Sec. 12. Conventional Symbols

Features and objects on the ground are shown on plans and maps by conventional symbols that are used in conformity with the stan­dards adopted for the definite; scale and type of plan or map.

Cartographic conventional symbols (USSR St. St. COST 21667-76) are divided into area, non-scale and linear symbols. Area symbols are

Fig. 12

the conventional signs used to fill areas of the terrain objects expressed in the scale of the plan or map. This sign makes it possible to determine from the plan (map) not only the position of an object or feature but also its size. If an object is so small at the given scale that it is impossible for it to be represented by an area symbol, a non-scale symbol is used. Objects represented by these symbols occupy more area on the plan than they ought to according to the scale. Non-scale symbols are often used on maps.

In order to represent linear objects on maps and plans, the objects lengths being expressed to meet the scale, it is usual to employ linear conventional symbols. These are shown on plans and maps to agree with the scale and position of the horizontal projection of an object's length, yet its width is somewhat exaggerated. Most inscrip­tions on a topographic plan or map are arranged parallel to the lower and upper margins. The names of rivers, streams and mountain ranges are inscribed along their directions.

To finish plans, they are coloured. The contours of bodies of water (rivers, lakes, ponds) are shown with a green line. Relief features and sands are coloured brown, local objects and inscriptions are black. Areas of water reservoirs are coloured pale blue for fresh water and violet for salt or bitter water. Highways are coloured red or orange, improved dirt roads yellow or light brown. Figure 12 contains the conventional symbols for a 1/5 000 scale. According to current standards they are a definite size for each representation and scale.

Sec. 13. The Contour Line Technique to Delineate Relief Features

There are various methods for showing relief features on plans and maps (hachuring, dashes, hill shading). However, the most common technique is to employ contour lines.

Fig.13

A contour line on the terrain can be thought of as being the locus of a level surface cutting the Earth's physical surface over the area being represented (Fig. 13). The best illustration for a contour line on the terrain would be the shore line of a body of water located at various definite heights.

The difference between the heights of two consecutive contour lines is termed the contour (vertical) interval. Depending on the scale, relief forms and use to be made of the plan (map), contour intervals are taken equal to 1, 2, 2.5, 5, 10, 20 m and so on. The smaller the contour interval, the more precisely the terrain relief is delineat­ed on the plan and the more detail should be involved when survey­ing relief features. If it is necessary to show quite mild relief fea­tures, it is customary to use fractional contour intervals such as 0.25 or 0.5 m.