Air Fronts: FM 21-26, Advanced Map and Aerial Photograph Reading - Section 7.

SECTION VII: ELEVATION AND RELIEF

42. GENERAL. a. Most of the earth's geographic features are the result of erosion, the wearing away by freezing, thawing, and draining of water from high to low ground. Relief is the variations in the elevations or heights of these features, such as ridges and valleys, hills and hollows, which divide terrain into two interlocking systems. Small streams join to form larger streams, and these join to make rivers, the whole network of water courses being known as a drainage system. Between the streams is high ground, the noses and hills of which form a system of their own called ridge lines. Points of abrupt change in elevation or important change in direction of ridges or streams are called critical points.

b. Ground forms, then, must be measured vertically as well as horizontally. The vertical distance is called elevation and is usually measured in feet or meters above mean sea level (plus elevation) or below sea level (minus elevation). (See FM 21-25.)

c. There are three main systems by which maps show the rise and fall of the earth's surface. In order of their importance, these methods are contours, hachures, and the layer-tint system. Other methods include approximate contours, form lines, or hill shading.

43. CONTOURS. a. General. Contour lines are drawn on maps to represent ground elevations. Each contour line passes through points which are exactly the same height above sea level. See FM 21-25 for a discussion of contours. Contours have certain characteristics. Contours:

(1) Are smooth curves.

(2) Are approximately V-shaped in narrow valleys with the "V" pointing upstream.

(3) Are generally shaped as "U's" pointing down ridges.

(4) Are shaped as an "M" just above stream junctions.

(5) Tend to parallel streams.

(6) Tend to parallel each other, each approximating the shape of the ones above and below it. This reflects the fact that changes in ground form are usually gradual.

(7) Never cross or touch, except at overhanging or vertical cliffs and at waterfalls.

(8) Never fork.

(9) Always close, on or off the map as indicated at '"f,'' hill 305, and "g," hill 430 in figure 39(2). The contour 400 leaves the map at (1) and returns to map at (2).

b. Depth curves. A contour showing points of equal elevation below the level of any body of water is called a depth curve. These curves indicate depths below a certain point, usually mean-low-water level for the body of water concerned. The vertical interval is frequently expressed in fathoms. One fathom is 6 feet.

44 HACHURES (fig. 40). The hachure method of representing relief is used on the Strategic Map of the United States (scale 1:500,000) when relief data are inadequate to draw contours, and is frequently found on large- as well as small-scale foreign maps. Hachures are short parallel or slightly divergent lines drawn in the direction of slopes. They are closely spaced on

(1) Perspective view with schematic Position of contours.
Figure 39.

Steep slopes, wide apart on gentle slopes, and converge toward the tops of ridges and hills.

45. LAYER-TINT SYSTEM. In addition to contours and hachures, relief is shown on some maps by the layer-tint system, used by the Army on aerial navigation charts. It is also used on the 1 -1,000,000 International Map of the World (IMW). Different colors or different tones of the same color show different zones of elevation. Each zone is bounded by contours, usually shown on

(2)Contour map of area shown above.
Figure 39. - Continued.

the map; contours within the zones are sometimes shown. The map margin carries a key showing the elevation of zones according to color.

Figure 40. Hachure map.

46. RIDGE AND STREAM LINES. a. Purpose. To emphasize high and low ground, a system known as ridge lining and stream lining is often used. On ridge-lined or stream-lined maps or aerial photographs, the map reader neglects the great mass of detail to study key features. Three steps are followed in this process.

b. Stream lines. Study the map or aerial photograph and select the main streams and their tributaries. Draw over them in blue; this makes the drainage system stand out. In figure 41, the streams are marked with solid lines.

Figure 41. Contoured map with stream- and ridge-lining.

c. Ridge lines. Between the streams there must be higher ground or ridges. To emphasize this, draw lines on the map along the main ridges. These should be in brown so as not to obscure underlying features. Then select the minor ridges and trace their ridge lines. The number of minor ridges to be included depends on the emphasis desired. Do not carry the ridge lines all the way to the streams. Stop at the beginning of the flood plain as shown by the increase in space between contours. Ridge lines join in a systematic branching structure. In figure 41, ridge lines are marked with dotted lines.

47. SLOPES. a. General. A slope is an inclined ground surface that forms an angle with the horizontal plane. The degree of inclination is also called slope.

Figure 42. Types of slopes revealed by contour lines.

A knowledge of slope is important in selecting routes for the movement of units, in siting CP's and OP's and in choosing good fields of fire and areas defiladed from enemy fire. The trained map reader can visualize the slope of the ground by referring to contour lines on the map.

b. Types (fig. 42). There are three types of slopes: uniform, concave, and convex.

