Characteristics of contour lines

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CHARACTERISTICS OF CONTOUR LINES

A contour line is a line of equal elevation; therefore, two different lines must indicate two different elevations. So two different contour lines cannot intersect or otherwise contact each other except at a point where a vertical or overhanging surface, such as a vertical or overhanging face of a cliff, exists on the ground Figure 8-18 shows an overhanging cliff. You can see how the segments of contour lines on this cliff are made as dotted (or hidden) lines. Aside from the exception mentioned, any point where two different contour lines intersect would be a point with two different elevations-an obvious impossibility.

In forming a mental image of the surface configuration from a study of contour lines, it is helpful for you to remember that a contour line is a level line; that is, a line that would be formed by a horizontal plane passing through the earth at the indicated elevation. If you keep this concept of levelness in mind you can usually get the "feel" of the rise and fall of the ground as you study the contour lines on the map.

A contour line must close on itself somewhere—either within or beyond the boundaries of the map. A line

Figure 8-19.-Uniform, gentle slope.

Figure 8-20.-Unifrom, steep slope.

Figure 8-21.-Concave slope.

that appears on the map completely closed may indicate either a summit or a depression. If the line indicates a depression, this fact is sometimes shown by a succession of short hachure lines, drawn perpendicular to the inner side of the line. An example of a depression is shown in figure 8-18. A contour line marked in this fashion is called a depression contour.

On a horizontal or level plane surface, the elevation of all points on the surface is the same. Therefore, since different contour lines indicate different elevations, there can be no contour lines on a level surface. On an inclined plane surface, contour lines at a given equal interval will be straight, parallel to each other, and equidistant.

A number of typical contour formations are shown in figure 8-18. For purposes of simplification, horizontal scales are not shown; however, you can see that various intervals are represented. The arrows shown indicate the direction of slope.

Generally, the spacing of the contour lines indicates the nature of the slope. Contour lines (fig. 8-19) that are evenly spaced and wide apart indicate a uniform, gentle slope. Contour lines (fig. 8-20) that are evenly spaced and close together indicate a uniform, steep slope. The closer the contour lines are to each other, the steeper the slope. Contour lines closely spaced at the top and widely spaced at the bottom indicate a concave slope (fig. 8-21).

Figure 8-22.-Convex slope

Figure 8-23.-Hill.

Contour lines widely spaced at the bottom indicate a convex slope (fig. 8-22).

A panoramic sketch is a pictorial representation of the terrain in elevation and perspective as seen from one point of observation. This type of map shows the

Figure 8-24.-Valley and draw.

horizon, which is always of military importance, with intervening features, such as crests, woods, structures, roads, and fences. Figures 8-23 through 8-29 show panoramic sketches and maps. Each figure shows a different relief feature and its characteristic contour pattern. Each relief feature illustrated is defined in the following paragraphs.

A hill is a point or small area of high ground (fig. 8-23). When you are on a hilltop, the ground slopes down in all directions.

A stream course that has at least a limited extent of reasonably level ground and is bordered on the sides by higher ground is a valley (fig. 8-24). The valley, generally, has maneuvering room within it. Contours indicating a valley are U-shaped and tend to parallel a major stream before crossing it. The more gradual the

Figure 8-25.-Ridge and spur.

fall of a stream, the farther each contour parallels it. The curve of the contour crossing always points upstream. A draw is a less-developed stream course where there is essentially no level ground and, therefore, little or no maneuvering room within its sides and towards the head of the draw. Draws occur frequently along the sides of ridges at right angles to the valley between them. Contours indicating a draw are V-shaped with the point of the V toward the head of the draw.

A ridge is a line of high ground that normally has minor variations along its crest (fig. 8-25). The ridge is not simply a line of hills; all points of the ridge crest are appreciably higher than the ground on both sides of the ridge.

A spur is usually a short continuously sloping line of higher ground normally jutting out from the side of a ridge (fig. 8-25). A spur is often formed by two roughly parallel streams that cut draws down the side of the ridge.

A saddle is a dip or low point along the crest of a ridge. A saddle is not necessarily the lower ground between the two hilltops; it maybe simply a dip or break along an otherwise level ridge crest (fig. 8-26).

A depression is a low point or sinkhole, surrounded on all sides by higher ground (fig. 8-27).

Figure 8-27.-Depression.

Table 8-1.-Recommended Contour Intervals–Topographic Map

Cuts and fills are man-made features that result when the bed of a road or railroad is graded or leveled off by cutting through high areas and filling in low areas along the right-of-way (fig. 8-28).

A vertical or near vertical slope is a cliff. As described previously, when the slope of an inclined surface increases, the contour lines become closer together. In the case of a cliff, the contour lines can actually join, as shown in figure 8-29. Notice the tick marks shown in this figure. These tick marks always point downgrade.