Air Fronts: FM 21-26, Advanced Map and Aerial Photograph Reading - Section 9.

SECTION IX: AERIAL PHOTOGRAPHS

54. GENERAL. Aerial photographs are used for many different purposes in military operations. In this manual they are considered principally in conjunction with or as substitutes for topographic maps. The ideal situation is to have an accurate topographic map and a recent aerial photograph or accurate photomap of the same area. A topographic map even a few years old can give a false picture of terrain because it shows conditions which existed when the surveys were made. Moreover on topographic maps, forest and woods are represented by their conventional signs regardless of their density; cultivated fields cannot be interpreted or recognized; and newly constructed roads, highways and bridges, and changes in features of the terrain due to storms, floods, and military operations are not indicated. A properly interpreted recent aerial photograph, however, shows all the above information and more. In fact, aerial photographs usually supply the only up-to-date map information available during the first few weeks of an operation in enemy territory. Photographs are used to determine distances and directions and to select routes in much the same manner as ordinary topographic maps are used. See FM 21-25 and TM 5-246 for interpretation of aerial photographs.

55. TYPES. The three general types of aerial photographs are vertical, oblique, and composite.

a. Vertical. Vertical photographs are taken with the axis of the camera perpendicular to the earth.

Each photograph covers a small ground area and shows ground features such as roads, railways, buildings, and rivers in much the same way as they are shown on topographic maps of similar scale. When the scale of the photograph is known, the distances along roads and widths of rivers can be measured accurately. Vertical photographs are used for local operation, making photomaps, and in pairs for studying terrain by stereovision (see par. 75). An example of a vertical aerial photograph is shown in figure 62, with camera in position as shown in figure 63 (1).

b. Obliques. Oblique photographs are taken with the camera tilted at an angle. The angle varies according to the mission, but is usually about 30° below the horizontal, as shown in figures 63 and 64. An oblique below the horizon is a low oblique; one including the horizon is a high oblique. Figure 63 (4) shows the

Figure 62. Vertical photograph.

(1), (2) and (3)  Diagrams  of vertical, low-oblique and bigh-oblique photography
(4)Oblique photograph of area compared with area's actual shape
Figure 63.

Figure 64. Low-oblique photograph.

Figure 65. High-obligue photograph.

relationship between the trapezoid ground area covered by an oblique photograph and the rectangular shape of the photograph itself. Distances on oblique photographs cannot be accurately scaled, but since obliques are taken from a view point similar to that of an observer on a high hill, terrain features have a more normal appearance than they do in a vertical picture. This makes obliques useful for studying terrain, vegetation, buildings, and other features indentified by their elevations. Obliques may accompany operations or field orders. As in figure 66, the line of departure, routes from bivouac to assault positions, assembly positions, objectives, boundaries between units, and information of the enemy, can be shown.

c. Composites. Composite aerial photographs are made with cameras having one principal lens and two or more surrounding and oblique lenses. The photographs resulting from these cameras are combinations of two, four, or eight obliques surrounding one vertical. These oblique photographs are corrected or transformed in plotting so as to permit assembly as verticals with the same scale. The air corps type T-3A five-lens camera may be used, but this system is being superseded by the new trimetrogon system which comprises three cameras operated simultaneously. For further information, see paragraph 25, TM 5-230.

56. PIN POINTS. Pin-point photographs are single vertical photographs, usually of large scale. Such features as airdromes, supply depots, dumps, road crossings, bridges, or bottlenecks on lines of communication are suitable objects for pin-point photographs. Adjacent pin-point photographs having proper overlap comprise a stereopair (see par. 75).

Figure 66. Oblique photograph used ax pbotomap to accompany oral field order.

57. STRIPS. When a flight is made to obtain photographic strips of an area, vertical pictures are made along a selected flight line or direction. The photographs of one run make a photographic strip or a reconnais-

 Figure 67. Line diagram showing relative positions of separate pictures  in a photographic strip.

sance run. Exposures are so timed that each vertical photograph in a strip overlaps 60 percent in the direction of flight (fig. 67). When a single strip of photographs will not cover the area desired, parallel strips are flown, each having an approximate 20 percent sidelap with the adjacent strip; in wide-angle mapping, sidelap of photographs should be 30 percent. The photographs of a strip may be used for stereoscopic study or matched and secured in place to form a mosaic.

