LAB 5: INTRODUCTION TO TOPOGRAPHIC (TOPO) MAPS
Introduction
A topographic (topo) map is simply a map
that depicts three-dimensional landform features
on a two-dimensional sheet of paper. Topo
maps use special lines called contour lines to
delineate points on the landscape having the
same elevation.
By studying the patterns
created by these lines, we can visualize how the
landscape looks in three dimensions. This kind
of visualization is an important component to
the success of many activities you are already
familiar with. A baseball fielder visualizes the
possible plays as each new batter steps up, just
as a carpenter visualizes what he or she wants a
structure to look like in a blueprint before any
nails are driven or lumber is purchased. Thus,
visualization results in a fuller understanding of
a situation, the circumvention of potential problems, sound decision-making, and successful,
efficient outcomes.
When geologists study a topo map, however, they see more than just the lay of the land in
their mind's eye. They also look for and identify patterns in the landscape. These patterns are
very significant because they yield important clues as to what processes are shaping the
landscape and how long these processes have been at work. While many of these patterns might
be apparent 'on the ground' when a geologist is in the field, the topo map is a valuable tool
because it allows the geologist to look for both large and small scale patterns over a large area
from a bird's eye perspective.
A standard set of map symbols is used to depict natural and cultural features on topo maps.
Other important information, such as the map's name, scale, and contour interval can be found on
the bottom margin of the map (see Figure 5.1).
Scale
Because a topo map is a two-dimensional scale model or representation of the landscape, dimensions of all features must be reduced proportionally. Common scale factors for topo maps are
1:24,000, 1:62,500, and 1:250,000. For example, on a 7-1/2 minute quadrangle topo map (scale
= 1:24,000), all features are reduced to one twenty-four thousandth of their actual size, or one
unit on the map (the numerator) equals a certain number of units in the field (the denominator).
A graphic (bar) scale, printed at the bottom of the map, will help you convert from map scale (in
inches) to actual scale (in miles and kilometers). The scales of topo maps can be described
relatively as "large-scale" or "small-scale." This simply refers to the relative numerical value of
the maps' fractional scales. For example, a 1:10 (1/10) map would be larger-scale than a 1:1000
(1/1000) map, and a particular feature on the 1:10 map would appear 100 times larger than it
would on the 1:1000 map.
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Figure 5.1. Information that appears in the margins of U.S.G.S. quadrangle maps.
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Questions - General
1.
Suppose you want to buy a topo map of your neighborhood in Tucson in order to determine
the elevation of your property. What scale would be more appropriate, 1:24,000 or
1:250,000? Why?
2.
If a pond is one inch across at its maximum width on a 1:24,000 scale map, how many feet
wide is it in actuality? How many miles? (5280 feet in a mile). Show your work.
3.
If the pond in question 2 were on a 1:250,000 map, how many feet wide is it? How many
miles? Show your work.
Latitude and Longitude
The Earth's surface is arbitrarily divided into a system of reference coordinates called latitude
and longitude. This coordinate system consists of imaginary lines on the Earth's surface called
parallels (latitude) and meridians (longitudes). Both of these are best described by assuming the
Earth to be represented by a globe with an axis of rotation passing through the north and south
poles.
Figure 5.2 The latitude/longitude grid system for the Earth,
showing a quadrangle.
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Meridians, or lines of longitude, are circles drawn on this globe that pass through the two poles.
Meridians are labeled according to their positions, in degrees, from the zero meridian, which by
international agreement passes through Greenwich near London, England. These longitudinal
lines, or meridians, are drawn every 10 degrees in an easterly direction from Greenwich (toward
Asia) and in a westerly direction from Greenwich (toward North America), creating a family of
great circles around the entire globe.
Another great circle passing around the Earth mid-point between the two poles is the equator. It
divides the Earth into the Northern and Southern Hemispheres. A family of lines drawn on the
globe parallel to the equator constitute the second set of reference lines needed to locate a point
on the earth accurately. These lines form circles that are called parallels of latitude, and are
labeled according to their distances in degrees north or south of the equator. The parallel that
lies halfway between the equator and the North Pole is latitude 45° North, and the North Pole
itself lies at latitude 90° North.
