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Conventional signs

This is the alphabet, the knowledge of which is necessary to read the map. Although the designations of conventional signs to a certain extent depend on the scale of the map, they are usually divided into three groups: large-scale, off-scale and explanatory. The former depict local objects (usually with a contour) that "fit" into the scale of the map: lakes, large cities, the latter depict objects that cannot be expressed on a given scale. By such a sign it is impossible to judge the actual size of the village, well or bridge shown on the map. The third characters include numbers, inscriptions and other designations.

With a decrease in the scale of the map, large-scale conventional symbols turn into off-scale ones. When reading conventional signs, you need to know that the map (plan) depicts, as a rule, the summer state of the area. The most common and necessary for the traveler conventional signs are shown in fig. 6.2.

Terrain relief

It is depicted on large-scale maps using contour lines, which are called curved closed lines connecting terrain points on the map that are the same in height above ocean level.

The distance in height between two adjacent horizontals taken on the same slope is called the height of the section, and the distance between the horizontals according to the plan is called the laying. The steeper the slope, the lower the amount of laying, and vice versa. To determine the steepness of the slope, the shortest distance between adjacent horizontals is measured and the corresponding position is selected on the scale, against which the steepness of the slope is read in degrees. To determine the direction of the slope along the horizontal lines, perpendicular dashes are placed on some of them - slope indicators (bergstrokes), "looking" with their free end downwards (Fig. 6.3).

Rice. 6.2. Conventional signs

Some landforms are depicted not by horizontal lines, but by special conventional icons. So, cliffs, ravines, ramparts, pits are shown by a line with denticles facing the steep slope. When using small-scale maps, tourists can see the image of the relief by the method of hillshade (thickening shadows) and hypsometric ("the higher, the darker").

Rice. 6.3. Depiction of landforms by contour lines

On maps where only a hydro-network is plotted (rivers, streams, lakes), the relief has to be "finished" in the mind, based on the density, direction and sinuosity of watercourses and elevations.

On the diagrams and plans of mountainous regions, the relief is often depicted only by watershed lines (ridges), marks of the main peaks, showing rivers and glaciers.

Classification of topographic maps

Russian topographic maps are nationwide. They are published in the scales indicated in Table. 6.1.

The topographic maps used by the troops are divided into large-scale (1:25:000,1), medium-scale (50:000, 1:100), and small-scale (000:1, 200:000).

Spherical surfaces do not develop on a plane without folds and breaks, and for this reason, distortions of lengths, angles, and areas are inevitable on maps. Only in some projections the equality of angles is preserved, but because of this, the lengths and areas are significantly distorted, or the equality of areas is preserved, but the angles and lengths are significantly distorted.

Projection of maps at a scale of 1:25-000:1. Topographic maps of the Russian Federation and many foreign countries are created in the Gaussian transverse cylindrical projection.

Table 6.1. Topographic maps at a scale of 1:25-000:1

Note. The first number of sheet sizes means the extent from north to south, this size is constant for any latitude. The second number is the length from east to west, this size gradually decreases with increasing latitude.

The projection of the earth's surface onto a plane in the Gaussian projection is carried out along zones elongated from the north pole to the south. The boundaries of the zones are meridians with longitude divisible by 6° (60 zones in total). Within each zone, the earth's surface is projected onto a plane by converting the geographic coordinates of points on the earth's surface into rectangular coordinates on the plane.

The line lengths are preserved only along the axial meridian; in other places they are somewhat exaggerated. The greatest relative distortions of lengths take place at the boundaries of the zones and within the Russian Federation reach 1/1000, the relative distortions of areas - 1/500. Distortions of distances during graphical measurements on topographic maps are not detected; they are taken into account only when performing special tasks related to the use of long ranges.

The corners within a small area are not distorted, the outlines of the contours on the ground and on the map are almost similar. Distortions of any directions on a map sheet at a scale of 1:100 do not exceed 000 . All sheets of maps of any scale within the same zone can be glued into one block without any folds or breaks.

Projection of a topographic map at a scale of 1:1 - a modified polyconic projection, adopted as an international projection for maps at a scale of 1:1. Its main characteristics are: the projection of the earth's surface covered by a map sheet is made onto a separate plane; parallels are represented by arcs of circles, and meridians by straight lines; the greatest distortion of lengths within the sheet reaches 000%, distortion of angles - up to 000%, distortion of areas - up to 0,14%.

When four map sheets are added at a scale of 1:1, located within latitudes of 000-000°, an angular discontinuity of the order of 40-60 and a linear discontinuity of 20-40 mm appear (the divergence of the sheets increases towards the poles). In one block, no more than 2 sheets are glued together without noticeable gaps.

Author: Mikhailov L.A.

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