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
PSC 121 Prince George's Community College

3. PSC 121 Prince George's Community College
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.
PSC 121 Prince George's Community College

4. PSC 121 Prince George's Community College
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
PSC 121 Prince George's Community College

5. PSC 121 Prince George's Community College
Contour Lines
Elevation is indicated by contour lines. A contour line is a line of equal elevations. All points on a contour line have the same elevation.
800 feet
700 feet
PSC 121 Prince George's Community College

6. PSC 121 Prince George's Community College
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
PSC 121 Prince George's Community College

7. PSC 121 Prince George's Community College
Contour Lines
The contour interval is usually printed on a topographic map.
However, you can always calculate the interval
from the lines with values.
PSC 121 Prince George's Community College

8. PSC 121 Prince George's Community College
Steep or flat?
You can tell whether a region on a map is steep terrain (hill/mountain) or relatively flat (plain) by looking at contour lines.
HOW?
We need to calculate a terrain gradient.
This is the change in elevation over the
change in distance along the ground.
PSC 121 Prince George's Community College

9. PSC 121 Prince George's Community College
Steep or flat?
Elevation change
= 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
PSC 121 Prince George's Community College

10. PSC 121 Prince George's Community College
Contours and Profiles
You can generate a profile of a landform from the contour lines on a topographic map.
Click on image
to see contours
develop
Simple contour map
PSC 121 Prince George's Community College

11. PSC 121 Prince George's Community College
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.
PSC 121 Prince George's Community College

12. PSC 121 Prince George's Community College
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
PSC 121 Prince George's Community College

13. PSC 121 Prince George's Community College
Map Symbols
Marsh
Secondary Highway
Houses
Forest
Stream
Intermittent Stream
PSC 121 Prince George's Community College

14. Tracking Flowing Water
How does water always flow?
DOWNHILL
How can we tell what direction (N,S,NW,ESE) is downhill?
PSC 121 Prince George's Community College

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
PSC 121 Prince George's Community College

16. Tracking Flowing Water
You can look at the contour lines as they cross the stream. They will always point upstream.
Downstream
Upstream
PSC 121 Prince George's Community College

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
PSC 121 Prince George's Community College

18. PSC 121 Prince George's Community College
Stream Gradients
You can determine a stream gradient along any section of a stream just like a terrain gradient.
2000 ft
2100 ft
0.4 miles
PSC 121 Prince George's Community College

19. PSC 121 Prince George's Community College
Bathymetric Maps
Maps of water depths are like topographic maps with contours. However, the contours are water depths, not elevations.
PSC 121 Prince George's Community College