(1) Uniform slope. A uniform slope is a smooth slope with equally spaced contours. On a steep, uniform slope the contours are equally and closely spaced; on a gentle, uniform slope the contours are equally spaced but far apart.

(2) Concave slope. This slope is caved-in; contours are close together at the top and far apart at the bottom.

(3) Convex slope. This slope is humped up; contours are spaced far apart at the top of the slope and close together at the bottom.

c. Slope in percent. The most common way to express slope is in percent. A 1% slope rises or descends 1 unit in a horizontal distance of 100 units; a slope of 10% rises or descends 10 feet in a 100 feet. Thus in figure 43, line XY represents a slope. If the horizontal distance from X to Y is 100 feet and the vertical distance at Y is 10 feet, the slope of XY = or .J. To change .1 to percent multiply by 100 = 10% Thus the formula for slope in percent is:

slope in percent

The horizontal distance is scaled from the face of a map; the vertical distance is the difference in elevation of the two points and may be interpolated from a contour map. A rising slope is plus and a descending slope is minus;

Figure 43. Expression of slope.

that is, the slope from X to Y is + 10%, but from. Y to X is - 10%. For example, in figure 44, the problem is to find the slope between A and B. First measure the horizontal distance, 220 feet. Determine the vertical distance by subtracting the elevation of A from B. The rise is 559 minus 530 or 29 feet. The slope is:

Figure 44. Slope interpreted from contour map.

%

 d. Slope in mils. The value of a slope in Mils (par. 16) is the angle in Mils between the horizontal plane and the inclined ground surface. For slopes up to 350 Mils the following approximate formula is used:

slope in mils

For example, in figure 45 (1) the vertical distance (difference in elevation) is 268 units and the horizontal distance is 1,000 units. The slope is mils.

In figure 45 (2), the slope is 100 Mils.

Figure 45. Slope expressed in mils.

e. Slope in degrees. Many instruments for measuring slope are graduated in degrees. A degree is a unit of angular measurement. It is the angle subtended by an arc of 1 unit on a radius of 57.3 units. The value of a slope in degrees is the angle in degrees between the horizontal plane and the inclined ground surface. The vertical distance is not exactly equal to the distance along the arc, but for slopes up to 20° the variations are negligible and may be disregarded. The formula for slope in degrees is:

57.3 = slope in degrees

For example, in figure 46 (1) the vertical distance is 16 units and the horizontal distance is 76.4 units. Slope in degrees is:

57.3 = 12°

In figure 46(2), the slope in degrees is:

= 7.5°

Figure 46. Slope expressed in degrees.

f. Grade. The elevation of the surface of roads and railroad lines is referred to as grade. The degree of inclination as compared with the horizontal is also called grade; it is usually given as percent slope and is calculated as described in c above.

g. Gradient. The rate of inclination from the horizontal is called gradient. It is expressed either as the

Figure 47. Slope expressed as gradient.

ratio of vertical to horizontal or horizontal to vertical distance. Hydraulic gradient is usually expressed as:

For example, a stream in figure 47 (1) has a vertical fall of 1 foot in a horizontal distance of 5,000 feet and has a gradient of 1 to 5,000 sometimes written as , or  1:5,000, or 1 in 5,000. The side slopes of earth fills or excavations are usually expressed as:

For example, the face of an earth fill in figure 47 (2) rises 5 feet in a horizontal distance of 15 feet. Its slope usually is expressed as 3 to 1, 3:1, or 3/1. To avoid misunderstanding when expressing slopes by rates, it is advisable always to state whether the slope is expressed as vertical

 
Figure 48. Conversion of slope expression units.

over horizontal or horizontal over vertical. A convenient method of showing this graphically is illustrated by the small triangle on the sloped line in figure 47 (2).

h. Conversion of slope expression units. Relations between angle of slope in degrees, percent slope, and slope ratio are shown in figure 48.

Figure 49. Explanation of profile.

48. PROFILE. a. General. The most satisfactory way to study slopes shown on maps is to make a profile. A profile is an exaggerated cross section of the earth's surface. For example in figure 49 (1), imagine a bayonet passed through the earth between points A and  and the front half of the hills and ridges removed. The outline of the surface of the remaining half is its profile as represented in figure 49 (2).

b. To draw a profile. Figure 50 (1) represents a portion of a contoured map. To construct a profile of the ground between points A and B, proceed as follows:

(1) Connect points A and B by a straight line and assume that a vertical plane is passed through this line.

(2) Take a piece of paper which has parallel horizontal lines equally spaced; cut or fold paper along one of these lines.

