Topography How to create and read a topographical map
BASICS Contour lines are lines drawn on a map connecting points of equal elevation. If you walk along a contour line you neither gain or lose elevation.
The shape of the island is shown by location shoreline on the map. Remember this shore line is a contour line. It separates areas that are above sea level from those that are below sea level. The shoreline itself is right at zero so we will call it the 0 ft. contour line (we could use m.,cm., in., or any other measurement for elevation).
But contour lines can have elevations other than sea level. We can picture this by pretending that we can change the depth of the ocean. The diagram below shows an island that is getting flooded as we raise the water level 10 ft above the original sea level.
By raising water levels to 20 ft and 30 ft above the original see level we can find the location of the 20ft and 30 ft contour lines. Notice our islands gets smaller and smaller
If we take each of the shorelines from the maps above and draw them on the same map we will get a topographic map (see map below). Taken all together the contour lines supply us with much information on the topography of the island. From the map (and the profile) we can see that this island has two "high" points
With practice we can picture topography by looking at the map even without the cross profile. That is the power of topographic maps
A common use for a topographic map is to determine the elevation at a specified locality. The map below is an enlargement of the map of the island from above. Each of the letters from A to E represent locations for which we wish to determine elevation. Use the map and determine (or estimate) the elevation of each of the 5 points. (Assume elevations are given in feet)
<ul><li>Point A = 0 ft </li></ul><ul><li>Point A sits right on the 0 ft contour line. Since all points on this line have an elevation of 0 ft, the elevation of point A is zero. </li></ul><ul><li>Point B = 10 ft. </li></ul><ul><li>Point B sits right on the 10 ft contour line. Since all points on this line have an elevation of 10 ft, the elevation of point B is 10 ft. </li></ul><ul><li>Point C ~ 15 ft. </li></ul><ul><li>Point C does not sit directly on a contour line so we can not determine the elevation precisely. We do know that point C is between the 10ft and 20 ft contour lines so its elevation must be greater than 10 ft and less than 20 ft. Because point C is midway between these contour lines we can estimate the elevation is about 15 feet (Note this assumes that the slope is constant between the two contour lines, this may not be the case) . </li></ul><ul><li>Point D ~ 25 ft. </li></ul><ul><li>We are even less sure of the elevation of point D than point C. Point D is inside the 20 ft. contour line indicating its elevation is above 20 ft. Its elevation has to be less than 30 ft. because there is no 30 ft. contour line shown. But how much less? There is no way to tell. The elevation could be 21 ft, or it could be 29 ft. There is now way to tell from the map. (An eight foot difference in elevation doesn't seem like much, but remember these numbers are just an example. If the contour lines were spaced at 100 ft intervals instead of 10 ft., the difference would be a more significant 80 ft.) </li></ul><ul><li>Point E ~ 8 ft. </li></ul><ul><li>Just as with point C above, we need to estimate the elevation of point E somewhere between the 0 ft and 10 ft contour lines it lies in between. Because this point is closer to the 10 ft line than the 0 ft. line we estimate an elevation closer to 10. In this case 8 ft. seems reasonable. Again this estimation makes the assumption of a constant slope between these two contour lines. </li></ul>
Unlike the simple topographic map used above, real topographic maps have many contour lines. It is not possible to label the elevation of each contour line. To make the map easier to read every fifth contour line vertically is an index contour. Index contours are shown by darker brown lines on the map. These are the contour lines that are usually labeled
<ul><li>Point A = 700 </li></ul><ul><li>Point B = 740 </li></ul><ul><li>Point C ~ 770 </li></ul><ul><li>Point c is not directly on a contour line. But by counting up from 700 we can </li></ul><ul><li>see it lies between the 760 and 780 contour lines. Because it is in the middle of the two we can estimate its elevation as 770. </li></ul><ul><li>Point D = 820 </li></ul><ul><li>Point D is outside the interval between the two measured contours. </li></ul><ul><li>While it may seem obvious that it is 20 above the 800 contour, how do we know the </li></ul><ul><li>slope hasn't changed and the elevation has started to back down? </li></ul><ul><li>We can tell because if the slope stated back down </li></ul><ul><li>we would need to repeat the 800 contour. </li></ul><ul><li>Because the contour under point D is not an index </li></ul><ul><li>contour it can not be the 800 contour, </li></ul><ul><li>so must be 820. </li></ul>