TOPOGRAPHIC MAPS
Introduction – Maps are used to show the spatial distribution of features in a region. The features
can range from political boundaries, roads, or cities to population density, annual
precipitation, or mineral resources. One of the most useful types of map is a topographic
map which is used to show the shape of the land surface. Topographic maps will be used in
many of the labs in this course and the ability to read them is an essential part of this
course. This skill will also prove valuable if you ever plan to head off into the woods,
mountains, or desert.
Purpose – This exercise is intended to illustrate the types of information presented on maps and
to develop your skill in constructing and interpreting topographic maps.
Method – You will first be introduced to the basic information included on maps. By
constructing topographic maps and profiles, you will learn how to quickly and accurately
interpret a topographic map in terms of the shape of the land surface.
Introduction – A map is a representation of a portion of the Earth's surface drawn to some scale,
usually on a flat two-dimensional surface. In special cases maps are produced on a spherical
surface, such as a globe of the Earth, or landscape features are modeled in three dimensions on
raised relief maps. Maps ordinarily represent land areas whereas representations of portions of
the oceans or large lakes are called charts.
Maps are included in some of the earliest historical records from the Middle East, and even
people without written language have used a type of map, such as those used by early
Polynesians to show the location of islands and ocean currents in the southwest Pacific.
Some of you may know that Christopher Columbus was a map-maker as well as a
navigator, and that it was maps made by him and other early explorers that enabled European
people to settle the New World. Over the half-millennium since Columbus' time, maps have
become diverse and highly specialized. Most students are more or less familiar with road maps,
but few students have seen a topographic map, a structure map, or a gravity map.
Maps are created for specific purposes. A geologist, for instance, is unlikely to consult a
road map in his search for an oil reservoir, but will commonly consult several other maps before
advising his company where to locate exploration wells that cost from thousands to millions of
dollars to drill. Typical maps a petroleum geologist might consult include a gravity map, a
magnetic map, a map of known oil reserves, and a variety of special geologic maps showing the
distribution of rock units and their structural arrangement.
What's on a map? – For maximum usefulness, a map must include certain essential types of
information. These include the following.
1.
Location – indicates what particular portion of the Earth's surface is portrayed on the map.
It is commonly included in the title. In addition, many maps include a coordinate system
for locating places plotted on the map.
Topographic Maps
2.
Direction – indicates the orientation of the map. It is commonly shown by a north arrow.
By convention, north is toward the top of the map unless a different orientation is indicated.
3.
Scale – indicates how much of the Earth's surface is portrayed on the map. Scale can be
presented in one of three ways.
0
a.
b.
c.
1
2
3
km
graphic scale
verbal scale
1 cm = 10 km
ratio scale 1:24,000 - one unit on the map = 24,000 units on the earth's surface
(e.g., one cm on the map = 24,000 cm, or 24 m, on the earth’s surface)
4.
Date – indicates when the map was made and thus how current the information is. Features
shown on maps can change through time.
5.
Legend – explains the symbols on the map.
6.
Source – indicates what individual, group, or agency produced the map.
Topographic maps show the three-dimensional geometry of the Earth’s surface in twodimensions (a map). Topographic features, such as hills and valleys, are shown through the use
of contour lines, which are lines of equal elevation. With a little practice, you will be able to
interpret the pattern of contour lines in terms of topography and will be able to use this readily
available and useful type of map. Knowing how to read a topo map may come in handy for
hiking, deciding whether to buy a piece of property, or assessing the flood potential of an area.
To build your skill in interpreting topographic maps, we will start by looking at a series of
related maps to show you how the shape of the earth’s surface can be represented with contour
lines. You will learn how to draw contour lines yourself and construct a topographic profile.
Finally, you will look at a topographic map and test your skill at visualizing the topography.
Terminology
Topography – shape of the land surface
Elevation – distance above or below sea level which has an elevation of 0 feet or meters
Contour line – line of equal elevation
Relief – difference between the highest and lowest elevations in an area
Contour interval – difference in elevation between adjacent contour lines
Slope – change in elevation over a certain distance
Topographic profile – a cross section of the topography along a specified line
2
Topographic Maps
Contour lines and topography
As mentioned earlier, a contour line is a line of equal elevation. It can be visualized by
imagining a series of horizontal planes intersecting with the land surface. Where each plane
intersects the land surface would be a contour line corresponding to the elevation of that plane
(Fig. 1). If you were to walk along a contour line, you would neither go up or downhill. Higher
elevations would always be on one side and lower on the other.
Topographic
map
200
Ocean
100
0
200
100
Imaginary
horizontal
surfaces
Figure 1. Creation of contour lines by intersecting different imaginary horizontal surfaces with
the topography. The intersection is a contour line and is projected onto the topographic map
above.
By looking at the shape, spacing, and location of contour lines, you can accurately interpret
the shape of the land surface.
Qualitative interpretation of topographic maps based on geometry of contour lines – The most
important skill in working with topographic maps is to be able to visualize the shape of the land
surface by looking at the pattern of contour lines. With just a few simple rules, you can make an
accurate description of the topography. The table below illustrates the important rules for
interpreting contour lines.
