A contour map is a simple way to represent three-dimensional shapes on a single sheet of paper. The shape and spacing of the map's lines convey large amounts of information in an easy-to-interpret graphical display. While topographic maps are the most familiar contour maps, the form is used throughout the sciences to display the three-dimensional distribution of many types of data.
Although many modern contour maps are computer-generated, the technique of making a contour map by hand is a basic skill taught in elementary science classes. No matter what sort of data will be contoured, the technique is the same.
- Skill level:
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Things you need
- Location map (paper) with geographic location information
- Observed data, including location information
- Eraser (optional)
Evaluate the observed data. Every point in the data must include both a data value and a location. Location information comes in many forms, including Cartesian coordinates, latitude and longitude, measurements from landmarks, and even street addresses. Inspect the location map and determine how to plot the locations in the data set on the map.
Select a contour interval, the difference between any two consecutive lines on the map. Contour intervals are usually "round" numbers (2, 5, 10, etc.). A good interval is one that produces 15 or 20 contour lines on a map.
Post all the data points on the map. Measure locations carefully with a ruler, since accurate locations are critical to the accuracy of the map. Place an X or dot on the map, and print the location's data value beside it.
Begin to contour by finding the highest number in the data set. Draw a light pencil line between this point and a nearby point. Estimate where the point (or points) that is an even multiple of your chosen contour interval lies on each line and make a small tick mark. For instance, if the contour interval is 10 and the two points are 5 and 15, the "10" point lies halfway between the two. If the value of a point is equal to a multiple of the contour interval, the line passes directly through it. Note that more than one contour line can pass between two points.
Repeat the line and estimation process for several nearby points surrounding the highest point. Make and label tick marks for every even multiple of the contour interval on each line.
Carefully sketch a smooth curve that connects all estimates of the same value. Your curve must follow two rules: first, it must pass through data points that have its value; second, the data points on one side must have higher values and points on the other side must have lower values. Do not mix higher and lower values on the same side. Contour lines always close on themselves, but may continue off the edge of a map and come back elsewhere.
Draw a second curve for the next multiple of the contour interval. This curve's shape should be similar to the first curve, but can never cross it. Label both contour lines with their value and check to see that the second line follows the rules stated above.
Continue drawing contour lines and labelling them until the entire map is full. A completed map has a line for every multiple of the contour interval between the highest and lowest values in the data set.
Tips and warnings
- It's normal to see contours bunch together in one area and spread apart in others.
- You can start with the lowest value in the data set, too.
- Drawing complicated contour maps often involves much erasing and redrawing of lines.
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