The topographic map of the heights of points above sea level (absolute heights) is determined using horizontal marks. If the point is located horizontally, then the task is reduced to determining the level of this horizontal. If there is no such mark on it, then it is determined by the marks of the nearest contour lines or points whose height is indicated on the topographic map.

## How to determine the height of terrain points on a topographic map, their mutual visibility, the amount of firewood, how many cubic meters of forest grows on a hectare and how much water in the river.

If the point is between the horizontals, then to determine its mark, you need to set the direction of the slope, determine the height of the lower horizontal nearest to it. Then add to it the excess of this point. It is determined by eye. The determination of the excess of one point over another (relative) is also determined using the contour marks.

## How to determine the mutual visibility of terrain points on a topographic map.

This should be known when choosing observation points, hidden approaches, as well as in cases where it is necessary to establish how the location is viewed from probable observation points of the enemy. The determination from the topographic map of mutual visibility reduces to the fact that, not being on the ground, to establish whether there is any elevation or local object in the direction of observation that will block your line of sight.

Determining the visibility of points from a topographic map can be determined most simply and accurately by constructing a triangle. For this purpose, connect the points of the NP (observation point) and C (target) on the map with a straight line. Mark on this line the point of possible shelter for target U. In a specific example, this may be a height with a horizontal of 180.

Having determined which of these three points (NP, C, Y) is the lowest, put a zero near it. The remaining points sign their excess in relation to this zero point. In our example, the target is the zero point, the shelter is 15 meters above it, and the observer is 25 meters.

From points having an excess above the zero point, restore the perpendiculars to the NP – C line and set the excess values (15 and 25) on them (at an arbitrary, but on the same scale). Then apply a ruler to the obtained points on the perpendiculars and draw a straight line (line of sight).

If this line passes above the zero point, the latter will not be visible. In our example, the target is not visible. In order for it to be visible to the observer, you must climb about 5-6 meters dashed line on the triangle.

## How to determine the amount of firewood that can be harvested in the forest shown on the topographic map.

Based on the characteristics of the forest, let’s say 20 / 0.3×5, it is known that the height of the trees is 20 meters, the thickness of tree trunks is 0.3 meters, and the distance between them is 5 meters. Assuming that the trunk of each tree has the shape of a cone, the base of which is a circle with a diameter of 0.3 m3, and a height of 20 meters. This data is quite enough to calculate the volume of the tree according to the well-known formula:

## How to find out how many cubic meters of forest grows on one hectare, depicted on a topographic map.

And to find out how many cubic meters of forest grows on one hectare, you need to determine the total number of trees in this area. For our example, the trees are 5 meters apart. So, 20 trees will be located at a distance of 100 meters, and 400 at an area of 100×100 meters. The volume of wood from such a forest per hectare will be 0.47×400 = 188 m3.

## How to determine how much water is in the river shown on the topographic map.

To answer this question, you need to know the average flow rate and the transverse area of the river. The speed of the river is indicated on the map, and the transverse area is absent. But if we take advantage of the width of the river and its depth and consider its transverse area as the area of a triangle or, closer to reality, a trapezoid (its lower base is equal to half the width of the river), then it is relatively simple to calculate the second water flow in the river.

These are the simplest calculations that can be performed on a topographic map without going out into the field. But the main task of the topographic map still comes down to studying the terrain depicted on it, and the ability to navigate it with the help of a map and a compass.

*Based on the book “Map and Compass My Friends”.*

* Klimenko A.I..*