For increased accuracy in locating a point, degrees may be subdivided into 60 subdivisions
known as minutes indicated by the notation '. Minutes may be subdivided into 60 subdivisions
known as seconds indicated by the notation ". Thus a position description might read: longitude
64°32'32" East, latitude 44°16'18" South. Longitude is always denoted first; latitude second.
Public Land Survey
We can use the latitude longitude system as a base for other methods from which to pinpoint
smaller features or locations on a map. One of these systems is the United States Public Land
Survey System (USPLSS), used in legal transactions in the Unites States. Each state has one or
more east-west running baselines and north-south running principal meridians. Additional lines
spaced six miles apart run parallel to the baselines and principal meridians. The east-west strips
created by these lines are called townships, and the north-south strips are called ranges.
Townships and ranges are numbered (and named) according to their position relative to the
baseline and principal meridian. Every 6 mile by 6 mile township is subdivided into 36 squares,
each with an area of one square mile. These squares are called sections and are numbered
sequentially in a serpentine manner starting in the upper right corner. Locations within sections
can be pinpointed to whatever level of precision is needed by quartering the section, and then
quartering the quarters repeatedly. All of these basics of the PLS system are illustrated in Figure
5.2.
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Fig 5.3: Township and Range system of land
subdivision.
Universal Transverse Mercator system
The Universal Transverse Mercator (UTM) system is used by the military and scientists alike to
describe locations in a simple grid system. The world is split into over 30 “zones”. One corner
of the zone is deemed the (0,0) point and then distance is counted away from these points in
meters. Topographic maps have the number of the zone, the datum used, and any other notes
printed in the bottom left corner of the map. The UTM coordinates are found along all four
edges of the topographic map. In addition to being relatively easy to use, this system allows
Pythagorean mathematics to calculate distance between two points. See attached handout for
more information on the UTM system.
Magnetic Declination
Longitudinal lines always lie in a true north-south direction, and parallels of latitude always lie
in a true east-west direction. Magnetic north, however, is the direction towards which the northseeking end of a magnetic compass needle will point. Because the magnetic poles are not coincident with the north and south ends of the Earth's rotational axis, magnetic north is different from
true north except on the meridian that passes through the magnetic north pole. The angle
between true north and magnetic north is called the magnetic declination, and is normally shown
on the lower margin of most U.S.G.S. maps for the benefit of those who use a compass in the
field to plot data on a base map. The magnetic declination varies in a systematic manner for
each location on Earth. Therefore, it is crucial that the magnetic declination of the area you will
be mapping or orienteering is set on your compass.
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Questions - Location
Vail, AZ 7.5’ Quad
4. What is the fractional scale of this map?
5. What is the magnetic north declination for this map (degrees and direction)?
6. There is one area in the world in which true north is the same as magnetic north. Can you
think of where it might be?
7. Use the Public Land Survey system to describe the location of the north shore of Rancho Del
Lago (near the center of the map).
8. Use longitude and latitude to describe the same location.
9. Use the UTM system to describe the same location.
10. Determine the exact distance between the buildings at Colossal Cave State Park to the peak
of Pistol Hill.
11. What is the elevation at the top of Pistol Hill?
12. What is the gradient (slope) between the top of Pistol Hill and the base of the Radio Tower
on the southwest flank of the hill? Show your work.
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Contour Lines
On a topo map, the third dimension, the dimension of height (elevation) is shown through the use
of contour lines. A contour line, or contour, connects all points on the map having the same
elevation above sea level. The contour interval is the vertical difference in elevation between
adjacent contour lines and varies according to the scale, purpose, and relief of the map. Index
contours are thicker and darker than regular contour lines and are labeled with the corresponding
altitude. Relief is the difference in elevation between two map points.
Rules for Contour Lines (applicable in both reading and making topo maps)
1. A contour line must never divide or split.
2. A contour line must never simply end. Somewhere (usually off the map) the two ends of a
contour line must join to enclose an irregularly circular region.
3. A contour line must represent one and only one elevation.
4. A contour line may never intersect other contour lines. (Overhanging cliffs are a rare exception, in which case the hidden contours are dashed.)
5. Contour lines form a V pattern when crossing streams. The V always points upstream
(uphill).
6. Closely spaced contour lines indicate a steep slope; widely spaced lines indicate a gentle
slope.
