A topographic map is a two-dimensional representation of a portion of the three-dimensional surface of the earth. Topography is the shape of the land surface, and topographic maps exist to represent the land surface. These maps are used for geologic studies because they show the configuration of the earth’s surface. Cartographers solve the problem of representing the three-dimensional land surface on a flat piece of paper by using contour lines, thus horizontal distances and vertical elevations can both be measured from a topographic map. Contour lines are curved, straight or a mixture of both lines on a map describing the intersection of a real or hypothetical surface with one or more horizontal planes. The configuration of these contours allows map readers to infer relative gradient of a parameter and estimate that parameter at specific places. The contour interval of a contour map is the difference in elevation between successive contour lines.

Topographic Map

A topographic profile of the earth surface is obtained from topographic profiling. A topographic Profile is a cross-sectional view along a line drawn through a portion of a topographic map.

Contour lines are curved, straight or a mixture of both lines on a map describing the intersection of a real or hypothetical surface with one or more horizontal planes. The configuration of these contours allows map readers to infer relative gradients of a parameter and estimate that parameter at specific places.

The following are general characteristics of contour lines:

1. Contour lines do not cross each other, divide or split.
2. Closely spaced contour lines represent steep slopes. Conversely, contour lines that are spaced far apart represent gentle slopes.
3. Contour lines trend up valleys and form a "V" or a "U" where they cross a stream.

How to Interpret Contour Lines:

• A glance at a topographic map (flat or mountainous) will provide you with an overview of the landscape.
• If each line represents an equal point of elevation, any elevation change must result in inconsistent line spacing. For example, several contour lines close together indicate steep terrain, whereas lines far apart indicate a gentler slope. On a map, there are three types of contour lines: intermediate, index, and supplementary.

1. Index lines: are the thickest contour lines, and they are labelled with a number at one point along the line. This tells you how high you are above sea level.
2. Intermediate lines: are the thinner, more common lines that connect the index lines. They usually don't have a label with a number on it. In most cases, one index line appears for every five intermediate.
3. Supplementary lines: are shown as dotted lines to indicate flatter terrain lines. In an index line, the elevation is easy to read because it is clearly labelled. Interval lines, on the other hand, are a little more complicated. Here contour intervals are required to figure out their elevation. Contour intervals indicate the difference in elevation between two contour lines. The contour interval can be found in the map key, usually located beneath the map's scale.

Commonly used contour line formations:

Peak Ring: The innermost ring at the centre of several contour loops represents a peak (highest elevation).
Depression Ring: The inner ring denotes a depression (lowest elevation), depicted on the map by a series of minor tick marks pointing toward the centre (called hachures).
Cliff: A cliff is a rock face that is extremely steep, vertical, or overhanging. Cliffs are represented by contour lines that are very close together. A cliff is formed when two or more lines converge until they appear as a single line.
Valley: A valley is a long, elongated depression in the landscape formed by water action (V-shaped) or carved out by glaciers (U-shaped). Valley bottoms are represented by "U" or "V" shaped contour lines, with the closed end pointing up.
Ridge: A ridge is a landform feature characterized by a continuous elevational crest with sloping sides.
Saddle: A saddle is a low-lying area between two higher elevation points. Identifying saddles can help you find the quickest path through hilly terrain.