Topographic Maps
A topographic map is a map that shows elevation and features
found on the earth's surface. Like any map it uses symbols to
represent these features.
• This section of a
topographic map
illustrates many of
the common
symbols used on
topo maps.
• Some of the more
common and important
topographic map
symbols have been
pointed out by the
purple arrows.
•Contour lines are lines drawn on a map connecting points of equal elevation. If you
walk along a contour line you neither gain nor lose elevation.
Picture walking along a beach exactly where the water meets the land (ignoring tides and waves for
this example). The water surface marks an elevation we call sea level, or zero. As you walk along
the shore your elevation will remain the same, you will be following a contour line. If you stray from
the shoreline and start walking into the ocean, the elevation of the ground (in this case the seafloor)
is below sea level. If you stray the other direction and walk up the beach your elevation will be
above sea level.
• Fortunately we do not really have to flood the world to make contour
lines. Unlike shorelines, contour lines are imaginary. They just exist on
maps. If we take each of the shorelines from the maps above and draw
them on the same map we will get a topographic map (see map below).
Taken all together the contour lines supply us with much information on the
topography of the island. From the map (and the profile) we can see that
this island has two "high" points. The highest point is above 30 ft elevation
(inside the 30 ft contour line). The second high point is above 20 ft in
elevation, but does not reach 30 ft. These high points are at the ends of a
ridge that runs the length of the island where elevations are above 10
ft. Lower elevations, between the 10 ft contour and sea level surround this
ridge.
• Point A = 0 ft: Point B = 10 ft: Point C ~ 15 ft:
Point D ~ 25 ft: Point E ~ 8 ft.
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Contour lines can be drawn for any elevation, but to simplify things only lines for
certain elevations are drawn on a topographic map. These elevations a chosen to be
evenly spaced vertically.
This vertical spacing is referred to as the contour interval.
For example the map above used a 10 ft contour interval. Each of the contour lines
was a multiple of 10 ft.( i.e. 0, 10, 20, 30).
The contour interval chosen for a map depends on the topography in the mapped
area.
In areas with high relief the contour interval is usually larger to prevent the map from
having too many contour lines, which would makes the map difficult to read. The
contour interval is constant for each map. It will be noted on the margin of the map.
You can also determine the contour interval by looking at how many contour lines are
between labeled contours.
•The example above is a section of a topographic map. The
brown lines are the contour lines. The thin lines are the normal
contours, the thick brown lines are the index contours. Notice
that elevations are only marked on the thick lines.
• Because we only have a piece of the topographic map we can not look at
the margin to find the contour interval. But since we know the elevation of
the two index contours we can calculate the interval ourselves.
• The difference in elevation between the two index contours (800 - 700) is
100.
• We cross five lines as we go from the 700 line to the 800 line. Therefore is
we divide the elevation difference (100) by the number of lines (5) we will get
the contour interval. In this case it is 20.
• We can check ourselves by counting up by 20 for each contour from the
700 line. We should reach 800 when we cross the 800 line.
Once we know
how to determine
the elevation of the
unmarked contour
lines we should be
able determine or
at least estimate
the elevation of
any point on the
map.
• Using the map above
estimate the elevation of
the points marked with
letters
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•
•
Point A = 700
Point B = 740
Point C ~ 770
Point D = 820
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A. 4400 ft
B. 4720 ft
C. 4236 ft
D. 4360 ft.
E 3800 ft
F. ~4780 ft.
G. 4080 ft
H. ~4100 ft
I ~3980 ft
• J ~ 3820 ft
A – 5760, B-5800, C-6120, D-5540, E – 5880, F – 5770,
G - 5590, H – 5990, I – 5710, J - 5850
A-5600, B-5520, C-5940, D-5780, E-6050, F-6330, G5870, H-5830, I-6030, J-5830
Map Scale
• The simplest form of map scale is a VERBAL
SCALE.
– A verbal scale states what distance on a map is equal to
what distance on the ground, (ex.1 inch = 10 miles).
– Though verbal scales are easy to understand, you usually
will not find them printed on topographic maps.
• Fractional scales are written as fractions (1/62500) or
as ratios (1:62500).
– Unlike verbal scales, fractional scales do not have units.
– Instead it is up to the map reader to provide his/her own units.
– Allowing the reader of the map to choose his/her own units
provides more flexibility but it also requires a little more work.
– Basically the fractional scale needs to turned in to a verbal scale
to make it useful.
Verbal Scale
• 1 inch on the map = 62500 inches on the
ground.
• 1 foot on the map = 62500 feet on the
ground
• 1 cm on the map = 62500 cm on the
ground
• 1 m on the map = 62500 m on the ground
Fractional Scale
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•
1:24000
1 in = 24000 inches = 2000 ft
1:62500
1 in = 62500 inches ~ 1 mi
1:100000
1 cm = 100000cm = 1 km
1:125000
1 in = 125000 inches = 10417
feet = ~2 miles
• 1:250000
1 in = 250000 inches = 20833
feet = ~4 miles
Bar Scale
Streams and Stream Valleys
Look at the
map at
left of the
Spruce
Knob
area. Wh
ere are
the
stream
valleys?
Streams and Stream Valleys
The
Rules
of V’s