A map with elevation points plotted that is ready for contouring
A topographic map shows the elevation of the earth's surface using lines that connect all of the places that are the same elevation. Such lines are called contour lines. To draw them by hand, you start with a series of points of known elevation that someone has measured and then infer where the lines would be between each pair of points. This process is called interpolation.
Instructions
1. Draw a triangle that connects three of the points of known elevation. The points should be near each other but not of the same elevation. Choose a contour interval (the spacing between lines) that is such that each side of the triangle crosses between one and three lines. You don't want it to be too large of an interval or else the map won't show enough detail, but you don't want it to be too small or it'll be cluttered. Once done, erase the triangle.
2. Measure the distance between two of the points. We'll call them A and B, where A has a lower elevation than B.
3. Draw a right triangle on a separate piece of paper in which the horizontal base is the same length as the distance between A and B.
4. Subtract A from B. Make that many equally-spaced marks--or a multiple, if it's a large number--on the vertical side of the triangle, starting with A at the bottom and ending with B at the top.
5. Find and mark where the contour lines would be on the vertical side of the triangle. For example, if A is 297, B is 318 and your contour interval is 10, you will have marks for 300 and 310.
6. Extend horizontal lines from the contour marks until they reach the hypotenuse. Then extend vertical lines down from the hypotenuse until they reach the horizontal base of the triangle. Measure where the points fall, then transfer them to their corresponding places between A and B on the map.
7. Repeat this process with every pair of points on the map. Use the same vertical scale for every pair of points. The equally spaced marks on the vertical side of the triangle should have the same spacing on every triangle. It doesn't have to be the same scale as the map. Some pairs can be omitted. As you gain experience, you'll get a feel for which ones can be eliminated.
8. Connect all the points that are on the same contour line. Make the line curved in between in a way that looks reasonable. Some artistic license will be involved.
Tags: side triangle, pair points, vertical side, vertical side triangle, contour interval, contour lines