Exposure interval (in seconds) for 60-percentoverlapGround gainedGround coverage per inch of negative andexposure intervalsWhen you intend to use the BM-38A computer,refer to the Photographic Computer Instruction Book,RC-025063, for detailed instructions.SCALEUsually, the area to be mapped is indicated on achart and maximum boundaries are provided. The scalefraction of this chart, or its linear scale, providesimportant information. The amount of area to becovered can be determined from one of these scales.The scale of a map is indicated as a commonfraction or as a ratio. For example, the scale may be1/10,000 or 1:10,000 on the map. In either case, thescale is read "one to ten thousand." This scale indicatesthat one unit of measure on the map is equal to 10,000of the same units on the ground.One problem in aerial mapping is locating the scaleof the mosaic map. When the required scale isprovided, then the altitude and focal length must bedetermined to get the required scale. The scale of aphotographic mosaic map is calculated as follows:S = Scale of the mapF = Focal length of the lensA = Altitude above the groundWith F (in inches), A (in feet) must be multipliedby 12 to convert to the same unit of measurement(inches).S=F12AExample: What is the scale of a map taken from analtitude of 5,000 feet, using a 6-inch lens.S=6==6112 5,00060,00010,000Therefore, the scale is 1/10,000. That means 1 inch onthe photograph equals 10,000 inches on the ground.FORWARD OVERLAPTo ensure complete coverage of the area, youshould take each photograph in each flight line or stripso it overlaps both the preceding photograph and thefollowing photograph. The amount of overlap on eachphotograph is approximately 60 percent. Creating thisoverlap ensures that the strip contains no blank areas(fig. 4-17).The overlap also serves another important function.In the construction of a mosaic map, only the centralarea of each print is used. Only the central area is usedbecause the middle areas of all vertical photographs arethe area of truest reproduction of terrain. (See fig.4-18.)In figure 4-18, the aircraft is flying over a mountainwhile making a series of vertical photographs. For allpractical purposes, when the aircraft is directly over themountain, a perfect reproduction of the mountain isobtained. Pictures taken before and after the onedirectly over the mountain show the near side of themountain clearly, but very little, if any, of the far side.This is caused by the different camera positions inrespect to the subject.Scale is affected by this difference of camerapositions. It is practically impossible to match theedges of prints when these distortions of the terrain arepresent. Therefore, the outer area (toward the edges ofthe print) is discarded and the inner 40 percent of eachprint is used. Another important reason for using onlythe center area of the prints is that stereoscopicmeasurement associated with either contour mapping orphotographic interpretation requires the highest degreeof accuracy.Since a 60-percent overlap is created, only 40percent of the ground-gained forward (GGF) is usablein each negative. For example, a 5- 5-inch negativehas a usable image area of 2 inches. (5.0 0.40 = 2.)To find the actual amount of usable GGF in eachnegative, multiply the ground coverage by 0.40. Forexample, using the IFGA formula, you have determinedthat the ground coverage for each negative is 9,000 feet.The usable GGF in each negative is 3,600 feet (9,000 0.40 = 3,600).SIDE LAPThe area that you are photographing for a mosaicmap may be wide and cannot be photographed in onestrip. The aircraft must fly a number of side-by-sidestrips to get complete coverage so none of the area is4-20