Air Fronts: FM 21-25, Elementary Map and Aerial Photograph Reading - CHAPTER 11. Aerial-Photographs and Photomaps

CHAPTER 11: AERIAL PHOTOGRAPHS AND PHOTOMAPS

Maps Made From the Air

The maps you have learned to use are topographic maps, with the ground features drawn to scale. Another kind of map, and one you are likely to see often, is a photomap, made by photographing the ground from an airplane. Your photomap may be a single aerial photo, or several put together to represent a larger area. It will have on its margin various items of information, the most important of which are the arrow indicating magnetic north, and the scale. The photomap may have a grid on it. If it has, you use it as you use the grid on a topographic map.

Photomaps are Up-to-Date

In some ways photomaps are not as easy to use as drawn maps. They do not indicate elevations clearly, because they do not have contour lines. Roads enter a wood, disappear beneath the trees, and reappear going in a different direction. On the other hand, the information of photomaps is more likely to be up-to-date. Probably your photomap was made from pictures taken only a few days ago. If it shows a wood or a road, you know it is there, while a wood shown on a topographic map have been cut down after the map was made, or a new road may have been built.

Figure 114 is a map of Sackville, half as a topographic map and half as a photomap. The photomap is at about the smallest scale you are likely to use. Notice the differences between the two kinds of map. By comparing the two maps you can easily follow the, roads, the railroad, and the river and you can see how the hilly terrain is shown by the contour lines on the topographic map.

Figure 114

Shadows Must Fall Toward You

To read a photomap or an aerial photograph, hold the photograph so that the shadows of objects fall toward you. Figure 115 is a picture of a trefoil with heavy shadows. As it appears here, you can see that the object seems to go down into the page. Now look away from the picture, turn the whole book upside down, and look at the picture again. Notice that the trefoil now appears to be coming up out of the page. There is only one correct way to look at an aerial photograph.

Figure 115

Figure 116 shows what happens when you hold an aerial photograph the right way and the-wrong way. In the upper part it is viewed correctly; the shadows fall toward you. You see a barren area near the Mareth Line in Tunisia, cut by deep stream lines. The lower portion shows the same area, but now the stream lines have become ridges. The earth heaped up by exploding bombs (upper right-hand corner) has sunk below ground level and you see deep holes (lower left).

Figure 116 (1)

Figure 116 (2)

Aids in Reading Photomaps

One of the qualities which help you identify things on a photomap is tone. This means shades of gray formed by comparative roughness or smoothness of surfaces. Rough surfaces, such as woods and brush, look dark.

Figure 117

Tall grass looks darker than close-clipped grass. Smooth surfaces, such as concrete roads, look light.

Color has some effect too. Thus, a roof painted white or some light color looks light, while a black or darkcolored roof may look dark, though both roofs may be smooth. Things that look dark or light to your eye look pretty much the same to the camera.

The shape of an object helps tell you what it is. Awood is irregular in shape, an orchard is regular, though both have trees. Roads and railroads both are long and narrow, but roads curve more sharply than railroads and have other roads joining them at right angles, something you never would see on a railroad.

Figure 118

Figure 119

The shadow of an object usually is a helpful telltale. Looking straight down from a plane the camera makes the top of a silo and the top of a water tower look alike; but the silo's shadow is a solid patch running away from its base, while the tank's shadow is a smaller dark patch some distance away, perhaps with the shadows of its supports also showing.

Shadow may tell you how high an object is, if there is another shadow of a familiar object nearby. If you compare the length of a steeple's shadow with that of a telephone pole you have some idea how high the steeple is. Shadows are especially useful for information purposes because an object may be camouflaged without hiding its shadow. Figure 117 illustrates how much can be learned from the shadows of objects.

Often you can identify an object by its relative size, for instance, a chicken house from a barn, or an irrigation ditch from a canal. The width of a road may be judged by comparing it with the width of vehicles on it.

Sometimes the relation of an object to nearby things gives you a clue to what they are. A large building with a baseball diamond near it is likely to be a school. A similar building with railroad tracks running close by it is more likely to be factory or warehouse.

Other objects may be recognized by their appearance on photomaps. Any light patch in a darker area may be fresh-turned earth, a plowed space, or a foxhole (that is why you conceal the fresh earth of your foxhole).

A small cluster of buildings may be a farm or a small village. If the buildings are of fairly uniform size and lie on both sides of a road probably it is a village. If they are mixed sizes, and most of them on one side of the road, it is likely to be a farm.

