Topographic Maps


Topographic maps are used by many different people. Geographers, geologists, archaeologists, biologists, historians, ecologists and other researchers use topographic maps routinely. Other users include the military, fish and game management workers, utility companies, real estate developers and hikers. The maps most commonly used for all these purposes are the United States Geological Survey (U.S.G.S.) 7.5 minute series topographic maps. In this lab you will begin to learn how to read topographic maps and how to use them to solve certain spatial problems.

Latitude and Longitude

You have learned how latitude and longitude are measured. The expression of these angles on the surface of the earth form parallels (lines of equal latitude, running east - west) and meridians (lines of equal longitude, running north - south) Together, parallels and meridians form a locational grid over the surface of the earth. The top and the bottom of your Cumberland map are both parallels of latitude. The sides of your map are both meridians of longitude. Parallels and meridians meet at right angles. Remember that the meridians converge at the poles. Thus, while the corners of the map should be right angles the meridians are converging, so that the length of the parallel on the north side of the map is a little shorter than that on the south side. This is why these are called quadrangles rather than rectangles.

Look at the upper right corner of your map. You will see two numbers: 78° 45' and 39° 45'. How do you know which is the latitude and which is the longitude? Look at the upper left corner of the map. The two numbers there are 78° 52' 30" and 39° 45'. Compare these numbers to those in the upper right corner. Since the top of your map is a parallel of latitude, the latitude angles will be the same in both upper corners. Which two numbers are the same, and so what is the latitude of the top of the map? It's the number that begins with the 39. What is wrong with saying that the latitude of the north of the map is 39° 45'? Remember, that latitude must be followed by north or south unless you are at the equator. The correct latitude of the northern boundary of your map is 39° 45' North. As a space saving convenience, this information is omitted from the map, but it must not be omitted from your interpretation.

You also could have determined which of the two numbers in the upper corner was a latitude by just thinking about it for a minute. Obviously, 78° 45' is not the latitude of Cumberland. Where would a latitude that high be located? Maryland is not THAT far north! Similarly, by comparing the numbers found in each corner of your map you should be able to answer the following questions:

A. What is the latitude of the bottom of the map? Answer
B. What is the longitude of the east (right) side of the map? Answer
C. What is the longitude of the west (left) side of the map? Answer

D. Subtract the latitude of the parallel forming the bottom of the map from the latitude of the parallel forming the top. This will give you the size of the angle portrayed on this map. Do the same for the longitudes of the west and east sides of the map. Why do they call these maps "7.5 minutes series"? Answer
E. In reference to the previous question, if the angles of latitude and of longitude shown on the map are both the same, why is the length of the sides and the bottom not the same? Answer

In addition to the numbers given in the corners, the map contains other latitude and longitude references. These are given in abbreviated form on the sides of the map. They are a little hard to see because they are mixed up with lots of other little numbers. Look closely for the two subdivision marks on the top, sides, and bottom of the map. Find the 40' mark on the right side of your map. This is an abbreviated latitude, missing the degrees. Recall the latitudes of the parallels forming the top (39° 45'N.) and the bottom (39° 37' 30"N.) of the map. The 40' indicates a latitude in between the top and bottom, namely, 39° 40'N. You will notice there is another 40' mark on the left side of the map. Draw a light pencil line straight between the two 40' marks. Label this line as the parallel 39° 40'N. Find the 42' 30" marks on both the right and left sides of the map. Draw and label this line as the parallel 39° 42' 30"N.

Longitudes are abbreviated on the map in the same way. The right side of the map has a longitude of 78° 45' W. and the left side of the map of 78° 52' 30"W. All the other longitudes on the map must lie in between those of the two sides. Find the 50' marks on the top and bottom of the map. This is an abbreviation for 78° 50'W longitude. Draw and label this meridian. Find the 47' 30" marks on the top and bottom of the map. The meridian defined by these two marks has a longitude of 78° 47' 30" W. Draw and label this meridian too. Note the little + that indicates the intersection of these interior parallels and meridians.

In total, our map identifies four parallels of latitude and four meridians of longitude. Together, these allow us to identify the position of any feature on the topographic map. For example, let's find the latitude and longitude of Lover's Leap in the Narrows of Wills Mountain. First, find the latitude by consulting the parallels you have drawn. The words Lover's Leap are very close to the parallel 39° 40'N, so we will say that is its approximate latitude. Now, find the longitude by consulting the meridians. Lover's Leap lies between the meridians of 78° 45'W and 78° 47' 30"W. You will have to estimate where the it falls between these two values.

Note that each of the lines you have drawn is separated by an angle of 2' 30". I expect you to be able to estimate longitudes and latitudes to 30", one-fifth of the distance between each line. By careful estimation, you can see that Lover's Leap is about four-fifths of the way between 78° 45'W and 78° 47' 30"W. It is about 1/5 of the way short of the left hand or greater coordinate. For this reason we will subtract 30" (one fifth of the 2' 30" between each marked meridian) from the longitude is 78° 47' 30'W. We now know that Lover's Leap on the surface of the earth at 39° 40'N; 78° 47'W, coordinates that would allow anyone on earth to locate this feature.

Practice your latitude and longitude skills by determining the position (to the nearest 30") of the following features:

A. Barrelsville Answer
B. Allegany Grove Answer

Identify the features located at these coordinates.

C. 39° 43'N, 78° 46' 30"W Answer
D. 39° 42'N, 78° 52' 30"W Answer

Continue with the lesson