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General Rules for Contour Lines

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General Rules for Contour Lines 6/30/2015 EASC111­LabE­For Printing LAB E: Topographic Maps (for printing) MAP READING PRE­LAB: Any type of map is a two­dimensional (flat) representation of Earth’s surface. Road maps, surveying maps, topographic maps, geologic maps can all cover the same territory but highlight different features of the area. Consider the following images from the same area in Illinois. Make a list of the types of features that are shown on each type of map. Topographic Maps The light brown lines on the topographic map are called contour lines. A contour line connects points of equal height above sea level, called elevation. For example, a 600’ (six hundred foot) contour line on a map means that every point on that contour line is 600’ above sea level. In order to understand contour lines better, imagine a box with a “mountain” in it with a clear plastic lid on top of the box. Assume the base of the mountain is at sea level. The box is slowly being filled with water. Since water automatically levels itself off, it will touch the “mountain” at the same height all the way around. If we were to peer down into the box from above, we could draw a line on the lid that marks where the water touches the “mountain”. This would be a contour line for that elevation. As the box continues to fill with water, we would draw contour lines at specific intervals, such as 1”. Each time the water level rises one inch, we will draw a line marking where the water touches the land. This does not mean that the contour lines will be one inch apart on the plastic lid. The contour lines mark where the land surface is one inch higher than the previous contour line. If the land surface is steep, the contour lines will be closer together. If the land surface is gently sloping, the contour lines will be farther apart. Because contour lines represent a specific height above sea level, certain rules apply to their use. These rules are highlighted for you in the box below: General Rules for Contour Lines 1. Contour lines connect points of equal elevation; therefore every point along a contour line is the exact same elevation. 2. Contour lines never intersect. (A point on the surface of the earth cannot be at two different elevations). 3. Contour lines never split or divide. Contour lines always separate points of higher elevation (uphill) from lower elevation 4. (downhill). 5. Contour lines always close to form an irregular circle. Note that sometimes contour lines extend beyond the area on a map so you may not see the entire closed circle. The closer spaced the contour lines are to one another, the steeper the slope. 6. A hill is represented by a concentric series of closed contours. 7. http://faculty.icc.edu/easc111lab/labs/labe/pre_labe_print.htm 1/3 6/30/2015 EASC111­LabE­For Printing 8. Depression contours are indicated by hachure marks on the downhill side. Contour lines “V” upstream when crossing a stream. The point of the “V” points uphill. 9. When reading a contour map, the elevation of a point between contour lines must be estimated. For example, the elevation of Point A is between 50’ and 60’. Elevations of specific points on topographic maps (tops of mountain peaks, survey points, etc.) are sometimes indicated directly on the maps beside the symbols used for that purpose. Click here for the map of Humphreys Peak in Arizona. What is the elevation of Humphreys Peak? The notation “BM” marks a benchmark, a permanent marker placed by the United States Geological Survey or the Bureau of Land Management at the point indicated on the map. Elevations are usually indicated for benchmarks. Contour intervals and index contours The spacing between contour lines is referred to as the contour interval. Depending on the scale of the map and the steepness of the land surface, each map has its own contour interval, written near the bar scale at the base of the map. If the contour interval is 20’, then there is a 20’ difference in elevation between one contour line and the next. It’s important to look closely at the map to determine if the next contour line is 20’ higher or 20’ lower than the previous one. Be sure to notice the contour interval given for the map you are using to answer questions in this lab exercise. To help with reading elevations, every fifth contour line is darkened with the elevation written on it. These darkened lines are called index contours and are a good reference point to begin interpreting your map. When reading contour lines, special notice should be given to contour lines that repeat on a map. These occur on opposite sides of ridges, valleys, or depressions (unless the depression is on a steep slope). Imagine yourself hiking up a hillside. As you climb, you are crossing contour lines at higher elevations. If you reach the top of a rise, you will start descending across those same contour lines. Likewise, with a depression, as you reach the lip of the depression and begin hiking down, you will cross the same elevations you just hiked up. As you come out of the depression, you cross those same lines again. If there is a depression on a steep slope, on the uphill portion of the slope the contour lines will not repeat. Print the following map and answer the Questions x­x with information from the map. You will turn this map in with your pre­lab exercises. Click on the map below to get a larger version that you can print You must print off the larger version to turn in !! http://faculty.icc.edu/easc111lab/labs/labe/pre_labe_print.htm 2/3 6/30/2015 EASC111­LabE­For Printing 3. Mark each contour line with its exact elevation, using a contour interval of 40 meters 4. What is the total relief of the area of this map? 5. Point A must lie at an elevation higher than _____ meters but lower than _____ meters. 6. Point B must lie at an elevation higher than _____ meters but lower than _____ meters. 7. Place an X on the map in the area with the steepest slope. Place a Y on the map where the flattest part of the land is. Explain your choices. http://faculty.icc.edu/easc111lab/labs/labe/pre_labe_print.htm 3/3.
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