Using a topographic map, you can solve a lot of practical problems without going to the area. From the topographic map you can determine: the scale of this map, the distance between any local objects, the size of any area, the slope of the slopes, the heights of any points in the terrain, the mutual excess of points, the visibility of points, the number of trees in the forest, the amount of water in the river, and much more.
How to determine the scale of a topographic map, measure the distance from it, determine the area, determine the slope of the slopes.
Typically, each topographic map is given a linear, numerical, and textual scale. But what if, for one reason or another, he was not there? An experienced specialist in the appearance of a topographic map can immediately name its scale. If you cannot do this, then you should resort to the following methods.
Determining the scale of a topographic map by kilometer grid.
Its side corresponds to a certain number of centimeters. If this distance is 2 cm, then the map scale of 1 cm is 500 meters, that is 1: 50,000. If 4 cm, then the map scale will accordingly be 1: 25 000.
Determining the scale of a topographic map along the length of the arc of the meridian.
In order to use this method, one must firmly remember that a single-geographical minute along the meridian is approximately 2 km (more precisely 1.85). Signatures of degrees and minutes are on the map, and in addition, each minute is highlighted with a checkmark. So, for example, in the figure below, the length of one minute is approximately 4 cm. This means that the scale of this map will be 1:50 000.
How to measure distance using a topographic map.
To determine the distance between two points, first measure this distance on the map, and then, using the numerical or linear scale of the map, determine the actual value of this distance on the ground. If you want to determine the distance not along a straight line, but along a winding road, use a special device – a curvimeter.
This is a device for measuring the length of curved lines. The basis of the curvimeter is a wheel, the circumference of which is known. The rotation of the wheel is transmitted to the arrow, which rotates on a dial. Knowing the number of revolutions of the wheel rolling along the measured line, it is easy to determine its length.
How to measure area by topographic map.
Geometrical area measurement.
The measured area is divided into a network of triangles, squares, trapezoids, the areas of which are calculated by known formulas. The sum of the areas of the known figures will give the total area enclosed in the contour.
Square measurement with a grid of squares.
It is very convenient to determine the area using a millimeter grid, which is applied to a transparent paper or film. Such a grid is applied to the map outline and the number of square millimeters is counted. Knowing what 1 mm2 of a topographic map on the ground is (for a scale of 1: 100,000 – 1 mm2 is a hectare, that is, 100 X 100 m), it is easy to determine the area on the map.
How to determine the steepness of slopes by topographic map.
The distance between the horizontal lines, the so-called laying, shows the slope of the ramp. The main methods for determining the slope slope from a topographic map are as follows.
How to determine the slope slope on the topographic map scale.
Typically, to determine the steepness of the slopes on the fields of the topographic map, a drawing is placed – a scale of occurrences. Along the lower base of this scale are numbers that indicate the slope of the ramps in degrees. On the perpendiculars to the base, the corresponding values of the laying on the map scale are plotted.
On the left side, the laying scale is built for the main section height, on the right – at five times the section height. To determine the slope of the ramp, for example, between points a-b, you need to take this distance with a pair of compasses and set aside on the scale and read the slope of the ramp – 3.5 degrees.
If it is necessary to determine the slope of the slope between horizontals thickened by n-m, then this distance must be postponed on the right scale and the slope of the slope in this case will be 10 degrees.
How to determine the slope of slopes by calculating.
Having measured the location d on the map and knowing the section height h, the slope a can be determined by the formula: a = h / d. Where a is the slope of the ramp in degrees, d is the distance between two adjacent horizontals in millimeters.
How to determine the slope of slopes using a ruler or by eye.
On Soviet topographic maps, the standard section height for each scale is set such that a slope of 1 cm corresponds to a slope of about 1 degree. From the above formula it is seen that how many times the laying is less than one centimeter, so many times the slope of the ramp is more than one degree. It follows that a slope of 1 mm corresponds to a slope of 10 degrees, a lay of 2 mm – 5 degrees, a lay of 5 mm – 2 degrees, and so on..
Based on the book “Map and Compass My Friends”.