Air Fronts: FM 21-26, Advanced Map and Aerial Photograph Reading - Section 10.

SECTION X: AERIAL PHOTOGRAPHS AS MAP SUBSTITUTES

62. COMPARISON OF AERIAL PHOTOGRAPHS WITH LINE MAPS. A topographic map is a line drawing of the land, showing objects and features by exaggerated conventional signs. An aerial photograph is an actual picture of the earth's surface which shows terrain features as they appear from the air.

a. An aerial photograph has the following advantages over a topographic map:

(1) It has a wealth of detail which no map can equal.

(2) It has accuracy of form.

(3) It can be obtained and developed in a short time.

(4) It may be made of areas otherwise inaccessible for physical or military reasons.

(5) It is up to date.

(6) Unobscured military features can be studied and interpreted.

b. The vertical photograph is inferior to a topographic map in the following ways:

(1) Important military features emphasized on a map are sometimes obscured or hidden by other detail.

(2) Neither absolute position nor absolute elevation can be obtained.

(3) Relative relief is not readily apparent.

(4) Displacement of position caused by relief and camera tilt usually do not permit the accurate determination of either distance or direction.

(5) Because of a lack of contrast in tone, it is difficult to read in poor light.

(6) Marginal data furnished on maps are generally lacking.

63. IDENTIFICATION. Topographic identification is the art of identifying visible features of the terrain from their images on a photograph. See FM 21-25 and TM 5-246 for the principles of aerial photograph reading.

64. SCALES. Scale affects the degree of interpretation possible. For example, note the differences in detail visible in figure 70 (1) and (2). The scale of aerial photographs can be determined in the following ways:

a. Focal length and lens height. To find scale by focal length and lens height, look in the marginal data for the focal length of the lens in the camera and the altitude at which the picture was made. The focal length is given first, in inches; the altitude is given next, in feet. The information may appear as (12-20,000), which means the photograph was made with a 12-inch focal length at an altimeter reading of 20,000 feet (see par. 59c(1)). The diagram in figure 71 shows the relation between the focal length of the camera, altitude of the plane, ground distance AB, and photo distance ab. The RF of the photograph is the focal length in inches divided by the altitude in inches. Thus, if the focal length is 12 inches and the altitude is 20,000 feet, the RF of the photograph is found by the formula:

RF =

The RF is . With a focal length of 6 inches, the RF would be or 1:40,000.

Figure 70. (1) Vertical photograph from high altitude.

Figure 70. (2) Low-altitude view of same terrain .

When altitude given is altitude above sea level, subtract the height of the ground above sea level from the altitude before using the equation. Thus, using examples just given, if the ground is 2,000 feet above sea level,

RF with 12-inch focal length would be , or scale 1:18,000; and with 6-inch focal length,    or scale 1:36,000.

b. By comparison with a map. The RF of a photograph can be determined by comparing it with a map. To do so, select two points on the photograph (such as crossroads or road junctions) that can be easily found on the map. A line between the points should pass as near the center of the photograph as possible, with the

Figure 71. Determining scale from focal length and lens height.

points about the same distance from the center of the picture and at approximately the same elevation. For example see line AD or line CB in figure 72. The distance along the line on the photograph is the photograph distance, or PD, in inches; this constitutes the numerator of the RF Ground distance, GD, is obtained by scaling the distance from the map; this is the denominator. RF is written as RF = or RF=. Thus, if the distance between two points along a line on the photograph is 6 inches, and the scaled ground distance from the map between the same points is 5,000 feet, then the RF =  or   and the scale of the photograph is 1:10,000.

c. By comparison with the ground. The RF of a photograph can be determined by comparison with ground distance if the two points selected on the photograph are accessible on the ground. This is done by measuring the distance between two points on the photograph and the actual ground distance between the same points. By the RF formula, if the distance on the photograph is 6 inches and the actual ground distance is 3,000 yards the RF of the photograph is or .

Figure72. Determining scale of photograph by comparison with map.

 d. Average RF. A more accurate scale is found by figuring the RF from several different scale lines, then taking the average of all the denominators. Thus, if  the RF's of two different scale lines were and , the average scale would be .

65. GRAPHIC SCALE. Preparing a photograph as a map substitute may include constructing a graphic scale in yards or other appropriate units. If the RF of the photograph is known, the graphic scale is constructed by the methods employed in constructing graphic scales for maps (see par. 12). On mounted photographs place the scale on the lower margin of the mount. On unmounted photographs place it on the back of the photograph where it will not obscure valuable detail.

66. DISTANCE. After the RF of a photograph is determined and a graphic scale is constructed, the map reader is ready to scale distances. The paper-strip method of measuring distances, discussed in FM 21-25, is the easiest method. The map measurer and the engineer scale may also be used.

67. DIRECTION. a. General. (1) For normal military uses, direction angles and azimuths on a photograph have the same significance as on a map and may be measured and laid out with the protractor in exactly the same manner. However, angles measured from the principal center point of the photograph more closely approximate their values as measured on the ground, since errors in position on a near vertical photo are radial from points falling near the center.

(2) To lay out or to measure directions on a photograph, magnetic, true, or grid north may be indicated on the photograph. Magnetic north is the most suitable base direction for photographs of unmapped country.

b. Finding north by shadows. (1) The shadow cast by objects in a photograph can be used to find true north on the photo. In the north temperate zone, shadows fall true north at noon, northwest in the morning, and northeast in the afternoon. If the note in the margin indicates the picture was made between 1000 and 1200, the shadows will be west of north . If made between 1200 and 1400, the shadows will be east of north. In the south temperate zone the directions are reversed.

 (2) In any particular locality, an hourly angular shadow variation may be obtained by measuring the angle through which a shadow moves in 1 hour, as in figure 73 (1). This value in degrees can then be applied as an adjustment to find true north on a photograph taken at any time. 

