Topographical Map Basics

Intro to Maps

The feature that most distinguishes topographic maps from maps of other types is the use of contour lines to portray the shape and elevation of the land. Topographic maps render the three-dimensional ups and downs of terrain on a two-dimensional surface. Topographic maps usually portray both natural and human made features. They portray works of nature including mountains, valleys, plains, lakes, rivers, and vegetation. They also identify the principal works of man, such as roads, boundaries, transmission lines, and major buildings. In the margins of a topographical (and most other types of) map, there is a lot of information that helps you read and interpret the symbols on the map. This is called, for obvious reasons, marginal information. The most common batch of marginal information is the map legend or sometimes called the map key. The legend or key will...

Map Colors

A topographic map uses certain colors to help tell you what a graphic or symbol is. The colors on a topo map are:

  • Black - human geography on a map (buildings, railroads, bridges, boundaries, and names)
  • Blue - any type of water (lakes, ponds, rivers, streams, water well, and marshes)
  • Brown - relief features (contour lines and elevation)
  • Green - areas of heavy vegetation (forest, woodlands, orchards)
  • Red - larger roads and surveying lines (two or four-lane roads, first and second level borders)
  • Purple - for overprinting (Revisions added from aerial photographs but not yet field-checked, or planned additions)
  • White - open area (mostly clear of trees, fields, meadows, rocky slopes, and other open country)

Map Symbols

The symbols used on a map are generally shown in the marginal information. These include roads, rivers, manmade objects like buildings or mines, and even sometimes the type of building, like a church or a school.

Elevation and Terrain Features

Contour Lines and Contour Intervals

Relief lines represent the shapes of hills, valleys, streams and other features of the earth’s surface. Most often, relief is shown by contour lines. Contour lines are what makes a map three-dimensional. They are lines that connect points of equal elevation on the earth’s surface, and are used to illustrate topography. On US maps, they are usually in feet. For example, numerous contour lines that are close together indicate hilly or mountainous terrain; when far apart, they represent a gentler slope.

There are three types of contour lines: index, intermediate, and supplementary. Index lines are the thick or dark colored contour lines. They are usually every fifth contour line, and are labeled with an elevation number. For example, if an index contour line is labeled with 100 (meaning 100ft in elevation above sea level), then usually the next index line will be 200.

Intermediate contour lines fall between the index lines and are NOT labeled with their elevation. In the previous example, the intermediate lines would represent elevations of 120, 140, 160, and 180.

Supplementary contour lines are generally dashed lines that show one-half the contour interval. They are often used on maps where the contour interval is large, and the terrain somewhat featureless overall. Keeping with our example, a dashed line would represent 150.

A contour interval represents the vertical distance between two contour lines. The interval measurement is given in the map’s marginal information. The numbered index contour lines, give that particular line’s elevation. Find an Index contour line to get a reference point for elevation, and then count up or down using the contour interval. Contour interval isn’t the same on all maps. When there is a lot of change in elevation on a map, and the slopes are steep, the contour interval tends to be bigger – otherwise there would be too many lines, and they would blend together too much. So the mapmakers increase the contour interval, and instead of 10 feet, it might be 20 or 50 between each contour line. What’s the contour interval on this map? Even though we don’t have the marginal information, you can tell by comparing the Index contour lines. You can see a 400’ Index Contour, then a 600’ Index Contour. 600-400 = 200 feet between Index Lines. 200 ÷ 5 = 40’ So the Contour Interval is 40 feet. Each contour line represents a point that’s 40 feet higher than the contour line below it. Can you determine the elevation of the Water Tank? It’s on the second contour line up from the 400’ Index, so it’s 480 feet. How about the hilltop? If the hilltop didn’t have its elevation marked (882’), you would estimate it by finding the contour line that defines the hilltop (no other contour lines within it), then add half of the contour interval. That gives us an estimate of 860 feet. It’s nice when they put the exact height of hilltops! Sometimes, you will see a notation such as “BM 214” on a map. That means that map makers have measured that particular spot on the earth’s surface to be exactly 214 feet above sea level. Some benchmarks have actual markers on the ground, but often the markers aren’t there anymore.

Terrain Features

Terrain features are identified the same way on all maps, regardless of the contour interval. However, a hill in the Rocky Mountains will be much bigger than one in south Florida. You must be able to recognize all the terrain features to locate a point on the ground or to navigate from one point to another.
The five major terrain features are: Hill, Ridge, Valley, Saddle, and Depression. The three minor terrain features are: Draw, Spur and Cliff. The two supplementary terrain features are: Cut and Fill.

Hill- a point or small area of high ground. When on a hilltop, the ground slopes down in all directions.



Ridge- a line of high ground with height variations along its crest. The ridge is not simply a line of hills; all points of the ridge crest are higher than the ground on both sides of the ridge.



Valley- reasonably level ground bordered on the sides by higher ground. A valley may or may not contain some type of moving water. Contour lines indicating a valley are U- shaped and tend to parallel a body of moving water before crossing it.



Saddle- a dip or low point along the crest of a ridge. A saddle is not necessarily the lower ground between two hilltops; it may be a break along an otherwise level ridge crest.



Depression- a low point or hole in the ground, surrounded on all sides by higher ground.



Draw- similar to a valley, except that it normally is a less developed stream course in which there is generally no level ground and, therefore, little or no maneuver room. The ground slopes upward on each side and toward the head of the draw.



Spur- a usually short, continuously sloping line of higher ground, normally jutting out from the side of a ridge. A spur is often formed by two thoroughly parallel streams cutting draws down the side of a ridge.



Cliff- a vertical or near-vertical slope. A cliff may be shown on a map by contour lines being close together, touching, or by a ticked "carrying" contour line. The ticks always point toward lower ground.



Cut and Fill- a feature where terrain has been cut away – essentially a minor cliff, and where it has been filled in, creating more level ground than would normally be seen on the natural terrain feature. Often done to create a stable platform for a road or railroad.