Topographic Maps—Chapter 8
Introduction—Maps
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2D representation of part of Earth’s surface that illustrate the shape, size, and
relative position of natural and cultural features.
Different maps are designed for different purposes, i.e. Highway maps emphasize
highway routes, have less detail on natural features.
Topographic maps: emphasize natural features such as streams, rivers; depict the
size, shape, elevation above mean sea level for any point; show cultural features
like highways, railroads, cities, buildings.
Topographic maps oriented such that the top is approximately parallel to latitude
and the bottom parallel to longitude. Cover 7.5’ or 7.5 minutes (latitude and
longitude are divided into degrees, minutes, seconds) of latitude and longitude, so
it’s called a 7.5’ quadrangle.
Uses of topographic maps:
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For geology: configuration of various landforms provides clues about
erosional/depositional processes that were important in their formation; provide
clues about the kind of rock and geologic structures that exist in subsurface
For hydrogeology: to study streams, determine gradients
For engineering: planning the least costly route for highway construction
(absence of high hills, steep slopes)
For recreation: hunters, fishermen, hikers/backpackers, canoeists, kayakers, rock
climbers all use topographic maps to become familiar with areas they frequent.
Sources of topographic maps:
Paper—USGS 7.5’ quads. Available from USGS. Mullins Library 4th floor
Digital—CAST (Arkansas data on GeoStor 4.0)—1:24,000 DRGs in TIF format can be
printed at any GACL computer lab
Topozone
Terraserver
National Geographic
Various softwares (ex. DeLorme, Maptech)
Scale:
 see Chapter 7 page 104 in lab manual
 7.5’ topo quads=1:24,000. This means that one inch on the map=24,000 inches
on the ground. This is the RF, or representative fraction. To figure the distance
in miles that map distance in inches equals,use the following equation:
(24,000 X (# of inches distance on map)) / (63,360 inches per mile) = # of miles
on map; for a 1:24,000 topo map, 1”=0.379 miles
 Measure map distance with a ruler; use a piece of string and ruler for curved
distances (i.e. streams, curved roads)
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For stream gradient: measure elevation at starting point and ending point (i.e.,
put-in and take-out for you boaters), subtract the ending point elevation from the
starting point; divide this difference by the number of miles between the starting
and ending point (measured using a piece of string and a ruler).
Symbols:
Your lab manual between pages 118 and 119 contains a guide to map information and
symbols published by the USGS in 1978 (a long time ago, but still applicable). This is
the best resource for you if you have questions about map symbols.
Location of points on maps:
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Refer to Figure 8-1, Page 117 in your lab manual
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Location important for defining property lines for legal reasons. Various grid
systems used…the one you need to be familiar with is called the Public Land
Survey System, inaugurated by Congress in 1784 and still in use today by all but
the original 13 states and a few southwestern states.
1) In this system, land is divided by north-south lines drawn parallel to a principal
meridian (line of longitude) and east-west lines drawn parallel to a line of latitude.
2) These intersecting lines form rectangles six miles on each side. Looking at the grid in
map view, a row of rectangles is called a township, and a column called a range.
3) Each township and range is numbered with respect to a nearby line of latitude or
longitude, and each cell in the grid identified by its intersecting township and range, i.e.
T2N, R3W.
4) Each of these 6 mile X 6 mile rectangles is further divided into 36 1-square-mile
rectangles, or sections, which are numbered. Each 1-sq-mi rectangle contains 640 acres
5) Each section is then divided into quarter-sections 160-acres each and these quarter
sections are quartered again to create 40-acre tracts of land. The methods of numbering
of sections and the quartering of sections are best explained by Figure 8-1 on page 117 in
your lab manual.
Elevations and Contour Lines:
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Elevation is vertical distance above an established datum—this is mean sea level
When areas surveyed, permanent points called benchmarks are established and
elevation precisely determined; other points not having permanent fixtures but
with a precisely figured elevation are called spot elevations.
Relief is elevation difference in a particular region found by determining the
highest and lowest elevations and finding the difference. Ex.—relief of Buffalo
River gorge, Grand Canyon, etc.
Contour line = imaginary line on ground, points on the line are all the same
elevation above sea level; not drawn for all elevations (map would get too
cluttered!)
Contour interval = vertical distance between adjacent contour lines; for most
USGS topographic maps, this will be 20, 40, 100, or 200 feet. It’s indicated at the
bottom center of the map, above the scale bar.
Every 5th line printed heavier than the others, and is labeled with its elevation.
These are called index contours
Steep slopes = closely spaced lines
Gentle slopes = widely spaced lines
Flat areas = no lines
Overhangs = lines of lesser elevations are seen to disappear beneath lines of
greater elevation