MAPS AND TOPOGRAPHY. John J. Thomas

Transcription

1 MAPS AND TOPOGRAPHY John J. Thomas Purpose An exercise to give the students experience reading and making topographic maps. The first exercise will be to orient the map and figure out the scale of the map (how big it is). They will then look for features on the map, find out what different symbols are used for different features. Finally, students normally have trouble reading a topographic map. Usually they have less trouble if they understand how a contour map is made. Included is an exercise in which the student can make a topographic map from a map of elevations. Definitions Scale - the size of the features on the map. Culture - a human made feature on the map. Topographic Map Symbols - a table of symbols used on the map. Contour Line - lines that show the contour of the land. Lines of equal elevation. Contour Interval - the change in elevation between each contour line. Topography - the shape of the land, the ups and downs. Topographic Map - a map that shows the topography by having contour lines. Quadrangle - the map, it has four corners. USGS - U.S. Geological Survey, the people who make topographic maps. Teacher Information It would be rare to find a student who has seen a topographic map and knows the wealth of information contained therein. Topographic maps show what a highway map shows culture and nature (towns, highways, rivers, etc) and more. Because the small-scale maps show more area than a highway map, and if the population is not too dense, topographic maps show all the buildings that were there at the time that the map was made. There are different symbols for schools, churches, houses, barns, and other features. The Topographic Map Symbols sheet shows you the variety of symbols. This sheet ought to still be available in color if you buy a topographic map. Make sure you ask for it. You can see these symbols more clearly if you go to the USGS web page: The feature that most people find new, and often frightening, is that topographic maps show the shape of the land by contour lines. By looking at the contour lines you can tell hills from valleys and steep slopes from gentle ones. The map that is included with this exercise is of Saratoga Springs, NY. You can use it for this exercise or you can use a map of your area and adapt this exercise to your map. The maps are available at most hunting, fishing, and camping stores. You can locate stores that sell maps by going to the USGS map page: You can also get them directly from the U.S. Geological Survey (U.S. G.S. Information Service, Box 25286, Denver CO 80225, but it frequently takes up to 6 months to get them. The USGS does have an Index to Topographic and Other Map Coverage booklet for each state that is free. It lists all the federal maps available for a state. If you buy a USGS map you can copy it and give the copies to the students. USGS maps are not copyrighted. Use the format of the map enclosed with this exercise as an example of how you might like to do your map. The first exercise you should do is orient the map. Anyone who uses a map ought to know where north is. The sides of the map (usually the long dimension) always point north. If you look at the lower left corner of the map, you will see a north arrow, there are even two. The one pointing straight up the map is True North, where the north pole of the earth is. The other one points to

2 magnetic north. The true north pole of the world and the magnetic north are not the same. On the Saratoga Springs map, the difference is about 14 degrees. The difference depends on where you are on the earth. The scale of the map is important. A small-scale map is one that shows a lot of detail. The smallest scale USGS topographic maps are called 7 1/2 minute quadrangles (the sides of the map are 7 1/2 minutes of a degree long, 8.6 miles). On 7 1/2 minute quadrangles, one inch equals 2000 feet. The AAA New York highway map has a scale of one inch equals 10 1/2 miles. This is a large-scale map because it shows a large area. To determine the scale of a USGS map look at the bottom center of the map. There will be bars that show feet and/or miles (meters and/or kilometers) and a ratio. The ratio is the scale. For the 7 1/2 minute quadrangle, such as the included map, the scale is 1:24,000. That means that one inch on the map equals 24,000 inches on the ground, one foot on the map equals 24,000 feet on the ground, etc. 1:24,000 does not mean much to most people, but you can do a scale conversion: 1:24,000 1 inch = 24,000 inches 12 inches/foot 24,000 inches 1 inch = inches/foot 1 inch = 2,000 feet To understand the culture on the map, look at the Topographic Map Symbols sheet for the symbols for different features: a house is a filled small square, A big building will be a filled shape similar to its floor plan, a barn is an open square, a school has a flag on its box, a church a cross, a synagogue a Star of David, etc. Different roads have different symbols, as do different bodies of water. Railroads and bridges have symbols. The map included with this exercise is in black and white (it is a copy of a USGS map). On real USGS maps, vegetation is green, water blue, contour lines are brown, and revisions from aerial photographs are in a pink overlay. All you have to do to find a feature is find its symbol. The culture on the map depends on the date the map was made. On the map of your area, if your house was built when the map was made (check the lower right had corner for the date) and the population density is not too great, you house will be there. Topography is another matter. The best way to understand a topographic map is to make one. I have included a simple topographic map exercise. There are dots with elevations marked next to them. Drawing a topographic map is like doing dot-to-dot. A few rules first: 1) All contour maps start from mean (average) low tide of the ocean. If the interval is 10 feet, the contour lines will be 0, 10, 20, 30, etc. 2) A contour line must have the same elevation everywhere. 3) Contour lines cannot ever cross themselves. 4) When you go over the top of a hill or through the bottom of a valley, you always cross the same elevation (contour line) twice.

