A very useful exercise for understanding what topographic maps represent is the construction of a topographic profile. A topographic profile is a cross-sectional view along a line drawn through a portion of a topographic map. In other words, if you could slice through a portion of the earth, pull away one half, and look at it from the side, the surface would be a topographic profile. Not only does constructing a topographic profile aid in understanding topographic maps, it is very useful for geologists when analyzing numerous problems.
To construct a topographic profile, you
must first decide on a line that is of interest to you. This could be an
area where you want to go for a hike and want to know how steep to expect it to
be, a line that shows the maximum relief (relief is the difference in elevation
between the highest and lowest points) in the map area, or any other area in
which you are interested. Once you have determined where you want to draw
your profile, use the following guidelines to construct your profile.
1) Pencil the line of your interest in lightly
on your map (or you can put mylar over the map and draw on it if you don't wish
to mark your map). If you use mylar, it may be a good
idea to mark the corners of the map on the mylar so you can reorient the mylar
on the map later if necessary.
2) Place a blank piece of paper
along the line you have drawn. You may want to tape the paper to the map
using drafting tape to keep them from moving relative to one another (don’t use
any other kind of tape unless you don’t mind taking some of the map off with the
tape later).
3) On both the blank paper and the
map (or mylar), mark clearly the starting and ending points of your line of
section. Below these marks, write down the elevation of the starting and
ending points of your section.
4) Make a tic mark wherever the paper crosses a contour line on the map, making larger tics for the index contours and smaller tics for the intermediate contours. Write the elevation of the index contours below their tics on your paper…you might want to start off writing the elevation of the intermediate contours as well to avoid confusion, but it will soon become tedious. Make a note of the highest and lowest points on the profile for use later. Be sure to keep track of the number of intermediate contours between the major contours; if there are more than four intermediate contours it means that there has been a change in slope and you need to check to see if you crossed a hill or a valley.
5) Once you are certain you have
all of the appropriate tic marks and elevations, remove your paper from the
map. Get a piece of graph paper that is at least as long as your line of
section (you can piece them together if you have to, but make sure all the grids
line up). If you are using a map with a scale of 1:24,000 you will want to
use graph paper that has one inch grids to make your life much easier (because
at a scale of 1:24,000, one inch on the paper is equal to 2000 feet).
Place your paper with the tic marks on the graph paper (once again, you may want
to tape it down) and mark the starting and ending points of your line of section
on the graph paper.
6) Draw vertical lines above your
starting and ending points, these will be the boundaries of your profile.
Use the maximum and minimum elevations along your line of section to determine
how long to draw these lines. For example, if your minimum elevation is
4320 ft and your maximum elevation is 6280 ft, you will want your vertical line
to be at least two inches long. Remember that one inch equals 2000 feet on
a 1:24,000 scale map. The difference between 6280 feet and 4320 feet is
less than 200 feet, so it would be possible to draw your profile in just one
inch. However, it is much easier to construct a profile if your lowest
elevation is a multiple of 2000, so you would want to start at 4000 feet and go
to 8000 feet (two inches).
7) Beginning with your starting elevation, go directly above the tic mark on your paper and make a small dot on the graph paper at the corresponding elevation (if your graph paper has one inch squares divided into tenths, each smaller square will represent 200 feet of elevation change; each index contour should lie along a horizontal grid line). Make a small dot for each tic mark on your paper.
8) Connect the dots on
the graph paper, and you have a topographic profile.
