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 x 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- x 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 Rl.

3. Place a straightedge on the point on Rl 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 Rl 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

Figure 4-21.-Number of flight lines nomograph.

is 1/10,000, the distance between flight lines drawn on the chart is 64,800 x 1/20,000, or 3.24 inches. Since it is difficult to measure the flight-line distances accurately, a multi-finger divider should be use fig 4-22 .

To use the multi-finger divider, first multiply the distance between flight lines by the number of fingers on the divider. This will give you the total flight-line plot width. Using a ruler, place the first divider finger on zero and the last divider finger on the mark corresponding to the total flight-line plot width. The individual fingers of the divider automatically space themselves to the correct distance for each flight line. The multi-finger divider may then be used to lay out the flight-line plots.