Transcript Topographic Maps

Topographic Maps
More than a Road Map
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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
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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.
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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
200 feet
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1 kilometer
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.
http://raider.muc.edu/~mcnaugma/Topographic%20Maps/contou
r.htm
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800 feet
700 feet
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 increase
20 ft elevation increase
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http://raider.muc.edu/~mcnaugma/Topographic%20Maps/contou
r.htm
20 ft elevation decrease
Contour Lines
The contour interval is usually printed on a
topographic map.
However, you can always calculate the interval
from the lines with values.
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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.
 Elevation (ft)
Gradient =
 Ground Distance (mi)
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Steep or flat?
Elevation change
0.5 mile distance Elevation change
2170-2150 = 20 feet
1380-780 = 600 feet
Gradient 20ft/0.5 mi = 40 ft/mi
Gradient 600ft/0.5 mi = 1200 ft/mi
Flat
Steep
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Contours and Profiles
http://reynolds.asu.edu/topo_gallery/images/hillmap.gif
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
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http://reynolds.asu.edu/topo_gallery/images/hillmap.gif
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.
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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
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Map Symbols
Marsh
Houses
Forest
Stream
Intermittent Stream
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Secondary Highway
Tracking Flowing Water
►How
does water always flow?
DOWNHILL
►How can we tell what direction
(N,S,NW,ESE) is downhill?
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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
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Tracking Flowing Water
You can look at the
contour lines as
they cross the
stream. They
will always point
upstream.
Downstream
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Upstream
Tracking Flowing Water
You can also
look at where
two streams
merge. The
merge will
form a V that
points
downstream.
Direction of flow
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Stream Gradients
You can determine a
stream gradient
along any section
of a stream just
like a terrain
gradient.
2000 ft
 elevation
distance along stream
100 ft
=
 250 ft/mi
0.4 mi
stream gradient =
2100 ft
0.4 miles
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Bathymetric Maps
Maps of water depths are like topographic maps
with contours. However, the contours are water
depths, not elevations.
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