximate mapping area.
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| Airbase (B)
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- This is the distance between two adjacent exposure stations. On photographs, it is the distance between successive principal points which is also called the 'advance'.
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| Base-height ratio (B/H ratio)
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- The ratio of airbase (B) and flying height (H) is called B/H ratio. Normally the vertical scale of stereo model than the horizontal scale. This scale disparity, which helps determination of heights and identification of objects better is called vertical exaggeration.
- The average adult eye base (b) is about 6.5 cm, the corresponding variable h is difficult to measure. Experiments have provided an approximate value of 42.5 cm. This gives approximate b/h value as 0.15. The vertical exaggeration V is defined as V = (B/H) x (h/b)
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| Spacing of flight line spacing
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- It is defined as the distance between two adjacent flight lines or strips, at the photographic scale or ground scale. The direction of flight lines is also important. If there are number of ridges and valleys, it is better to fly parallel to the ridges. The strips are arranged in N-S or E-W direction keeping in mind the movement of sun throughout the day and effects of shadows.
- It is better to run the strips in E-W course, as the dip of the magnetic north towards north (or south) makes accurate course of flight difficult if navigation is with compass only.
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| Number of flight lines
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- It depends on total width of area to be photographed. To avoid any possible gap in the flight line spacing and extra line can be added at ends
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| Number of photographs per flight line
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| Exposure interval
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- This is the time interval between two successive exposures and is a function of longitudinal overlap and aircraft velocity. It is equal to the time taken by aircraft to cover airbase. This can be done with a device known as intervalometer, which automatically make an exposures at fixed interval of time.
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| Maximum exposure time
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Larger diaphragm opening (f - stop setting) allows more light to enter the camera, giving better image. A low shutter speed allows light for long time. The time interval which is small fraction of a second, during which the diaphragm is kept open is called exposure
time. A small value of exposure time results
in poor illumination and darker image. But a higher
value is also problematic since it gives streak
in an image instead of a point while imaging. So
one needs just sufficient exposure time (shorter)
without any appreciable image movement.
- Assuming a permissible image movement of about 0.02 mm, the maximum exposure time can be calculated. The image movement allowed on the photograph, when converted to ground scale and divided by the aircraft speed gives exposure time, and is normally expressed as 1/t seconds.
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