Topographic Maps
120
140
Hill – closed contours with highest elevation in center
140
Depression – closed contours with lowest elevation in
center. Will have hachures (short lines) on
some contour lines
100
Ridge – V-shaped contours with higher elevations in
middle of V
100
80
Valley – V-shaped contours with lower elevations in
middle of V. Contour lines V upstream
80
120
140
Uniform slope – parallel contours. The more closely
spaced the contours, the steeper the slope
120
Contouring a map from spot elevations – Another valuable way to gain an understanding of
topographic maps is to draw your own contour lines from a series of spot elevations. These
elevations may have come from a surveyor or from an automated process using aerial
photographs. The process of contouring is used in a wide range of applications, not just creating
a topographic map. For example, you might want to contour a map of population density to
show where the population centers are located.
1.
2.
3.
4.
Start by locating the greatest elevation(s) and marking with an X.
Find an even number elevation corresponding to one of the contour lines near your X and
connect it with other locations with the same elevation.
When drawing a contour line between two elevations, estimate its position between the two
points by using proportions. For example, a 580 contour line would be closer to an
elevation of 584 than 568.
Continue with remaining contour lines making sure that the contour lines follow the rules
below
Topographic Maps




Rules for contour lines
Contour lines never cross.
Contour lines never merge except for a vertical cliff.
Contour lines V when crossing a stream. The direction of the V points upstream.
Contour lines never end: they form closed loops (they may not close on your map, but will
on adjacent maps).
Try contouring the simple map below. Start by drawing the 500 m contour, then draw a contour
line every 50 m. How would you describe the topography?
400
500
450
500
500
525
575
450
610
550
500
500
400
575
550
475
500
400
475
500
425
425
Topographic profiles – A topographic profile is a cross section of the topography (Fig. 3). They
are useful for showing the nature of the terrain along a straight or curved line. For example,
constructing a profile along the route of your planned hike will show you what sort of relief you
will face on your hike. A profile along a river shows where you may encounter rapids.
Topographic Maps
100
120
140
Profile
Line
140
120
100
160
140
120
100
Figure 3. Construction of a topographic profile.
Constructing a profile is simple and requires only a pencil and ruler.
1.
Draw the profile line on the contour map.
2.
Lay a piece of paper along the line and mark where each contour line crosses the profile
line and indicate the value of the contour line.
3.
Transfer these markings to a line drawn on another piece of paper.
4.
Measure up a scaled distance corresponding to the elevation of each marked point and put a
point.
5.
Connect all your points.
600
v – above 620 m, less than 640 m elevation
620
640
w –w 640 m elevation
y
z
v
x
x – above 640 m, less than 660 m elevation
640
y – back down, on slope within depression to
640 m elevation
y
z
zx – wbelow 640m,
640 more than 620 m elevation
v
620
600
Lab Exercises
1. Contour the map below and construct a topographic profile along A-A’. The numbers are the
elevation above sea level of the locations shown by dots. The dashed lines represent streams.
Using a contour interval of 100 feet, draw contour lines connecting all points of equal
elevation above sea level in even 100's of feet. Use a pencil only. Sketch lightly so you can
make adjustments easily. When sketching is complete, darken the contour lines and label
each line with numerals to indicate its elevation.
Topographic Maps
270
60
100
350
517
450
450
505
520
395
310
602
515
500
A
A'
30
200
500
200
420
520
450
603
510
150
525
300
350
20
210
480
390
490
125
200
OCEAN
400
250
325
315
150
150
75
250
150
100
50
175
120
0
1000'
2000'
250
500
3000'
Contour interval = 100 feet
Construct a topographic profile along line A-A' using the graph below.
A
A'
600'
400'
200'
2.
0'
Interpret the topographic map of Paw Paw, MD/WV .
a) What is the contour interval on this map? ___________ feet
b) What is the highest elevation along Purslane Mountain? __________ feet
c) What is the relief on Purslane Mountain? __________ feet
d) What is the lowest elevation on the map? __________ feet
Topographic Maps
e) Which direction does the Potomac River flow? __________
f) What is the length of the longest straight stretch of the Potomac River? ___________ feet
g) Estimate the gradient (slope) of the Potomac River throughout the area. __________
feet/mile
h) Which way is Town Hill elongated? __________
i) Estimate what proportion of the map is in Maryland and West Virginia
j) __________ Maryland
___________ West Virginia
k) How would you describe the topography in the map area?
Sketch (i.e., general shape to approximate scale) a topographic profile from the southeast corner
to the midpoint on the western map boundary. Label the ends of your profile with compass
directions and mark major topographic features (e.g., Town Hill, Potomac River, etc.).
Pre-Lab Questions
1.
In the most general sense, what does a map show?
2.
What does a topographic map show?
3.
What is a topographic contour line?
Topographic Maps
4.
A topographic map has contour lines that are very closely spaced. What does that tell you
about the topography?
5.
Sketch a contour line crossing the river below. The direction of flow is indicated by the
arrow.