MSL101L04 Basic Map Reading SR.pdf lesson 4.pdf

Figure 9 2 points on contour lines and contour

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Figure 9-2. Points on contour lines and contour interval note 9-10. There may be times when elevation points need to be determined to a greater accuracy. To do this, determine how far between the two contour lines the point lays. However, most military needs are satisfied by estimating the elevation of points between contour lines. (See Figure 9-3.) If the point is less than one-fourth the distance between contour lines, the elevation is the same as the last contour line. In Figure 9-3, the elevation of point A is 100 m. To estimate the elevation of a point between one-fourth and three-fourths of the distance between contour lines, add one-half the contour interval to the last contour line. Point B is one-half the distance between contour lines. The contour line immediately below point b is at an elevation of 160 m. The contour interval is 20 m; one-half the contour interval is 10 m. In this case, add 10 m to the last contour line of 160 m. The elevation of point b is about 170 m.
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Chapter 9 9-4 TC 3-25.26 15 November 2013 Figure 9-3. Points between contour lines 9-11. A point located more than three-fourths of the distance between contour lines is considered to be at the same elevation as the next contour line. Point C is located three-fourths of the distance between contour lines. In Figure 9-3, point c is considered to be at an elevation of 200 m. 9-12. To estimate the elevation to the bottom of a depression, subtract one-half the contour interval from the value of the lowest contour line before the depression. In Figure 9-4, the lowest contour line before the depression is 240 m in elevation, making the elevation at the edge of the depression 240 m. To determine the elevation at the bottom of the depression, subtract one-half the contour interval. The contour interval for this example is 20 m. Subtract 10 m from the lowest contour line immediately before the depression. The result is that the elevation at the bottom of the depression is 230 m. The tick marks on the contour line forming a depression always point to lower elevations.
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Elevation and Relief 15 November 2013 TC 3-25.26 9-5 Figure 9-4. Depression 9-13. In addition to the contour lines, bench marks (BMs) and spot elevations are used to indicate points of known elevations on the map. Bench marks, the more accurate of the two, are symbolized by a black X; for example X BM 214. The 214 indicates that the center of the X is at an elevation of 214 units of measure (feet, meters, or yards) above mean sea level. To determine the units of measure, refer to the contour interval in the marginal information. 9-14. Spot elevations are shown by a brown X and are usually located at road junctions, hilltops, and other prominent terrain features. If the elevation is shown in black numerals, it has been checked for accuracy; if it is in brown, it has not been checked.
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