7. Concentric circles of contour lines indicate a hilltop or mountain peak.
8. Concentric circles of hachured contour lines indicate a hollow or closed depression.
Figure 5.4. Construction of a contour line from points of known elevation.
Figure 5.4 above illustrates how a contour line is constructed from surveyed points of known
elevation. To add contour lines at 490' and 510', we would have to interpolate between the
known points as shown below.
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When ridge tops or valley bottoms are depicted on topographic maps, contour lines of the
highest or lowest elevation are repeated to show that the orientation of the slope has changed.
Figure 5.5. Repetition of contour lines when slope orientation is changed.
Likewise, contours are repeated to show the changes in slope orientation found in depressions.
Note, however, that in cases where a depression is found on a hillside, slope orientation changes
only on the downhill side of the depression or crater.
Figure 5.6. Repetition of contour lines when slope orientation is changed in depressions.
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Questions - Contours
Nankoweap, AZ 15’ Quad
13. What is the scale of this map? Are images smaller or larger compared to the previous map?
14. What is the contour interval on this map? Don’t forget units.
15. Near the UTM location 4021000N, 423000E which side of canyon is the steepest?
16. What direction do the streams in Saddle Canyon flow? Why is the water represented by dotdashed lines?
17. What is the gradient of the river between Nankoweap Rapids and President Harding rapids?
Measure your distance along the river, not straight from point to point.
18. Which way is the “Tilted Mesa” tilted? The location of the mesa is 4017500N, 419000E.
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19. Draw in all necessary contour lines on the map below. Use a contour interval of 10 feet.
20. Match the hill cross-sections on the right to their corresponding contours.
1 _____
2 _____
3 _____
4 _____
5 _____
6 _____
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Topographic Profiles
Topo maps depict the terrain of an area as viewed from above. It is possible to convert the aerial
view of a topo map into a view that shows the topography along a given line as viewed from
ground level by constructing a topographic profile. The profile is a cross sectional graph with
elevation plotted vertically and distance plotted horizontally, and it clearly outlines the relief and
slope along a given line on a topo map. Topo profiles may be readily constructed using the
following steps (from Pipkin and Cummings 1983):
1.
Choose the line of the profile on the map. Note the relief along the profile line so you can
estimate the values for the vertical axis.
2.
Lay a strip of paper along the line of the profile. Mark the ends of the profile line on the
paper using the designation of the profile line (e.g., A-A', NE-SW).
3.
Wherever a contour line intersects the strip of paper, mark a short dash at the edge of the
paper. Label the index contours. It is also a good idea to label the points where streams,
ridge crests, or other topographic features and certain man-made features such as roads and
railroads cross the profile.
4.
Select a vertical scale (other than the map scale if appropriate). Label the vertical scale such
that the highest and lowest elevations (relief) along the profile are shown. It is not
necessary to start from sea level.
5.
Plot the elevations noted on the bottom edge of the strip of paper at their appropriate height
according the scale on the vertical axis.
6.
Connect the points with a smooth curve rather than straight lines. Interpolate for hill tops
and valley floors.
7.
Label the streams, peaks, and cultural features.
8.
If you are exaggerating the vertical scale, calculate the vertical exaggeration (VE) with the
equation. Do not forget to convert feet to inches!
Representative fraction - vertical
VE = Representative fraction - horizontal
1"/12 000"
1"/24 000" = 2
Therefore
9.
VE = 2 x actual vertical relief
Identify the cross-section by writing on it the title, vertical scale or exaggeration, horizontal
scale, name of map, and your name.
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The basics of topographic profiling are illustrated in Figure 5.7. The contour interval on this
map is 40’, and Old Caves Crater in northeastern Arizona near Flagstaff has been translated into
two topographic profiles; one with vertical exaggeration (VE) of 2 and one with VE of 4. To
demonstrate the technique for preparing topographic profiles, several representative points are
plotted.
Figure 5.7. Example of topographic profiling.
Topographic Profile for the Lost Continent
After you review the introductory information on topo maps with your lab instructor, construct a
topographic profile for section A to A' on a topo map of the Lost Continent. Use the map on the
next page and draw a profile on the page following the map.
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Topographic Profile — Lost Continent
A
A'
Contour interval = _______ meters
Calculate vertical exaggeration.
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