1. PSC 121 Prince George's Community College
Topographic Maps
More than a Road Map
PSC 121 Prince George's Community College

2. What is a Topographic Map?
Scaled representation of features on the surface of the Earth such as roads, rivers, and plains
Provides information about land elevations and landforms such as mountains, hills, and depressions

3. Map Scale
Maps always have a scale printed somewhere on the map. For USGS maps it is at the bottom.
On USGS the map portion below, the scale is 1:24,000. That means that 1” on the map is 24,000” on the ground or 1’ on the map is 24,000’ on the ground.

4. Map Scale
There are also scales that can be used to determine distance for many different units using a ruler and the map scale.
1 mile
1 mile
1/2 mile
2000 feet
1 kilometer
200 feet

5. Contour Lines Elevation is indicated by contour lines.
All points connected by a contour line have the same elevation.
800 feet
700 feet

6. Contour Lines
The elevation difference from one line to another is called the contour interval.
On the map portion below the contour interval is 20 ft.
20 ft elevation decrease
20 ft elevation increase
20 ft elevation increase

7. Contour Lines
The contour interval is usually printed on a topographic map.
However, you can always calculate the interval from the line values.

8. Steep or flat? We need to calculate a terrain gradient.
How can you tell whether a region on a map is steep
or flat by looking at contour lines.
We need to calculate a terrain gradient.
This is the change in elevation over the
change in distance along the ground.

9. Steep or flat? Elevation change Elevation change 0.5 mile distance
= 20 feet
Gradient 20ft/0.5 mi = 40 ft/mi
Flat
Elevation change
= 600 feet
Gradient 600ft/0.5 mi = 1200 ft/mi
Steep
0.5 mile distance

10. Contours and Profiles
You can generate a profile of a landform from the contour lines on a topographic map.
Simple contour map

11. Contours and Profiles From a contour plot we can draw an axis line.
We can then drop
lines straight down
to a grid labeled with
elevations to the
correct value.
Connecting the
points generates
a profile or
cross-section of
the map.

12. Map Symbols
Topographic maps usually have a separate sheet with an explanation of map symbols. There is no room on the map for all the symbols
Sample page from USGS Topo Map Symbol Pamphlet

13. Map Symbols Marsh Secondary Highway Houses Forest Stream
Intermittent Stream

14. Tracking Flowing Water
What direction does water always flow?
DOWNHILL
How can we tell what direction (N,S,NW,ESE) is downhill?

15. Tracking Flowing Water
You can look for elevation changes since water will flow from higher to lower elevations.
Here the stream flows toward the northwest.
2000 ft
2100 ft

16. Tracking Flowing Water
You can look at the contour lines as they cross the stream. They will always point upstream.

17. Tracking Flowing Water
You can also look at where two streams merge. The merge will form a V that points downstream.
Direction of flow

18. Stream Gradients
You can determine a stream gradient along any section of a stream just like a terrain gradient.
2000 ft
0.4 miles
2100 ft

19. Bathymetric Maps
Maps of water depths are like topographic maps with contours. However, the contours are water depths, not elevations.