(3) Refer to map and determine highest and lowest elevation along the line AB; number the lines on the paper to correspond with the elevations, beginning with the highest elevation at the top of the paper as shown in figure 50 (2).

(4) Place top edge of the paper along the line AB, and where the edge cuts each contour, drop perpendiculars to the horizontal line on the paper corresponding to the elevation of the contour.

(5) On the paper, connect points of intersection of the perpendiculars and the elevation lines by means of a smooth curved line. The profile is now complete.

(6) Elevations of intermediate areas such as valleys c and d in figure 50 (2) are determined by estimation between adjacent contours of the same elevation.

(7) Where a line crosses a crest or a depression, an elevation is sometimes given on the map; this assists in completing the profile. Where such elevation numbers are missing, interpolate necessary elevations from the spacing of the contours.

(8) To make a profile of a winding line such as a road or trench, divide it into a series of approximately straight sections and plot as above; turn the paper at each angle to make a continuous profile.

Figure 50. Constructing a profile.

c. Vertical scale. The horizontal scale of a profile is ordinarily the same as that of the map while the vertical scale is considerably exaggerated, as shown in figure 50. In the figure, the lines represent 10-foot vertical intervals; for easier interpretation they could represent 5-foot intervals, thus further exaggerating the profile.

49. VISIBILITY. a. General. One of the important uses of maps for military purposes is to determine whether a point, a route of travel, or an area is visible from a given point or position.

b. Defilade. When two points are visible one from the other, they are intervisible. If there is a feature between them higher than both, such as a hump in slope, vegetation, or man-made works, they are in sight-defilade. A feature that interferes with visibility between points is called a mask. The term height of mask is the height of the feature above the line of sight between two points. In figure 51, points B and C are intervisible; points B and A are sight defiladed, A cannot be seen from B; point C is the mask between points B and A and the ground between C and A is defiladed from B; the "height of mask" M is the height of point C above the line of sight from B to A. Other terms like topographical crest (the top of a hill), military crest (the highest part of the hill from which

Figure 51. Defilade diagram.

all of the valley can be observed), and defilade D are shown in figure 51.

c. Determination. (1) By inspection. To determine visibility from a map the following points are helpful:

(a) Points on opposite sides of a valley and well above intervening ground are intervisible.

(b) Two points separated by a feature higher than both are not intervisible.

(c) If two points are separated by a feature higher than one of the points, the points may or may not be intervisible.

(d) If the slope of the ground between the two points is convex, they are not intervisible.

(e) If the slope of the ground between the two points is concave, they are probably intervisible.

(f) When the ground between the two points is level, intervisibility depends on the vegetation and works of man.

(2) By profile. To determine by profile whether or not B is visible from A, proceed as follows:

(a) Construct profile as described in paragraph 48b and as shown in figure 52 (1) and (2).

(b) In figure 52 (2), draw a line from a to the crest of c and thence to h. That portion of the ground between c and h, including b, is not visible from a. In the figure this area has been shaded.

(c) In the above example, points a and b were both at ground level. To determine whether a man at a, eyes 5 feet above the ground, could see a truck 8 feet high at b, it would be necessary to plot a in the profile with an elevation of 1,135 feet and b with an elevation of 1,108 feet. The map reader, however, is seldom concerned with the eye level of a man in a standing position, since observers in combat operations observe from as near the ground as possible.

Figure 52. Determining visibility by profile method.

(3) By hasty profile. Many times it is necessary to make a hasty profile to determine whether a point can be observed from a particular location. Figure 53 shows a hasty profile. The problem is to determine if point P can be seen from point A. For a hasty profile, only the points that may mask the line of sight are plotted. These points are B', C', D', E', and F'. Plot A' and draw the lines of sight A'B' and A'F'. It is clear that the RJ at P cannot be seen from A; it is masked by the ridge at F.

Figure 53. Determining visibiliy by hasty profile method.

d. Defiladed areas. Defiladed areas may be located by a map and a series of profiles. With an OP or gun position located on the map, draw a line in the principal direction of observation and call this the line XY; then lay off other lines radiating from the OP at 10° intervals. Thus in figure 54 (1) from observation post X, the lines of sight are XY, XY², and X.Y³,  Figure 54 (2) shows the profiles of these lines of sight. These profiles show defiladed areas at ab,  a²b², a³b³, cd, c²d², c³f³, ef, and e²f². Project these points from the profiles on to the map along lines XY, XY², and XY³, connect them, and shade them in as shown in figure 54 (1). These areas cannot be seen from X.

e. Floating line. Paragraph 79 explains a way to determine intervisibility by a "floating line" on aerial photographic stereopairs.