Topographic Maps

Lesson Objectives

Explain how to read and interpret a topographic map.
Explain how bathymetric maps are used to determine underwater features.
Describe what a geologic map shows.

Vocabulary

bathymetric map
contour interval
contour line
geologic map
topographic map

Introduction

Maps are extremely useful to Earth scientists to represent geographic features found above and below sea level and to show the geology of a region. Rock units and geologic structures are shown on geologic maps.

What is a Topographic Map?

Mapping is a crucial part of Earth science. Topographic maps represent the locations of geographical features, such as hills and valleys. Topographic maps use contour lines to show different elevations on a map. A contour line is a type of isoline; in this case, a line of equal elevation. If you walk along a contour line you will not go uphill or downhill. Mathematically, a contour line is a curve in two dimensions on which the value of a function f(x, y) is a constant.

Contour Lines and Intervals

Contour lines connect all the points on a map that have the same elevation and therefore reveal the location of hills, mountains, and valleys. While a road map shows where a road goes, a topographic map shows why. For example, the road bends in order to go around a hill or stops at the top of a mountain. On a contour map:

Each contour line represents a specific elevation and connects all the points that are at the same elevation. Every fifth contour line is bolded and labeled with numerical elevations.
The contour lines run next to each other and NEVER cross. After all, a single point can only have one elevation.
Two contour lines next to one another are separated by a constant difference in elevation (such as 20 ft or 100 ft). This difference between contour lines is called the contour interval. The map legend gives the contour interval.

A topographic map of Stowe, Vermont.

How would you calculate the contour interval on the map of Stowe, Vermont (see Figure above)?

Calculate the difference in elevation between two bold lines.
Divide that difference by the number of contour lines between them.

On the Stowe map, the difference between two bold lines is 100 feet and there are five lines between them, so the contour interval is 20 feet (100 ft/5 lines = 20 ft/line).

The Value of a Topographic Map

Swamp Canyon in Bryce Canyon National Park, Utah (shown in Figure below) is very rugged, with steep canyon walls and a valley below.

View of Swamp Canyon in Bryce Canyon National Park.

The visitor’s map of the area in Figure below shows important locations. What’s missing from this map? This map does not represent the landscape.

Swamp Canyon Loop in Bryce Canyon National Park. The green line indicates the main road, black dotted lines are trails, and there are markers for campsites, a picnic area, and a shuttle bus stop.

With contour lines to indicate elevation, the topographic map in Figure below shows the terrain.

Topographic map of Swamp Canyon Trail portion of Bryce Canyon National Park.

Interpreting Contour Maps

How does the map of Bryce Canyon reveal the terrain of the region? Several principles are important for reading a topographic map:

1. Contour lines show the 3-dimensional shape of the land (Figure below). What does the spacing of contour lines indicate?

Closely-spaced contour lines indicate a steep slope, because the elevation changes quickly in a small area.
Contour lines that seem to touch indicate a very steep rise, like a cliff or canyon wall.
Broadly spaced contour lines indicate a shallow slope.

A portion of a USGS topographic map of Stowe, Vermont. Just to the right of the city of Stowe is a steep hill with a sharp rise of about 200 ft that becomes less steep toward the right.

2. Concentric circles indicate a hill. When contour lines form closed loops all together in the same area, this is a hill. The smallest loops are the higher elevations and the larger loops are downhill. On the Stowe map, which hill has an elevation of 1122 feet? If you found Cady Hill, on the left side of the map, you are right.

3. Hatched concentric circles indicate a depression, as seen in the Figure below. The hatch marks are short, perpendicular lines inside the circle. The innermost hatched circle would represent the deepest part of the depression, while the outer hatched circles represent higher elevations.

On a contour map, a circle with inward hatches indicates a depression.

4. V-shaped expanses of contour lines indicate stream valleys. Where a stream crosses the land, the Vs in the contour lines point uphill. The channel of the stream passes through the point of the V and the open end of the V represents the downstream portion. If the stream contains water, the line will be blue; otherwise, the V patterns indicate the direction water will flow. In the map of Stowe, where does a stream run downhill into a lake?

