Understanding Topographic Maps
A topographic map, simply put, is a two-dimensional
representation of a portion of the three-dimensional
surface of the earth. Topography is the shape of the land
surface, and topographic maps exist to represent the land
surface. Topographic maps are tools used in geologic
studies because they show the configuration of the earth’s
surface. Cartographers solve the problem of representing
the three-dimensional land surface on a flat piece of paper
by using contour lines, thus horizontal distances and
vertical elevations can both be measured from a
topographic map.
General Information
The terms below indicate what information is contained
on a topographic map, and where it can be found.
Maps come in a variety of scales, covering
areas ranging from the entire earth to a city block (or
less).
Map Scale:
All maps have a
horizontal scale. Topographic maps also have a vertical
scale to allow the determination of a point in three dimensional space.
Vertical Scale (contour interval):
Contour lines are used to determine elevations and are lines on a map that
are produced from connecting points of equal elevation (elevation refers to height in feet,
or meters, above sea level).
Contour Lines:
The following are general characteristics of contour lines:
1. Contour lines do not cross each other, divide or split.
2. Closely spaced contour lines represent steep slopes, conversely, contour
lines that are spaced far apart represent gentle slopes.
3. Contour lines trend up valleys and form a "V" or a "U" where they cross
a stream.
On most topographic maps, index contour lines are generally darker and are marked with
their elevations. Lighter contour lines do not have elevations, but can be determined by
counting up or down from the nearest index contour line and multiplying by the contour
interval. The contour interval is stated on every topographic map and is usually located
below the scale.
Remember that topographic maps represent a view of the
landscape as seen from above. For producing a detailed study of a landform it is
necessary to construct a topographic profile or cross-section through a particular interval.
A topographic profile is a cross-sectional view along a line drawn through a portion of a
topographic map.
A profile may be constructed quickly and accurately across any straight line on a map by
following this procedure:
Creating topographic profiles:
a. Lay a strip of paper along a line across the area where the profile is to
be constructed.
b. Mark on the paper the exact place where each contour, stream and hill
top crosses the profile line.
c. Label each mark with the elevation of the contour it represents.
d. Prepare a vertical scale on profile paper by labeling the horizontal lines
corresponding to the elevation of each index contour line.
e. Place the paper with the labeled contour lines at the bottom of the
profile paper and project each contour to the horizontal line of the same
elevation.
f. Connect the points.
Stream Gradient: Stream gradient can also be determined from a topographic map. The
gradient of a steam or river is determined by measuring a section of a stream or river and
dividing the distance (in miles) into the vertical difference (in feet) between the two
points. The result is expressed in feet per mile (ft./mi.). The equation used is:
drop in elevation between two chosen points (feet)
Gradient =
distance between the two points (miles)
Tips for Interpreting Topographic Maps
Vertical exaggeration: Vertical exaggeration is the effect that is created when the
horizontal and vertical scales on your topographic profile are not the same.
Determining hillslope: Among other things, a topographic map can be used to measure the
average slope of a hill (or hills).
Topographic Map Example
As an example, look at a map of the Sulphur-Boundary Creek
area along the Middle Fork of the Salmon River This map is a
geologic map of glacial geology in the area, drawn on a
topographic map base. The map has a contour interval of forty
feet, which means that every place between the marked 6800
foot line and the next lowest line (which is 6760 feet, and not
marked) has an elevation equal or greater than 6760 feet, but
less than 6800 feet. You can figure out the elevation of any
point by finding the nearest labeled line, counting the number of
lines above or below it, multiplying by the contour interval, and
adding or subtracting the result from the nearest marked contour Click on image for a larger view.
line. The more closely spaced the contour lines, the steeper the
slope. You can find out exactly how steep the slope of the area you are interested in by
subtracting the lowest elevation from the highest, and dividing the result by the horizontal
distance. Horizontal distance is found on the scale. As you look at the map, notice that
the contour lines enclose smaller and smaller areas. The smallest circles represent the
tops of peaks, and some are marked with x’s with numbers next to them. The numbers are
the elevation at the top of the peak.
Follow a contour line along its length. Notice the indentations. As the contour lines cross
gullies or stream drainages, they "vee" uphill. Drainages that have water in them yearround have solid lines connecting the points of the vees. Drainages that have water only
part of the year are marked with dashed lines.
PROFILE CONSTRUCTION
A topographic map is a two-dimensional representation of our three-dimensional world.
On a map, the visual impact of elevation is sacrificed so that horizontal relationships can
be immediately seen. A topographic profile is a restatement of spatial relationships which
emphasizes the vertical axis and deemphasizes the horizontal axes. A topographic profile
has a vertical axis (elevation) and a horizontal axis (any line on the topographic map).
Profiles are used to emphasize slopes and changes in slope angles across a map. They are
usually aligned perpendicular to major structural/topographic trends. They may be
constructed with a horizontal scale in common with the base map, larger, or smaller, and
with no or significant vertical exaggeration.
Vertical Exaggeration. The vertical exaggeration is the ratio between the vertical scale
and the horizontal scale. If the two are the same the ratio is 1 and there is no vertical
exaggeration. If the horizontal scale is 1:24000 and the vertical scale is 1:2400, the
vertical exaggeration is:
(1/2400)/(1/24000) = 10
NOTE: Vertical exaggeration is NEVER used unless there is a definite need for it. If
there is no vertical exaggeration, slopes will be shown to scale and approximate slope
angles can be scaled from the profile. Use vertical exaggeration only if there are subtle
slope features which must be shown, and even then use the smallest exaggeration which
has the desired results. Large vertical exaggerations are the prime cause of difficulty in
the interpretation of profiles.
Common scale profiles. Topographic profiles generated at a common scale with the base
map are the most frequently used. They have the advantage of being able to be placed on
the map along the line of profile and compared to the base map. This is particularly
important in the preparation of geological cross-sections (below).
To generate a common-scale profile:
1. First draw a light, thin line on the base map
between the two end points of the profile.
2. Place a piece of precise graph paper (such as
20 x 20 to the inch) such that its edge lies
along the line, with one end of the line even
with a (heavy) starting line on the graph
paper.
3. Draw the vertical axis along that line,
perpendicular to the edge of the paper. The
units on the vertical axis should bracket the
highest and lowest elevations to be shown on
the profile, and should be at the same scale
as the horizontal scale, which is that of the
map. (This is the hardest part - if your map
scale is 1:24,000 your profile scale should be
the same. This translates to 1 inch = 2000
feet, so if you are working in English units
you can use 20 x 20 to the inch graph paper
and each division will represent 100 vertical
feet.)
4. Mark the starting elevation on the vertical
axis and proceed along the profile line,
marking an elevation whenever you cross a
significant contour line. If only a generalized
profile is required you can use only the index
contours - this results in a smoothing or
averaging of the topography. If a detailed
profile is required, use each contour line.
5. Connect the marks to yield a profile which is
at a common scale to the base map. Note that
interpretation is required here. At the
bottom of a narrow valley the profile might
make a sharp "V", whereas equally spaced
contours at the top of a hill might imply a
broad crest.