What is a Topographic Map

A topographic map is the three-dimensional representation of the Earth's surface in a two-dimensional surface. It depicts the shape and elevation of the area with contour lines. The contours are imaginary lines drawn by joining the points of equal elevations with reference to the mean sea level. Each of the contours represents a line of similar elevations, hence they never cut or cross each other. In a topographic map, these contours lines indicate plain surfaces, slopes and/or depressions present on the surface of the Earth. Like any other maps, the magnification value is written on the topographic map.

History

The first topographic map was produced by the United States Geological Survey (USGS) in 1879. Since then, the topographers mapped manually (plot with hand) until the intervention of aerial photography in the 1940s. In the present scenario, aerial photographs are used for creating the topographic maps. A multi-lens camera captures the images of a particular area, both vertically and obliquely. The images are then joined to produce a continuous photograph.

The topographers first identify the natural and artificial features with the help of aerial photographs. Based on the features, contours lines are plotted for matching the elevations. The distance between two successive contour lines is called contour interval. The intervals may vary depending upon the map; it may be 50 feet, 100 feet and so on. After the completion of plotting contours, the map is then printed. It requires about 5 years for mapping a complete topographic map. Till now, thousands of topographic maps representing various countries have been produced with the help of aerial photography.

Details

The major advantage of topographic maps is that they show both natural landscapes and man-made (artificial) features. Natural landscapes such as mountains, rivers and lakes and artificial features like roads and buildings are highlighted by small symbols, pictures and colors. The three basic representative symbols used in the topographic maps are point, line and polygon.

Points are used for small structures such as buildings and bridges, whereas lines depict linear features like rivers, roads, highways and railway tracks. Lines may be straight, curved, dotted, solid, dashed or a combination of any of these. Vast areas of land and water like lakes, forested areas, agricultural fields and barren lands are illustrated by making use of different colors. For example, lakes are represented by blue color and deep forested areas by dark green color.

Uses

Topographic maps are used to describe the configuration of a particular landscape. It allows the map user to illustrate an area by analyzing the arrangement, shape and spacing of the contour lines. A river valley is represented by the contours of upward 'V' shape. Similarly, contour lines for ridges have downward 'V' shape. Speaking about the contours for slope, a steep slope is represented by closely spaced contours, whereas a gentle slope is illustrated by widely spaced contour lines.

By referring contour elevation and interval of topographic maps, the height of hills or mountains, steepness of slopes and depth of seas or oceans can be measured without going to the actual site. In day-to-day uses, topographic maps are used for the mining purposes, engineering works, building architecture and planning recreational activities like camping, paragliding, driving, hiking and cross-country skiing.