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Land Nav Task 6 - Determine the Elevation of a Point on the Ground Using a Map

Standards: Determined the elevation of the designated point to within half the value of the contour interval.

Conditions: Given a standard 1:50,000 scale military map, a pencil, and a designated point on the map.

Standards: Determined the elevation of the designated point to within half the value of the contour interval.

 

Performance Steps

1.   Locate contour lines and contour intervals.

a. The brown lines on the map are called contour lines. Each line shows the height above sea level. Contour lines never cross one another. Printed at the bottom of the map is the contour interval, which is the difference in height (elevation) between one brown line and the one on either side of it. On a map with a scale of 1:50,000, the contour interval is usually 20 meters. This would make point A 80 meters higher or lower than point B (figure C-23).


Figure C-23. Contour lines


b. You can easily tell from the brown lines the direction of uphill or downhill because every fifth line is heavier and has a number that gives its elevation. Let's say the contour interval is 20 meters again. Now you can tell that point B. Also, if you know the distance between A and B, you can get an idea of the steepness of the slope (figure C-24).


Figure C-24. Contour intervals

 

c.  Widely spaced contour lines show a gentle slope. When they are close together, the slope is steep (figure C-25).


Figure C-25. Slope

 

d. When the contour lines are close together at the top of the hill, the hilltop is pointed. When the contour lines are widely spaced, the hilltop is flat (figure C-26).


Figure C-26.  Elevation

 

2.   Determine elevation.

a. Locate the point on the map. (It may already be plotted on the map, or given as eight-digit coordinates).

b. Determine the contour interval of the map from the marginal information.

c.  Locate the index contour line nearest the point for which the elevation is being sought.

d. Count the number of contour lines, up or down, that must be crossed to go from the numbered lines to the point, and note the direction to the point. If the point is on a contour line, its elevation is that of the contour line. For points between contour lines-

(1)  Points less than one-fourth the distance between lines are considered to be the same as the elevation of the nearest line.

(2)  Point one-fourth to three-fourths the distance from the lower line are considered to be at an elevation half the contour interval above the lower line.

e. To estimate the elevation of the top of an unmarked hill, add half the contour interval to the elevation of the highest contour line around the hill.

f.   To estimate the elevation of the bottom of depression, subtract half the contour interval from the elevation of the lowest contour around the depression.

g. On maps that do not show elevation and relief as much detail as needed, supplementary contour lines may be used. Marginal information indicates the interval, and the supplementary lines are used exactly like solid contour lines.

h. Benchmarks and spot elevation also indicate points of known elevation.

Evaluation Preparation: 

Setup: Provide the soldier with a 1:50,000 scale military map, a designated point on the map labeled "A", and a pencil.

Brief Soldier: Tell the soldier to determine the elevation of the point that is labeled "A" on the map.

Performance Measures

GO

NO GO

1.   Determined the correct elevation within half the value of the contour interval.

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Note. If the contour interval of the map is in feet, the answer must be in feet. If the contour interval is in meters, the answer must be in meters.

 

 

Evaluation Guidance:  Score the soldier GO if the performance measure is passed. Score the soldier NO GO if the performance measure is failed. If the soldier scores NO GO, show what was done wrong and how to do it correctly.

References

 

Required

Related

 

FM 3-25.26