A hedge may look like a brush-grown fence, but it will have a more uniform width from being trimmed.

Military objects and activities in an area are noticeable in aerial photographs, both because of the shape and layout of the objects and because of their tracks.

For example, in figure 118, the mess made by an army unit in bivouac is very noticeable. One look at this photograph and you can tell what is here. Field Artillery guns are often in groups of four. When four track patterns are made in an area likely to hold artillery, the marks are visible on an aerial photograph (fig. 119).

Figure 120 shows what trenches look like in an aerial photograph, while figure 121, shows clearly a battery of railroad artillery.

Figure 120

Figure 121

Growing crops appear in roughly rectangular patches of different shades of light and dark, with the edges sharply marked. Figure 122 is such an area, the kind, you often find in European countryside.

Figure 122

Different kinds of trees and woods can be detected in an aerial photograph. Figure 123, for example, is an aerial view of jungle terrain.

Figure 123

Figure 124 shows a number of objects and forms in an aerial photograph. These are identified as follows:

Figure 124

No. 1 is a plowed field.

No. 2 is a bridge. There is another bridge just below it. Note the shadow of its spans on the water.

No. 3 is a railroad.

No. 4 is at the crossing of a large highway and a country road. Roads are usually light-toned and even in width.

No. 5 is a building. It hard to see without a magnifying  glass, but you can see its shadow.

No. 6 is a neck of woods, bordering on a meadow. The denser the woods, the darker it appears.

No. 7 is a footpath crossing the meadow. You can tell it from a road, because it is so much narrower than the light-colored lines which are roads.

No 8 is an-orchard, with evenly spaced trees.

No. 9 is a mud flat. Usually a mud flat is darker than the water, and has light spots caused by pools of standing water.

No. 10 is a small stream, identified by its meandering course. When a small stream flows through a wood, you can tell it is there, even though you cannot see it, by the wavy line of heavier vegetation along it.

No. 11 is a brick factory. The spoked round buildings are kilns.

No. 12 is a fence line. Usually the texture of the ground on two sides of a fence is different, so one side is lighter than the other. Also, a fence line is darker than the space around it, because brush grows close to it.

No. 13 plainly is a village. You can see several of its streets, which make squares.

Grid coordinates

Your  photomap usually will have a grid like the one you have learned to use on a topographic map (fig. 125) but sometimes you may be given an aerial photo with a grid like the one shown in figure 126. These grid lines are always 1.8 inches apart, no matter what the scale of the map. The grid lines in fig. 126, however, are not 1.8" apart, because we had to reduce the size of the map to get it in this manual. Here you see the grid lines identified by letters instead of by numerals. - The first step in locating a point on this grid is to give the letter of the vertical and then the horizontal line crossing at the lower left corner of the square in which the object lies. Then, by eye, divide the sides of the square into tenths. Now, just as you learned before, count right on the bottom line of the square the number of tenths the point is from the left grid line of the square, and then up the number of tenths it is above the bottom grid line of the square. Notice that the north arrow in photo maps does not always point to the top of the map.

Figure 125

For example, there is a large building in square AN. It is about four tenths right from line A and about four tenths up from line N. The coordinates of the building terefore, are AN44, without either decimal or dash.

Figure 126

This type grid is only for locating points. Since lines are not spaced at fixed ground distances apart, you cannot use them to scale distance as you can those on topographic maps.

To Orient an Aerial Photograph

If you have a topographic map of the same area, turn your photograph until roads and other features on it lie exactly parallel to those on the map; then draw a north arrow exactly parallel to the one on the map.

If you have no topographic map, orient your photograph by turning it until its roads parallel those shown on the ground. Then lay your compass on it and draw a line in the direction your compass needle points when the dial is at rest. That gives your magnetic north line.

A rough-and-ready way to orient your aerial photo-graph is by its shadows. 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 photograph's margin tells you the photograph was taken between 1000 and 1200 hours, orient the map by pointing the shadows a little west of north. If made cetween 1200 and 1400 hours, point them a. little east of north. If the photo time was earlier than 1000 or later than 1400 hours, point its shadows a little more west or east of north.

If you are south of the equator, simply reverse the above directions.

You learn the time the area was photographed by reading the information in the top margin. You'll see, among other things, something like this: 12:28:1330. That means the ground was photographed at 1330 hours on December 28. The hour figure-1330--is the one you use in orienting your aerial photograph by its shadows.