What is a Topographic Map? - Mapping is a crucial part of earth science. Topographic maps represent the locations of major geological features. Topographic maps use a special type of line, called a contour line, to show different elevations on a map.
Contour lines are drawn on a topographic map to show the location of hills, mountains and valleys. When you use a regular road map, you can see where the roads go, but a road map doesn’t tell you why a road stops or bends.
Topographic map will show you that the road bends to go around a hill or stops because that is the top of a mountain. You need a special type of map to represent the elevations in an area. This type of map is called a topographic map. What makes a topographic map different from other maps? Contour lines help show various elevations.
Contour lines connect all the points on the map that have the same elevation. Let’s take a closer look at this.
- Each contour line represents a specific elevation and connects all the places that are at the same elevation. Every fifth contour line is bolded. The bold contour lines are labeled with numerical elevations.
- The contour lines run next to each other and NEVER cross one another. That would mean one place had two different elevations, which cannot happen.
- Two contour lines next to one another are separated by a constant difference in elevation (e.g. 20 ft or 100 ft.). This difference between contour lines is called the contour interval. You can calculate the contour interval. The legend on the map will also tell you the contour interval. ( – Take the difference in elevation between 2 bold lines. – Divide that difference by the number of contour lines between them. If the difference between two bold lines is 100 feet and there are five lines between them, what is the contour interval? If you answered 20 feet, then you are correct (100 ft/5 = 20 ft) )
Interpreting Contour Maps
How does a topographic map tell you about the terrain? Well, in reading a topographic map, consider the following principles:
- Contour lines can indicate the slope of the land. Closely-spaced contour lines indicate a steep slope, because elevation changes quickly in a small area. In contrast, broadly spaced contour lines indicate a shallow slope. Contour lines that seem to touch indicate a very steep or vertical rise, like a cliff or canyon wall. So, contour lines show the three-dimensional shape of the land.
- Concentric circles indicate a hill.
- Hatched concentric circles indicate a depression. The hatch marks are short, perpendicular lines inside the circle. The innermost hatched circle would represent the deepest part of the depression, while the outer hatched circles represent higher elevations.
- V-shaped portions of contour lines indicate stream valleys. Here the V- shape of the contour lines “point” uphill. The channel of the stream passes through the point of the V and the open end of the V represents the downstream portion. Thus, the V points upstream. A blue line will indicate the stream if water is actually running through the valley; otherwise, the V patterns will indicate which way water will flow.
- Like other maps, topographic maps have a scale on them to tell you the horizontal distance. The horizontal scale helps to calculate the slope of the land (vertical height/horizontal distance). Common scales used in United States Geological Service (USGS) maps include the following: ( 1:24,000 scale – 1 inch = 2000 ft , 1:100,000 scale – 1 inch = 1.6 miles , 1:250,000 scale – 1 inch = 4 miles )
So, the contour lines, their spacing intervals, circles, and V-shapes allow a topographic map to convert 3-dimensional information into a 2-dimensional representation on a piece of paper. The topographic map gives us an idea of the shape of the land.
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