Creating a vertical cross section from a topographic map
Topographic maps (often called “topo maps”) represent the three-dimensional landscape
on a two-dimensional piece of paper. Topography – the elevation of the earth’s surface – is
represented on a topo map by contour lines. Contour lines are lines of equal elevation. The
elevation change between each line (“contour interval”) varies from map to map, but each map
will have only one contour interval, which is listed on the map. Ordinary contour lines always
enclose higher ground; where a contour line encloses a depression, the line will have tick marks
on the inside. Figure 1 shows how a three-dimensional surface can be represented on a topo map
using contour lines.
For obvious reasons, topo maps, like other types of maps, show a scaled-down version of
the earth’s surface. Each map typically shows two types of scales on the map: a numerical scale
and a bar scale. Numerical scales are given as representative fractions, or ratios. For example,
on a 1:24,000 map one inch on the map represents 24,000 inches on the ground. Bar scales
measure distance by displaying a simple line or bar showing represented distance in feet, miles,
Figure 1. Topographic map and corresponding block diagram.
When looking at a topo map, you are seeing the landscape from directly above (often
referred to as “map view”). Sometimes it is useful to visualize how the landscape would look in
profile. Constructing a topographic cross-section gives us a side view of the landscape along a
line. You can think of a cross-section as the topography you would encounter as you walked a
straight line between two points. A profile is unexaggerated if the horizontal and vertical scales
are the same. To emphasize the differences in relief in a comparatively flat area, the vertical
scale may be expanded relative to the horizontal scale, producing a cross-section with vertical
exaggeration. Vertical exaggeration is given by:
E=H/V where E = vertical exaggeration
H = denominator of the horizontal scale fraction
V = denominator of the vertical scale fraction
One efficient way to construct topographic cross-sections is to outlined below:
1. Lay a strip of blank paper along the line you want to profile.
2. Mark on the edge of the paper endpoints of the profile line and the exact place that
each contour, stream, and hilltop crosses the profile line. Where contour lines are
closely spaced, it is sufficient to label just the index contours (every fifth contour,
drawn bold on topo maps). Double-check yourself every five lines or so. (Figure 9.2)
3. Label each mark with the contour elevation or other feature it represents.
4. Determine the total relief of the profile line. Relief is the difference in elevation
between the highest and lowest points.
5. Prepare a vertical scale on a sheet of graph paper or lined paper based on the relief.
Label the horizontal lines on this sheet to correspond to the elevation of each index
6. Write down the vertical exaggeration of your profile. Compare what an inch
represents on each of your axes using the equation given above. Be sure to
exaggerate only when needed for clarity.
7. Place the plain paper with labeled marks at the bottom (or top) of the profile paper
and carefully project each contour onto the horizontal line of the same elevation.
8. Connect all of the points with a smooth line. Mark streams, peaks, benchmarks
(points of exact elevation, as surveyed), etc.
9. Check that your profile matches the general topography along your transect line.
For the Torrey pines project, use a vertical/horizontal exaggeration of 2x.
After you make your vertical cross-section, put the geology from your map on the top surface of
the cross section. Then by using what you know about the lateral extent of the various
geological units, how they dip along the cross section or where they pinch out, sketch the
geology onto your cross section.
Figure 2. Drawing a profile. Notice the tick marks along some contours indicating a depression.
The vertical exaggeration is 25 times.