Start at the “T” in Stowe. A blue stream goes downhill (northwest) into a lake. Coming out of the T on the other side, you can follow the blue stream uphill (southeast). Where the water flow is light or nonexistent, there is no longer a blue line, but the contour lines point uphill indicating that the stream channel is still there (see the map of Stowe in Figure above).

5. Scales on topographic maps indicate horizontal distance. The horizontal scale can be used to calculate the slope of the land (vertical height/horizontal distance). Common scales used in United States Geological Service (USGS) maps include the following:

1:24,000 scale – 1 inch = 2000 ft
1:100,000 scale – 1 inch = 1.6 miles
1:250,000 scale – 1 inch = 4 miles

An animation showing contour lines and the slopes they represent.

Bathymetric maps

The bathymetric map in the Figure below is like a topographic map with the contour lines representing depth below sea level, rather than height above. Numbers are low near sea level and become higher with depth. Bathymetric maps help oceanographers visualize the landforms at the bottoms of lakes, bays, and the ocean as if the water were removed.

Loihi volcano growing on the flank of Kilauea volcano in Hawaii. Black lines in the inset show the land surface above sea level and blue lines show the topography below sea level.

Geologic Maps

A geologic map shows the geological features of a region (see examples in Figure below and Figure below). Rock units are color-coded and identified in a key. In the map of Yosemite (Figure below), volcanic rocks are brown, the Tuolumne Intrusive Suite is peach, and the metamorphosed sedimentary rocks are green. Structural features, such as folds and faults, are also shown on a geologic map. The area around Mt. Dana on the east central side of the map has fault lines.

Geologic map of Yosemite National Park.

On a large scale geologic map, colors represent geological provinces.

Lesson Summary

Topographic maps are 2-dimensional representations of the 3-dimensional surface features of an area.
Topographic maps have contour lines that connect points of identical elevation above sea level.
Contour lines run next to each other. Adjacent contour lines are separated by a constant difference in elevation, usually noted on the map.
Topographic maps have a horizontal scale to indicate horizontal distances.
People use topographic maps to locate surface features in a given area, to find their way through a particular area, and to determine the direction of water flow in a given area.
Oceanographers use bathymetric maps to depict the features beneath a body of water.
Geologic maps display rock units and geologic features of a region of any size. A small scale map displays individual rock units; a large scale map shows geologic provinces.

Review Questions

On a topographic map, contour lines create a group of concentric, closed loops. Which of the following features could this indicate?
1. a stream channel
2. a hilltop
3. depression
4. a cliff
Describe the pattern on a topographic map that would indicate a stream valley. How do you determine the direction of water flow?
On a topographic map, five contour lines are very close together in one area. The contour interval is 100 ft. What feature does that indicate? How high is this feature?
On a topographic map, describe how you can tell a steep slope from a shallow slope.
On a topographic map, a river is shown crossing from Point A in the northwest to Point B in the southeast. Point A is on a contour line of 800 ft and Point B is on a contour line of 900 ft. In which direction does the river flow?
On a topographic map, six contour lines span a horizontal map distance of 0.5 inches. The horizontal scale is 1 inch equals 2,000 ft. How far apart are the first and sixth lines?
On a geologic map of the Grand Canyon, a rock unit called the Kaibab Limestone takes up the entire surface of the region. Down some steep topographic lines is a very thin rock unit called the Toroweap Formation and down more topographic lines into the canyon from that is another thin unit, the Coconino Sandstone. Describe how these three rock units sit relative to each other. Which is oldest and which is youngest?

Points to Consider

How might a civil engineer use a topographic map to build a road, bridge, or tunnel through the area such as that shown in Figure above? What topography would be best for a bridge? Which areas might need a bridge? Where might a tunnel be helpful?
If you wanted to participate in orienteering, would it be better to have a topographic map or a road map? How would a topographic map help you?
If you were the captain of a ship, what type of map would you want and why?

Studying Earth’s Surface