58. MOSAICS. A mosaic is formed by joining several vertical photographs (fig. 68). When several photographs of a single strip are joined together, the result is called a strip mosaic. Mosaics may be either controlled or uncontrolled.

a. Uncontrolled mosaics are produced when the photographs are put together by matching detail along their borders. This type of mosaic gives a good pictorial representation of the ground but contains errors in scale and direction because of differences in scale and distortion around the edges of each photograph. Care in mounting will improve the accuracy, but the only way an accurate scale can be obtained is by ground control.

b. Controlled mosaics are prepared by adjusting vertical aerial photographs to the plotted positions of ground control points. Unless an accurate map of the same scale as the photographs is available, an actual survey must be made in the field, and control points easily identified on photographs plotted on a mounting board to the average scale of the photographs. When these control points on the photographs are placed directly over the corresponding control points on the mounting board, the photographs are in true relation to each other. The controlled mosaic is accurate enough in scale and direction for most practical purposes.

Figure 68. Matching three overlapping vertical photographs of a strip to form a mosaic.

However, the ground features where the photographs are joined will not match because of distortion and variation in scale. For further reference see TM 5-230.

59. MARGINAL DATA. a. General. The titling and identification of aerial photographs is the responsibility of the Army Air Forces but the data are actually placed on the negatives by the units making the photographic mission. Technically excellent photography may be rendered valueless because of improper or inadequate titling'

b. Minimum title data for all aerial photographs. The minimum data on all aerial photographs are the cataloging information which appears at the top. Following is an example: 137PS-3M109-1 1-V-121 (fig.69).

(1) 137PS. Photograph made by 137th Photo Reconnaissance Squadron USAAF.

(2) 3M109. The year is 1943, indicated by the fourth digit of the calendar year, that is, 3 for 1943. The letter M stands for mission, 109 for the mission number.

(3) 11-V. Eleven (11) is the roll number; V means the camera was in a vertical position. Negatives from cameras used in the trimetrogon assembly indicate the positions of the cameras by having in their marginal data, the letters L for left camera, V for vertical camera, and R for the right camera. Negatives from the fivelens camera are numbered as follows with D closest to the front of the airplane:

   D
A B C
    E

Letters A, C, D, and E are obliques, while B is a vertical.

(4) 121. The negative number is 121. Negatives are numbered consecutively starting with 1.

c. Additional title data to be placed on photographs used for intelligence and interpretation purposes. The following data are added to the cataloging information and are the minimum data required on photographs used for intelligence and interpretation purposes. Thus immediately following 137PS-3M109-11V-121 are the data, 12:28:1330-12:20000-V-842Sl4826EBuna New Guinea-Confidential. The explanation is as follows:

(1) 12:28:1330. Date and hour, 28 December at 1330.

(2) 12:20000. Focal length of the camera, 12 inches, and altimeter reading, 20,000 feet. If possible, altimeter reading should be adjusted to give height of plane above terrain photographed.

(3) V. V for vertical. If, however, the photograph were an oblique made with the camera tilted 45° from the vertical, it would read O-45°.

(4) 842S14826E. Geographic coordinates of the center of the photograph to the nearest minute: 8° 42' south latitude, 148° 26' east longitude.

(5) Puna, New Guinea. Descriptive title, including the locality and country or island group.

(6) Confidential. Security classification; only confidential and secret classifications are shown.

d. Further titling data for continuous strip mapping, charting and reconnaissance photography. In addition to the minimum titling that appears on all photographs, other data are added to the key negatives at the beginning and end of each strip of mapping, charting, and reconnaissance photography. Thus, immediately following are the

Figure 69. Aerial photograph with titling data.

data 12:28:1330-6.03:20000-842S14826E/816AI4745E. The explanation is as follows:

(1) 12:28:1330. Date and hour, 28 December at 1330.

(2) 6.03:20000. Focal length of camera in inches to nearest hundredth (6.03 inches) and altimeter reading 20,000 feet.

(3) 842S14826E/816SI4745E. Geographic coordinates of the beginning and end of each continuous strip of photography to the nearest minute; beginning of strip is at 8° 42' south latitude, 148° 26' east longitude; end of strip at 8° 16' south latitude, 147° 45' east longitude.

60. FIDUCIAL MARKS. Fiducial marks appear on nearly all aerial photographs. These are ticks, notches, lines, or half arrows, at the middle of each of the four sides of the negative. They are registered during exposure by a template in the camera. The use of fiducial marks to place a grid on the photograph is discussed in paragraph 69b.

61. SOURCE OF ERROR. All aerial photographs contain some error resulting from the tilt of the airplane in flight and from variations in ground elevations. The amount of error in any photograph is less at the center and increases toward the edges. These unavoidable errors may be disregarded for ordinary field use. However, photographs taken when the airplane was obviously badly tilted should be discarded.