Regents Prep: Earth Science: Mapping: Topographic Maps

Fields
A field is a region with a measurable quantity at every location.

 The picture to the right is an example of a field map. In this example, the numbers on the map represent the concentration of gasoline found at that location. So the map simply shows how much of something (in this case gasoline) is found at many locations.

Isolines
Isolines
are lines that are drawn on a field map to connect all of the points on that map that have the same value.

 The image on the left  is an example of a gasoline field map with the 40 isoline drawn in. Notice that the line connects points that all have the same value. In this example, the circle connects all of the points with a value of 40. That means that every point on that line has a value of exactly 40.

 This map has all of the isolines drawn in, at an interval of 10 (each line is 10 units apart) is shown to the right:

Topographic Maps
Contour lines
are isolines that are drawn on an elevation map. These lines connect all points with the same elevation, and give a great picture of the lay of the land. The resulting map is called a Topographic Map.

 Above is an example of a Topographic Map. Notice that isolines are drawn every 20 meters to connect points with the same elevation.

There are several important rules to remember when reading a topographic map:

 Close line =Steep Slope The steepest slopes on the map can be seen by looking for lines that are very close together. Since each line represents a change in elevation of a set amount (20 meters in this example), lines that are close together indicate steep gradients. The area circled in red is very steep, because it has many lines crowded close together. Contour lines bend upstream when crossing a river Since contour lines must remain at the same elevation, they must bend when they cross riverbeds. A riverbed drops downward, so the contour line must bend uphill to stay at the same elevation. The highlighted lines show that the direction of the Mill River must be northeast. Highest possible elevation of a hill The highest elevation of a hill can be calculated by finding the last (highest) contour line on that hill, and then figuring out the next line that would be drawn. The highest possible elevation of the hill is just below the value of  that next line. The highest possible elevation of the hill indicated by the arrow is 239 meters. The last line indicated on the map is 220 meters. The next line would be 240 meters (remember that the contour interval is 20 meters). Since there is no 240 meter line, the hill cannot be higher than 239 meters! Depressions are shown by small marks pointing inward off the contour line The arrow is pointing to a depression, or hole. The contour line with the marks, or hatchers, has the same elevation as the line before it. In this case the hatchered line has a value of 140 meters, and the depression must be less than 140 meters.

Profiles
A profile is a picture of what the landscape would look like from the side. It is a cross section of the landscape. You should be able to identify a correct profile, as well as draw one.

 Identify a Profile Given the topographic map above, which of the four choices below accurately depicts the correct profile between points X and Y? The best way to approach this is by the process of elimination. First of all, point X appears to be approximately 290 meters in elevation. Let's eliminate all the choices that don't begin at 290 meters: That eliminates choices 1 & 3 right off the bat! Next, we look at point Y, which appears to be approximately 130 meters. That eliminates choice 4. That means choice 2 must be correct!

 Draw a Profile Given the topographic map above, lets draw a profile map from point P to point Q. First, we need a horizontal grid that represents the distance from P-Q Next, transfer the points from the contour map to the grid. Then connect the points with a smooth line to draw the profile. And there! We have a cross section of the landscape.

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