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Figure  417.Forward  overlap
Interval Between Exposures

Photography (Advanced) - Advanced manual for photography and other graphic techniques
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possible error in lining up each successive flight line. The longer dimension of the film is always used for the GGS. Figure 4-20 shows the usable portion of a 9- 9-inch negative after the GGF and GGS have been factored  in. NUMBER OF EXPOSURES When  you  are  flying  for  mosaic  mapping  purposes, the  flight  strips  are  usually  made  along  the  long dimension  of  the  area  being  photographed.  This practice reduces the number of turns the aircraft must make to photograph the strips. For example, if the area to be photographed is 5 nautical miles east and west by 10 nautical miles north and south, the strips should be flown  north  and  south. To determine the number of exposures per strip, you should divide the ground-gained forward into the length of the map. When the unit of measurement is in nautical miles, you must convert it into feet (1 nmi = 6,080 ft). Therefore, if the area to be photographed is 10 nautical miles, the area when converted to feet is 60,800 (10 6,080). You add four additional frames to each strip. Two additional  photographs  should  be  taken  just  before reaching the beginning point and two just after the ending  point.  These  four  photographs  allow  for possible errors in reading the beginning point and the Figure 4-20.—Usable portion of a 9- 9-inch negative. ending point of the run on the ground (from the data shown  on  the  flight  chart). You must first calculate the total number of flight strips required to cover the area. Next, divide the ground-gained sideways (GGS) by the total width of the area to determine the total number of strips. Always add  one  additional  strip  to  your  calculations.  To determine the total number of photographs (frames) required for the entire mosaic mission, multiply the number of photographs required for each strip by the number  of  strips. If the camera can hold enough film for the entire mission, you should have no problem. However, if the camera does not hold enough film for the entire mission, you either have to change film between strips or be prepared to make several flights. FLIGHT  LINES Before the mapping flight, you should plot the flight lines for each run and draw them on the flight chart with a color that is easily recognizable. Draw the first flight line along the border of the area to be photographed. The remainder of the flight lines should be evenly spaced  and  parallel  to  one  another. Figure 4-21 shows a nomograph that can be used to determine the number of flight lines required to cover the  target.  This  nomograph  is  for  low-altitude  coverage only. The nomograph (fig. 4-21) is used as follows: 1. Place a straightedge on the width of the area to be  searched  and  another  along  the  altitude  to  be  flown. 2. Note the intersection on line R1. 3. Place a straightedge on the point on R1 and another along the field of view of the camera lens. 4. Note the intersection on line R2. 5. Move to R3, keeping the same relative positions on segments R1 and R2. 6. Place a straightedge on the point on R3 and another along the side lap required. 7. Read the number of flight paths (to the largest whole number). To determine the distance between the plotted lines on the chart, you must change the ground-gained sideways into inches and multiply it by the scale (fraction) used on the chart. For example, if the GGS is 5,400 feet, or 64,800 inches, and the scale of the chart 4-22







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