Determining the width of a river, other local object or site, for example a large ravine, is as follows. The observer becomes at the starting point A. For example, on the bank of the river, and selects a clearly visible landmark on the opposite bank or side. For example, bush, stone, tree and the like.
After that, going in the direction perpendicular to the AB line, it passes to point B, remote from point A more than the estimated width of the river or the measured area. At point B sets a milestone (stick, peg). Then, going in the same direction, it passes to point C. Which is located at the same distance from point B as point B from point A.
Further from point C, it goes at right angles to the AC line to point D, that is, until the milestone installed at point B is in alignment with the selected landmark (tree) on the opposite bank of the river. Or the point of the measured area. From the equality of the triangles GVA = WBA, it can be seen that the segment SD = AB, that is, the width of the river or the measured area. In the example shown in the figure, it is 45 meters.
A method for determining the width of a river, other local object or site using a blade of grass.
Determination of the width of the river, another local object or site may be made using a conventional blade of grass. Tear it and select on the opposite bank of the river or side of the site, two-conspicuous objects. Stand facing these objects, stretch out your hands with a blade of grass and use it to measure the distance between the objects, looking at them with one eye.
After that, fold the blade of grass in half (twice). And then move away from the coast or the conditional border of the site until the distance between the selected objects is closed (laid out) by a doubled blade of grass. Measure this distance in steps, convert it to meters. This will be the distance equal to the width of the river or the measured area.
The method of determining the range of the visible horizon.
In order to better develop the eye, it is necessary to know how far the horizon lies from the observer. For this purpose, use the formula: horizon = 113 x square root h, where h is the observer height (in km).
For example, standing on a plain, a person 1.6 meters tall sees the surrounding area at a distance equal to: 113 x square root of 0.0016 = 4.52 kilometers. And sitting in a boat and rising only 1 meter above the water, a person can survey the surrounding area at a distance equal to: 113 x square root of 0.001 = 3.58 kilometers.
Based on the book “Map and Compass My Friends”.