We are explaining importance of relief representation in a map and methods of various relief representation in detail.This is a Free Photogrammetry Study material supplied for photogrammetry students, photogrammetry professionals and GIS professionals.The relief representation on topo map is not only important to give information about terrain but it is also used for all types of engineering, planning and development activities.
Methods for representation of relief are
1. Hatchering: Representation of mountains by short lines 1r to contour in the direction of the steepest slope is called hatchering. The technique of slope hatchers is based on the principle that under 1r light, horizontal surface receives more light than the sloping surface. Degree of darkness is directly proportional to steepness terrain. In this method slope differences are expressed by difference in thickness of black lines drawn. The length of line is in principle determined by distance between 2 adjacent contours. The thickness of hatchers is based on principle that a horizontal surface is represented as full white and slope of 45o or more is shown with full black. All other slopes between 0o & 45o are represented in a tone ranging between white and black.
The ratio of black and white is black: white = x: 45-x [x is slope angle].
2. Shading: In hill shading technique, terrain is supposed to be illuminated obliquely, instead of vertically. The light source in supposed to be on northwest corner at 30o incluation. The shading is done by pencil dust (gray color) sand dues are also shown by sets of different thickness with proper pattern to create the effect of shading which brings out the relief perception and differentiation from flat sand. The method of hill shading is mostly used in geographical maps and is also preferred in topographical maps with contours on the background.
3. Layering: Here relief is shown in color pattern. The land surface is divided into height belts. Sometimes even natural water up to a distance of 5km beyond shore is also included.
e.g. 0 to 1000 m
1000 to 2500m
2500 to 5000m
According to convenience and need of depiction separate color wash are given to bring out physiological effect of relief.
E.g. showing 0 to 1000 in green color,1000 to 2500 with light brown parched land, higher regions by dark brown, glacial region by bluish tint layering with contouring makes a very good combination.
4. Spot Heights : spot heights are points on map, the heights of which above or below the datum surface is given. In topographic maps, not heights are used as supplement to contour lines and indicates height of important points on terrain. These points must be clearly readable on maps. The number of spot heights in map is determined by any fixed rule. Then number depends upon scale of map and type of area. At least one spot height should be near the village area to indicate the general height of village above datum surface. In case of place areas, spot height should be more to represent relief as accurately as possible.
5. Contours : Contours are imaginary line passing through contiguous points on same elevation from a reference surface called as datum. Such lines are drawn at constant interval. Fixing of contour interval for any particular map being surveyed and drawn will depend on following
1. Scale of map
2. Purpose and use of map
3. Nature of ground
4. Availability of time and money
In general each contour is congested area is separated from its neighbors by at least 0.5mm. To facilitate reading, every fifth contour line commencing from zero is made thicker than the rest.
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