What is a topographic map?

Topographic map
Topographic map

A topographic map (also called a “tapa” map) is a field map that represents a scale model of part of the Earth’s surface. Using special symbols and lines, it shows the three-dimensional shapes of the surface using two dimensions. Topo maps are used a great deal by geologists in the field, primarily to gather information about features, map certain interesting rock areas, and to generally get around rougher terrain.

What are contour lines on topographic maps?

The most important features on topographic maps are contour lines: the brown lines of differing widths that represent points of equal elevation. These lines symbolize the shape of the Earth’s surface, with each line representing the land as if it were sliced by a horizontal plane at a particular elevation above sea level. Thicker contour lines called index contours-usually shown as every fifth contour line-make it easier for the user to determine elevations. Contour lines that are very close together represent steep slopes, while widely spaced contours or no contours at all-represent relatively level ground. The contour lines also represent a distance called the contour interval, or the difference in elevation represented by adjacent contour lines. Each map has a different contour interval listed on the map’s legend. For example, a relatively flat area may have a contour interval of 10 feet (3 meters) or less, meaning that the difference between each contour line will be 10 feet (3 meters) up or down in elevation; a mountainous area may have a contour interval of 100 feet (30 meters) or more.

What are some of the major rules for contour lines on a map?

There are several major rules contour lines must follow. In particular, contour lines must not cross (except in the rare case of an overhanging cliff); spacing represents either a very steep slope (lines close together) or wide plains (lines wide apart); a hill is represented by a series of closed contour lines “stacked” on one another like a lopsided bull’s eye (depressions are the same, but contain hatch marks within the closed contour lines-or the downhill side); contour lines must never diverge; and when contour lines cross stream or river valleys, they must form Vs that point upstream.

What are bathymetric contours?

Bathymetric contours are similar to regular contours, except they depict the elevations, shape, and slope of marine features offshore (usually the bottom floors of bays, seas, and oceans). These contours are in black or blue, and they are usually written in meters at various intervals, depending on the map scale. They should not be confused with maps that depict depth curves, which usually represent water depths along coast~ lines and inland bodies of water. The contours of these maps are usually show in blue, with the data coming from hydrographic charts and depth soundings.

How do you determine the scale of a topographic map?

A topographic map’s scale-no matter which scale is used-represents the horizontal distances on the map (not elevation distances, which are shown by contour lines). Similar to a street or highway map, the scale can vary widely, depending on the map. But the topographic map’s scale differs in a major way, by allowing the easy interpretation of each map’s distances. Topographic maps are notorious for using different scales, depending on how much detail is desired. Each scale comes with a map ratio. For example, a map with a scale of 1:25,000 means one inch on the map is equal to 25,000 inches on the ground. And because both numbers use the same units, it can also be interpreted as any unit measure. For example, the same map could also be interpreted as 1 centimeter equals 25,000 centimeters on the ground. For those who prefer to measure in miles and kilometers, most topographic maps also offer a graphic scale in the legend.

What is a geologic map?

Surface geologic map of France
Surface geologic map of France

A geologic map is actually a form of topographic map, but in this case it shows the type of sediment or rock outcrops exposed at the Earth’s surface, along with the contour lines. The information on these maps can range from the rock type and age to the orientation of rock layers and major (and sometimes minor) geologic features. Who uses these maps? Most geologists involved in almost every phase of field geology use geologic maps. For example, petrologists use these maps to determine the location of possible economic resources, such as metal ores, water, or oil. Ceomor· phologists use such maps to detect potential hazards in various areas, such as areas prone to earthquakes, flooding, or landslides. Occasionally, geologic profiles are also provided on these maps to help scientists understand, for example, the rock underlying an area.

How do geologists use strike and dip while in the field?

Strike and dip are not baseball terms; rather, they are used by geologists in the field to determine how rock layers and/or outcrops lean (or don’t lean) in certain directions. Both are very useful to geologists as they map rock outcrops and geologic features. Dip is the angle at which a layer or rock is inclined from the horizontal. It is usually measured with a clinometer. This instrument contains a straightedge that is lined up against the dip of the rock; a weight is used to measure the angle. Strike is the opposite-a line that a dipping rock layer makes with the horizontal (one way of visualizing it is to think how a waterline would form if the rock layer dipped into a lake). Geologists often use a compass to measure strike.