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Topographic Maps. More than a Road Map. 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. Map Scale.

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topographic maps

Topographic Maps

More than a Road Map

PSC 121 Prince George's Community College

what is a topographic map
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

map scale
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

map scale1
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

contour lines
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

http://raider.muc.edu/~mcnaugma/Topographic%20Maps/contour.htm

700 feet

PSC 121 Prince George's Community College

contour lines1
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

http://raider.muc.edu/~mcnaugma/Topographic%20Maps/contour.htm

20 ft elevation increase

PSC 121 Prince George's Community College

contour lines2
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

steep or flat
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

steep or flat1
Steep or flat?

Elevation change

2170-2150 = 20 feet

Gradient 20ft/0.5 mi = 40 ft/mi

Flat

Elevation change

1380-780 = 600 feet

Gradient 600ft/0.5 mi = 1200 ft/mi

Steep

0.5 mile distance

PSC 121 Prince George's Community College

contours and profiles
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

http://reynolds.asu.edu/topo_gallery/images/hillmap.gif

Simple contour map

PSC 121 Prince George's Community College

contours and profiles1
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.

http://reynolds.asu.edu/topo_gallery/images/hillmap.gif

Connecting the

points generates

a profile or

cross-section of

the map.

PSC 121 Prince George's Community College

map symbols
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

map symbols1
Map Symbols

Marsh

Secondary Highway

Houses

Forest

Stream

Intermittent Stream

PSC 121 Prince George's Community College

tracking flowing water
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

tracking flowing water1
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

tracking flowing water2
Tracking Flowing Water

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

Downstream

Upstream

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tracking flowing water3
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

stream gradients
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

bathymetric maps
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