The Visual Presentation of Spatial Data

Contouring Rules

The map below displays the pH of rainfall in the United States from March 25 to April 21 in 1991. Clearly, it is difficult to visualize patterns to the data when they are displayed in this manner. We will use this data set to demonstrate how to create a contour map for any type of spatially distributed data.

To construct a contour map from spatially plotted data:

1.Determine the maximum and minimum values plotted on the map.

2.Determine a contour interval that best describes the range of data and the detail you wish to show. If possible, use an interval that produces contour lines with whole numbers. Contour intervals of 2, 5, 10, 20, 50, and 100 work well for a variety of topographic maps.

3.Write out the contour lines necessary to accommodate your data. The highest contour line should be slightly less than the maximum value plotted and the lowest contour line less than the minimum value.

4.Find and mark all the points with values equal to or greater than the highest contour line.
 a. Draw a line through all the points with values equal to that of the contour line. b. Circle those points which are higher, and exclude points with values lower than the contour line. c. Draw the contour line close to points with values similar to that of the contour line and far away from those points significantly different.
5.Mark all points with values between that of the contour line you just drew and the next lower contour line.
 a. Circle this group of points with the next lower contour line. b. Position the line as described in step 4.

6.Repeat this procedure until you have contoured the entire map.