topographic maps

PSC 121 Prince George's Community CollegePSC 121 Prince George's Community College

Topographic MapsTopographic MapsMore than a Road MapMore 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 Scaled representation of features on the surface of the Earth such as roads, surface of the Earth such as roads, rivers, and plainsrivers, and plains

► Provides information about land Provides information about land elevations and landforms such as elevations and landforms such as mountains, hills, and depressionsmountains, hills, and depressions


Map ScaleMap Scale

Maps always have a scale printed somewhere Maps always have a scale printed somewhere on the map. For USGS maps it is at the on the map. For USGS maps it is at the bottom.bottom.

On USGS the map portion below, the scale is On USGS the map portion below, the scale is 1:24,000. That means that 1” on the map 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 or 1’ on the map is 24,000’ on the ground.24,000’ on the ground.


Map ScaleMap ScaleThere are also scales that can be used to There are also scales that can be used to

determine distance for many different units determine distance for many different units using a ruler and the map scale.using a ruler and the map scale.

1 mile1 mile

1/2 mile

2000 feet

200 feet

1 kilometer


Contour LinesContour Lines

Elevation is indicated by contour lines. AElevation is indicated by contour lines. A contour line contour line is ais a line of equal elevations. line of equal elevations. All All points on a contour line have the same points on a contour line have the same elevation.elevation.

800 feet

700 feet

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The The elevation difference from one line to elevation difference from one line to anotheranother is called the is called the contour intervalcontour interval. On . On the map portion below the contour interval the map portion below the contour interval is 20 ft.is 20 ft.

20 ft elevation increase

20 ft elevation increase

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The contour interval is usually printed on a The contour interval is usually printed on a topographic map. topographic map.

However, you can always calculate the intervalHowever, you can always calculate the interval

from the lines with values.from the lines with values.


Steep or flat?Steep or flat?

You can tell whether a region on a map is You can tell whether a region on a map is steep terrain (hill/mountain) or relatively steep terrain (hill/mountain) or relatively flat (plain) by looking at contour lines.flat (plain) by looking at contour lines.

HOW?HOW?

We need to calculate a terrain gradient. This is the change in elevation over the change in distance along the ground.

Elevation (ft)Gradient =

Ground Distance (mi)


Steep or flat?Steep or flat?

0.5 mile distanceElevation change2170-2150 = 20 feetGradient 20ft/0.5 mi = 40 ft/mi Flat

Elevation change1380-780 = 600 feetGradient 600ft/0.5 mi = 1200 ft/mi Steep


Contours and ProfilesContours and Profiles

You can generate a profile of a landform from You can generate a profile of a landform from the contour lines on a topographic map.the contour lines on a topographic map.

Simple contour map

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From a contour plotwe can draw anaxis line.

We can then drop lines straight downto a grid labeled withelevations to the correct value.

Connecting thepoints generatesa profile or cross-section ofthe map.


Map SymbolsMap Symbols

Topographic Topographic maps usually maps usually have a have a separate separate sheet with an sheet with an explanation explanation of map of map symbols. symbols. There is no There is no room on the room on the map for all map for all the symbolsthe symbols

Sample page from USGS Topo Map Symbol Pamphlet


Map SymbolsMap Symbols

Marsh

Stream

Forest

Intermittent Stream

Secondary Highway

Houses


Tracking Flowing WaterTracking Flowing Water

►How does water always flow?How does water always flow?

DOWNHILLDOWNHILL►How can we tell what direction How can we tell what direction

(N,S,NW,ESE) is downhill?(N,S,NW,ESE) is downhill?


Tracking Flowing WaterTracking Flowing WaterYou can look for You can look for

elevation elevation changes since changes since water will flow water will flow from higher to from higher to lower lower elevations.elevations.

Here the stream Here the stream flows toward flows toward the northwest.the northwest.

2100 ft2000 ft



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

Downstream Upstream



You can also You can also look at look at where two where two streams streams merge. The merge. The merge will merge will form a V that form a V that points points downstream.downstream.

Direction of flow


Stream GradientsStream Gradients

You can determine You can determine a stream a stream gradient along gradient along any section of a any section of a stream just like stream just like a terrain a terrain gradient.gradient.

2100 ft2000 ft

0.4 miles

elevationstream gradient =

distance along stream

100 ft = 250 ft/mi

0.4 mi


Bathymetric MapsBathymetric Maps

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

topographic maps

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