ESCI-215 Introduction to Earth and Physical Science
Assignment 2: Topographic Maps, Contours, and Making Cross-sections.
Background information
Topographic Maps
A map is a scaled down two-dimensional representation of some portion of the surface
of the Earth. A large map may represent an exceedingly small area on the Earth's
surface; such a map would show considerable detail. Conversely, a small map might
represent a large area on the surface of the Earth, showing only regional scale features.
It is important to note the scale of a map, as the scale indicates how much of the Earth's
surface is represented by the map.
The easiest way to define the scale of a map is by means of a line or a bar which
represents a given distance on the Earth. The bar scale is especially convenient
because it can be easily subdivided, and it will remain useful if the map is enlarged or
reduced. For example, if the map is reduced to one half its original size, the graphic or
bar scale will also be reduced by one half. Thus, the map and the scale will remain in
proportion to one another.
A fractional scale such as 1:24,000 indicates that one unit on the map (inches,
centimeters, feet) equals 24,000 of the same units on the surface of the Earth.
Fractional scales will become invalid after a map is photo-reduced or enlarged.
The contour or topographic map is one of the most useful types of maps in
environmental geology. These maps depict, by means of contour lines, the 3D
dimensional aspects of the Earth's surface. Hills, valleys, plains, and rugged mountains
can be discerned on a topographic map. Roads and important man-made structures
(houses, bridges, railroad tracks) are illustrated by symbols. A contour line connects
points of equal elevation. The elevations are measured above a base or datum plane
which is almost always sea level.
Carefully read the following statements that are pertinent to all contour maps and refer
to the examples given on the sample topo map, Figure 1.2.
——RULES OF CONTOURING——
A. The contour interval (CI), the difference in elevation between two succeeding
contour lines, is always given at the bottom of the map. In rugged mountainous
terrain the contour interval is typically large (maybe 50 feet) and in flat terrain it
is smaller (maybe 10 feet). Can also be expressed in meters.
B. Steep slopes are represented by closely spaced contours, gentle slopes by
widely spaced contours.
C. Contour lines never cross, but they may merge to indicate vertical or
overhanging cliffs.
D. Contour lines always close (the contour must meet itself). Frequently, contours
are truncated by the map boundaries, but if we had the adjacent maps, we
would see the contours are always continuous.
E. If a contour line is crossed ascending a hill, the same contour must eventually
be crossed descending the hill.
F. Hachured contour lines indicate local depressions. The hachure marks point
toward the center of the depression.
G. Contours "v" upstream. The contours must head upstream as they enter the
stream valleys to remain at a constant elevation.
Topographic Profile/Cross-Section
A topographic profile (also known as cross-section) illustrates the topography that is
intersected by a vertical plane. The profiles are useful because they are cross-sections
which clearly show the changes in the shape of the land along the line of the profile.
These profiles may be constructed using the information given on a topographic map.
To construct a topographic profile:
A. Determine the map features to be included in the profile and draw a line on the
topographic map that includes these features.
B. Lay a piece of graph paper along the line of the profile and record the positions
and elevations of contours that intersect the paper (Fig. 1.3).
C. Plot contours and sketch a smooth profile through the points (Fig. 1.3).
Topographic profiles usually are drawn with vertical exaggeration. To show topographic
details the vertical scale (Fig. 1.3) is usually expanded or exaggerated. The amount of
vertical exaggeration can be determined by comparing the vertical to the horizontal
scales (Fig. 1.3).
Figure 1.3 Topographic profile along A - A' in Fig. 1.2.
WARM-UP THOUGHT EXPERIMENT
CAREFULLY OBSERVE THE TOPOGRAPHIC MAPS (1 - 6) ON THE LEFT IN THE
FOLLOWING FIGURE AND TRY TO MATCH THEM TO THE CORRECT LANDFORM
ON THE RIGHT (A TO F).
If you are stuck here---let me know. If you are confident you have these all correct, you
are ready for this exercise!
Example!
The next page shows a topographic map with a river that is similar in some respects to your
assignment. It also shows what a completed profile (cross-section) from gamma to gammaprime looks like. Note, that the uppermost horizontal line of the profile is the place/line where
your topographic map data get annotated. (e.g., 240, 230, etc.).
From there you drop down vertical guidelines that should be at right angles to the top horizontal
line. In this way you keep the relative distances between elevation data points accurate. Then,
you place the data POINTS FIRST at the intersection points of the guideline and the proper
elevation above sea level (noted on the Y-axis). Finally, you smoothly connect the points into a
logical slope and note that the river channel dips below the two adjacent contour/elevation
points. But, the curve you draw for the river should not dip so low that it touches the next lower
contour (in this case 190).
PRACTICE MAP AND CROSS SECTION
C
A=230-240m
B=230-240m
C=210-220m
River flows North
A
B
FIRST TASK—Label the contours—you will need this information to answer the
questions!
SECOND TASK—On the worksheet, give the elevation at the following points (give
elevation as a range, 220-230m, for example). On a topo map you can only state that a
point on the map is between one value and the adjacent value (unless a landmark is
marked with a discrete number). The locations of points a, b, c, through l, are noted on
the map. (See Figs. 1.1 and 1.2 above for more on this).
THIRD TASK—Now that all the contour elevations are known, find out which direction
the river is flowing by identifying where the contours of the highest elevations are and
where the lower elevation-contours cross the river. Which way does a river flow? Up or
down?! Once you know, write in that direction (like flows to the north, flows to the
southeast, etc.)
FOURTH TASK—Construct a topographic profile (also known as (aka) a cross-section)
along the line A-A'.
FIFTH TASK—Construct a topographic profile along the line B-B'.
You will need to refer to Figs. 1.1, 1.2, and 1.3 above to understand how to correctly
make a profile or cross-section. Pay close attention to the way vertical guidelines
(dashed) are needed to make sure that your profile data is an accurate depiction of
what is shown on the map. We call this “ground truthing”. Once all your needed
elevation values are noted, you then draw a smooth profile through the points.
Remember your experience walking across fields and up and down hills?! Most breaks
in slope are rather gradual, which is why you want you profile to be a relatively smooth
line connecting relatively flat areas (plateaus, etc.) to more steeply dipping areas.
SIXTH TASK—Calculate the gradient or slope (rise over the run) or the amount of
elevation increase per unit horizontal distance, of the hill between points n and m on the
topographic map. Gradient equals difference in elevation between two points divided by
map distance between those two points. Important! You need to express your answer in
meters per kilometer (m/km). Please show your math work. Note: points n and m are in
the southwestern section of the map indicated by the “X” on each end of the blue line..
GOOD LUCK!