1. Reading Contour Lines
“The Basics”

2. 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.

3. Picture walking along a beach exactly where the water meets the land
Picture walking along a beach exactly where the water meets the land. The water surface marks an elevation we call sea level, or zero. As you walk along the shore your elevation will remain the same, you will be following a contour line. If you stray from the shoreline and start walking into the ocean, the elevation of the ground (in this case the seafloor) is below sea level.  If you stray the other direction and walk up the beach your elevation will be above sea level (see diagram at right).  

4. The contour line represented by the shoreline separates areas that have elevations above sea level from those that have elevations below sea level. 
We refer to contour lines in terms of their elevation above or below sea level. In this example the shoreline would be the zero contour line ( it could be 0 ft., 0 m, or something else depending on the units we were using for elevation).  

5. The two diagrams below illustrate the same island.
From the profile it is clear that the topography of the island is more complicated than what is indicated by a map with one contour line.

6. If we flood the island with 10 feet of water, we can see that the island appears to be smaller.
Only the land with an elevation greater than 10 ft is above the water line.
New contour line is 10 ft.

7. What will happen to the island as we continue to add water 10 ft at a time?
Notice the island gets smaller and smaller.

8. Taken all together the contour lines supply us with much information on the topography of the island. 
With practice we can picture topography by looking at the map even without the cross profile.

9. Reading Elevations
The map below is an enlargement of the map of the previous island.  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.

10. Contour Interval The contour interval is constant for each map.
It will be noted on the margin of the map.
You can also determine the contour interval by looking at how many contour lines are between labeled contours.

11. Using Index Contours
What is the contour interval?
20 meters

12. What are the elevations of points A-D?

13. Depression Contours Shows decreasing elevations
What is the lowest possible elevation?
31 meters

14. Relief to Contours

15. River Contours
Contour lines that cross river beds bend in the opposite direction of river flow
They Point upstream