Topographic Mapping
Basic Rules and Definitions
What is a Topographic Map?
1. A topographic map is a map that shows the elevation of an area by using special isolines called
contour lines. By using these lines and other symbols, a 2-dimensional aerial (top-view) picture
of a 3-dimensional area is constructed. The topographic map can be used to create a profile or
side view of the area.
2. Isolines are lines that are drawn on a map that connect all of the points on the map that have the
same value.
3. Contour lines are isolines that are drawn on an topographic map. These lines connect all points
with the same elevation, and create an aerial picture of the defined area.
Above is an example of a Topographic Map. Notice that isolines are drawn every 20 meters to
connect points with the same elevation. Notice that the contour lines drawn at 100, 200 and 300
meters are darker than the others. These are special contour lines called index contour lines.
What are the rules for using a topographic map?
1. Close lines mean steep slope.
The steep slopes on the map can be
found by looking for lines that are very
close together. Since each line represents
a change in elevation of a set amount (20
meters in this example), lines that are
close together indicate steep gradients.
The area circled in red is very steep,
because it has many lines crowded close
together. The closer the lines are, the
steeper the slope.
2. Contour lines bend upstream at the points where they cross a river.
Since contour lines must only connect
points at the same elevation, they must
bend when they cross riverbeds. A
riverbed drops below the level of its
bank, so the contour line must bend
uphill to stay at the same elevation. In
other words, the lines bend in the
opposite direction as the flow of the
river. The highlighted lines show that
the direction of the Mill River must be
northeast.
3. Closed, unmarked circles inside each other indicate a hill.
Small hills can be located on a
topographic map by looking for areas
where the contour lines connect
together and form circles. As the
circles get smaller, the elevation is
getting higher. Hill contour lines are
smooth and not marked with tiny lines
(see next rule).
4. Depressions are shown by small marks on a closed contour line.
The arrow is pointing to a depression,
or hole. The contour line with the
marks, or hatchers, has the same
elevation as the line before it. In this
case the hatchered line has a value of
140 meters, and the depression must be
less than 140 meters.
What is a profile?
A profile is a picture of what the landscape would look like from the side. It is a cross section
of the landscape. You should be able to identify a correct profile, as well as draw one.
Identify a Profile
Given the topographic map above, which of the four choices below accurately depicts the correct profile
between points X and Y?
The best way to approach this is by the process of
elimination. First of all, point X appears to be
approximately 290 meters in elevation. Let's
eliminate all the choices that don't begin at 290
meters:
That eliminates choices 1 & 3 right off the bat!
Next, we look at point Y, which appears to be
approximately 130 meters.
That eliminates choice 4. That means choice 2
must be correct!
Draw a Profile
Given the topographic map at left, let’s
draw a profile map from point P to point Q.
First, we need a horizontal grid that
represents the distance from P-Q
Next, transfer the points from the
contour map to the grid.
Then connect the points with a
smooth line to draw the profile.
And there! We have a cross section of the
landscape.