Topographic maps are useful because they represent the three-dimensional configuration of the land by using lines and symbols.
Topographic maps show the location and shape of mountains, valleys, plains, streams, wooded areas, roads, buildings, and many other features.
Topographic maps are useful tools for planning highways, recreation areas, airports, housing developments, industrial sites, selecting pipeline and powerline routes, for agricultural research, property surveys, and natural resources management. For you, topographic maps may be useful for recreational activities such as hiking, boating, camping, fishing or hunting. Many of you have probably already used topographic maps.
Most topographic maps of the U.S. are printed and distributed by the U.S. Geological Survey. Topographic maps of Georgia may be purchased from the Georgia Geologic Survey in Atlanta, or from various map companies and hiking, camping, or nature stores.
A contour line is an imaginary line on the surface of the Earth connecting points of equal elevation.
Contour lines would merge to form a single line only at a vertical cliff (very rare). In the extremely rare case of an overhang, contour lines would cross, but the hidden contours are dashed.
Check the topographic map symbol sheet for more information (in your lab manual, in the appendix to your textbook, and on the web). You will find a list of many common map symbols for things such as boundaries, roads, buildings, railroads, types of vegetation, marshes, quarries and mines, water and coastal features, etc.
Latitude and Longitude
The edges of many topographic maps are bounded by lines of latitude and longitude. The large maps available from the U. S Geological Survey or the Georgia Geologic Survey in Atlanta are called topographic quadrangle maps.
The size of the quadrangle is given in degrees, minutes, and seconds.
Look for the latitude and longitude, given in degrees, minutes, and seconds, at the corners of the map.
The lines that run east and west, bounding the top and bottom of the map are latitude lines.
Look at the left and right top corners of the map to see the latitude of the line that forms the top (northern) edge of the map.
The lines that run north and south, bounding the left and right sides of the map are longitude lines. Look at the top and bottom corners on the left to see the longitude of the line that forms the left (western) edge of the map.
Intermediate latitude and longitude lines (for various seconds or minutes) are found along the edges of the map. Please note that the degrees may have been left off (as an abbreviation), and you may see only minute and second designations. The degrees are listed only at the corners on most maps.
Also note that there are other numbered lines on the map and tick marks along the map edges. Some of these are other systems of measurement, such as Universal Transverse Mercator (UTM) lines. If you look carefully, these are NOT parallel to the edges of the map or the latitude and longitude lines. If you are looking for latitude and longitude markings, you will need to ignore these other lines and tick marks. Look for the tick marks labelled with minutes (') and seconds (").
Maps covering 7.5 minutes (7.5' or 7'30") of latitude and longitude, and maps covering 15 minutes (15') of latitude and longitude are common. Maps covering a large region are typically 1o x 2o quadrangles, or 30' x 60' quadrangles.
Topographic maps at the equator that cover 7.5' x 7.5' are basically square. The 7.5' x 7.5' maps become narrower as you approach the poles because the lines of longitude converge poleward.
For Georgia, the 7.5' x 7.5' maps are about 2 feet wide (east to west) and 3 feet tall (north to south).
The topographic maps in your lab manual are, for the most part, small portions of larger topographic maps, and may or may not include informaiton on latitude and longitude (although they should).
For latitude or longitude:|
1o = 60 minutes (60')
1 minute (1') = 60 seconds (60") .
For more information on latitude and longitude, see your lab manual and the appendix in your textbook.
We often want to know:
"How many feet (or miles) are equal to one inch on the map?"
Example: Ratio scale 1:24,000
This says 1 unit on the map = 24,000 units on the ground.
If we want to know how many miles on the ground would be equal to one inch on the map, we set the problem up like this:
1 inch = 24,000 inches x (1 foot/12 inches)
In essence, we are multiplying one side of the equation by 1, because 1 foot = 12 inches.
1 inch = 2000 feet
1 inch = 2000 feet x (1 mile/5280 feet)
You divide 2000 feet by 5280 feet, then cancel out "feet".
1 inch = 0.379 miles
This is the answer you were looking for. On a map with scale 1:24,000, one inch = 2000 feet, and one inch = 0.379 miles.
The compass angle between true north and magentic north is called declination, and varied depending on where you are on the globe.
In the Atlanta area, the difference is less than 2o.
In New York, the compass needle points about 10o west of true geographic north.
In California, the compass needle points about 20o east of true north.
Georgia Perimeter College, Clarkston Campus, is located on the Stone Mountain, GA 7.5' topographic quadrangle.
Atlanta is covered on four maps, Northeast Atlanta, Northwest Atlanta, Southeast Atlanta, and Southwest Atlanta.
The names of the surrounding quadrangles are present in small letters along the edges at at the corners of the map.
Be sure that you are very careful and very precise when drawing topgraphic profiles. Make sure that your dots show. Make sure that you know the elevation of every contour line and every dot.
Also make sure that the spacing of your dots is exactly the same as the spacing of the contour lines in the particular area. If your dots are all spaced equidistant, then you have done something wrong.
Connect your dots carefully. Make the hilltops and valley bottoms slightly rounded (but do not cross any extra lines).
If you have three points in a line, with the same elevation, the land is NOT flat there. It goes slightly up between one pair of dots, and slightly down between the next pair of dots.
This is an exercise in being very careful and very precise. Do not let your paper slip.
Your topographic profile will not be vertically exaggerated ONLY when the vertical scale you use on your graph paper is the same as the horizontal scale on the map.
The amount of vertical exaggeration can be calculated using the following formula:
|Vertical exaggeration = Vertical ratio scale/Map ratio scale|
So for the profile on the left, above (where it was originally drawn on graph paper with a distance of one inch between 0 and 1000 feet, across the bottom):
First convert 1 inch = 1000 feet to a ratio scale.
That gives a ratio scale of 1:12,000
Then covert the vertical scale of the graph paper (1 inch = 200 feet) into a ratio scale.
The ratio scale of the vertical scale is 1:2400.
V.E. = 1:2400/1:12,000 = 12000/2400 = 5
So the vertical exaggeration is 5 times.
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Copyright 2000 Pamela J.W. Gore
Page created by Pamela J. W. Gore
Georgia Perimeter College,
May 28, 2000