3 5) Contour lines are always smooth and well rounded. 6) Contour lines always come full circle and close somewhere, usually off the map. 7) The shape of any two adjacent contour lines is similar. 8) Where the lines are close together, the slope is steep. Where they are far apart the slope is gentle. 9) A good assumption is that the slope between any two points is constant. Try the contouring exercise (Turtle Island). a) Look for the contour interval, on this exercise it is 10 feet. That means that the contour lines will be at 0, 10, 20, 30 and 40 feet. b) Look for points with these elevations. The contour lines will pass through these points. If the points are not at even 10's, take two adjacent points that are above and below the elevation you seek. For example, if you are looking for an elevation of 30 feet, take two adjacent points whose elevations are 25 and 35. Halfway between these two will be an elevation of 30. Put a dot at this point and write the elevation next to it. Even if the numbers are not as nice as these, you can always estimate where the 10's elevation is. The points may be far enough apart that more than one contour line passes between them. Estimate both lines. c) Once you have enough points to tell where the lines will go, draw in the lines by following the dots. It is usually best not to start at the zero line or the highest one. Start somewhere in the middle where you have lots of points to fix the line. Keep your lines smooth and well rounded. Keep the shape of adjacent lines similar.

4 LOOKING AT A MAP 1) Hold the map so that North is pointing away from you. Notice the North arrow in the lower left corner of the map. There are two arrows. One is magnetic north and the other is the true North Pole. The two are not the same. The map points toward true north. 2) What is the scale of the map, for example 1:36,580 (look at the center of the bottom of the map)? 3) This means that 1 inch on the map equals how many inches on the ground (for example 1 inch = 36,580 inches on the map) 1 inch on the map = inches on the ground. 4) To find out how many inches on the map equals how many feet on the ground, divide the right hand side of the ratio by 12 (1 foot = 12 inches). 1 inch on the map = feet on the ground. 5) Look for and circle the following cultural (human made) features (look at the Topographic Map Symbols supplied with this exercise to see what symbol goes with which feature): Caroline Street School St. Clements Church Skidmore College A house on Lake Avenue A house on Broadway Saratoga Hospital Loughberry Lake Water Tank Quarries High Rock Spring Spa Baths Saratoga Racetrack Saratoga Raceway 6) How far is it from the Fire Station to Caroline Street School? feet. miles. 7) What is the elevation near the County Building and the Post Office? A carefully measured elevation point is called a Bench Mark (BM). feet.

5 ANSWER SHEET LOOKING AT A MAP 1) Hold the map so that North is pointing away from you. Notice the North arrow in the lower left corner of the map. There are two arrows. One is magnetic north and the other is the true North Pole. The two are not the same. The map points toward true north. 2) What is the scale of the map, for example 1:36,580 (look at the center of the bottom of the map)? 1: ) This means that 1 inch on the map equals how many inches on the ground (for example 1 inch = 36,580 inches on the map) 1 inch on the map = 24,OOO inches on the ground. 4) To find out how many inches on the map equals how many feet on the ground, divide the right hand side of the ratio by 12 (1 foot = 12 inches). 1 inch on the map = 2,000 feet on the ground. 5) Look for and circle the following cultural (human made) features (look at the Topographic Map Symbols supplied with this exercise to see what symbol goes with which feature): Caroline Street School J-K 10 St. Clements Church J-K 9 Skidmore College F-G 12,13 A house on Lake Avenue F-N 9 A house on Broadway H1-E,18 Saratoga Hospital B-C 9 Loughberry Lake K2-J6 Water Tank F3 Quarries D1 High Rock Spring F-G 8 Spa Baths D-E 16 Saratoga Racetrack I-J 14 Saratoga Raceway J ) How far is it from the Fire Station to Caroline Street School? 4,200 feet. 0.8 miles. 7) What is the elevation near the County Building and the Post Office? A carefully measured elevation point is called a Bench Mark (BM). 316 feet.

6

7

8

9

10

11