(3) For example, assume that the position is north of the equator and that in 1 hour the sun moves the shadow through 13°. Then if the aerial photograph of that area shown in figure 73 (2) was made at 0900, the shadows would point 13° X 3 (hours) or 39° west of north. Therefore, the approximate true north-south line would be 39° to the right of the line of shadows. Draw a line on the photograph in the direction of the shadows and measure 39° to the right with a protractor. The dashed line in figure 73 (2) is approximately true north. If this same photograph had been made at 1500, true north would point 39° to the left of the line of shadows.

Figure 73. Finding north by shadows in photograph.

South of the equator the directions are reversed. All time must be sun time.

c. Locating magnetic north on a photograph from a map. A map can be used to locate magnetic north on a photograph. First, choose two points (such as A and B in figure 74) which appear plainly on both photograph and map and are so located that a line joining them passes through or near the center of the photograph. With a protractor, find the grid azimuth of the line on the map. In this case, it is 45°' and the magnetic azimuth is 54°. From the corresponding line AB on the photograph, measure 54° counterclockwise. A line drawn through the index of the protractor along this new direction is the same magnetic north as on the map.

68. ORIENTATION. Like a map, an aerial photograph must be oriented. This is done by turning the photograph until some well-defined line on it, such as a road, lies parallel to the same line on the ground. To

Figure 74. Using map to locate magnetic north on photograph.

check the orientation, place the compass on the magnetic line, found by methods used in paragraph 67, and rotate the photograph horizontally until the compass needle points along the line. Other methods of orienting photographs are covered in FM 21-25.

69. POINT-DESIGNATION GRID. a. Printing accurate grids on photographs is impracticable because of scale distortion. A special grid, known as the pointdesignation grid, may be used. This grid has no relation to the actual scale or orientation of the photograph; it serves only for point or target designations and normally is not suitable for measuring distance or azimuth. For convenience, the dimension of the grid square is 1.44 inches. The 1:25,000 scale then can be used for determining and plotting the coordinates of points.

b. A point-designation grid may be printed on a photo or it may be printed on a transparent sheet-the point-designation grid template-for use on photographs not gridded. It is essential that all concerned place the grid or template on a photograph in exactly the same manner. For vertical photographs the procedure is, first, to turn the photograph so the marginal information, whether at the top or bottom, is in the normal reading position. Then draw grid line A through the fiducial marks at the top and bottom of the photograph and grid line M through the fiducial marks on the sides of the photograph (fig. 75). Additional grid lines are drawn parallel to grid lines A and M at 1.44-inch spacing. Those parallel to and above M are lettered from M to the top of the photograph N, O, P, 2, and so on. Those below M are lettered from M toward bottom of photograph in the order Z, Y, X, W, and so on. Lines to the right of A are lettered B, C, D, et cetera, and those to the left L, K, J, I, et cetera. When

Figure 75. Point-designation grid.

an oblique photograph is used, it is placed in the normal photo reading position with the foreground at the bottom, and grid lines are placed as described above. North does not necessarily point to the top of the picture (see north arrow in grid KN, fig. 75).

c. To name the coordinates of a point, first indicate by two letters the intersecting grid lines at the lower left corner of the grid square in which the point is located; then read right and up to get the two numerical coordinates. For example, the coordinates of building indicated by the white arrow in figure 75 are AN4535, reading in hundredths. If less accuracy is sufficient, the coordinates in tenths are AN54.

70. EMPHASIZING AND CLARIFYING DETAIL. a. General. It is difficult to write on the face of a photograph; moreover, marks clutter up and hide important detail. Nevertheless, it may be desirable to emphasize certain points and lines such as routes of march, water and supply points, and other important features. The general pattern of the terrain may be clarified by tracing stream lines in blue, ridge lines in brown. Particular objects or points may be noted by placing a small number beside the item on the photograph and explaining its meaning in a key on a separate sheet of paper. An alternate method is to use an overlay.

b. Overlay. An overlay is a tracing of a photograph's detail on transparent material such as a light, tough, tissue-thin paper, or on materials such as tracing cloth and cellulose acetate. Cut the tracing paper a little larger than the photograph (fig. 76). With the photograph on a hard, flat surface, place the overlay on it and fasten it securely, leaving the bottom portion of the paper loose so it may be lifted in order to refer to points on the photograph. Register the overlay by tracing the outline of the photograph, by indicating the fiducial (collimation) marks, or by marking clearly defined terrain features such as road junctions, crossroads, and streams. These marks allow the overlay to be exactly registered over the photograph. When the overlay paper is registered properly, the detail required is traced. Finally, a north arrow and title block are added. The latter includes title of overlay, and date, name, grade, and organization of the sketcher, and the sketcher's location. The photograph's serial number, RF, graphic scale, and classification should also be shown. For further information on this subject see FM 21-35.

Figure 76. Use of overlay on a photograph.

71. TO INDEX AND PLOT AERIAL PHOTOGRAPHS.

Examination and study of photographs is facilitated by first outlining on a map the area or areas covered by the photograph or photographs as indicated in figure 77. This is done by constructing templates or tracing paper which represent the area of the photograph at the scale of the map. For example, if the map has an RF of 1:20,000, and the ground area represented by a photograph is 7,000 by 9,000 feet, the template would measure:

 or 4.2 by 5.4 inches

The template is placed on the map and shifted about until its outline includes the details shown on the photograph. The area then is outlined on the map by marking around the template, and the serial number of the photograph is entered in the outline. The process is repeated for any number of photographs. The result is an index sheet of the photographs, (sometimes called "sortie plot") showing the relation of the photographs one to another and to the area on the map.

Figure77. Method of indexing and plotting aerial photographs.