Quick Look Techniques

Odd Number of Contours

A basic rule of contouring is that ALL contours on a continuous surface must close or end at the edge of the map. This rule seems so obvious and simple that no one could break this rule of contouring.

Figure 1 is a relatively simple structure map with a few faults. Consider the area to the right of the major down-to-the-east fault. Is there a contouring problem?

Fig. 1

Starting at the -10,300 foot contour, try to go around the small finite fault and return to the -10,300 foot contour. Can it be done? The answer is no. Five contours terminate against the finite fault; therefore, a contour is dangling. In other words, one contour does not close. One contour is missing.

All contours must close. There must be an even number of contours around a finite fault such as the one shown in Figure 1. This type error is very common. A quick way to check a map with a small fault, which dies in both directions, is to count the number of contours intersecting or terminating against the fault. If there is an odd number of contours, the construction is wrong. This may be a minor mapping bust, but if you find several on one map, it may be wise to question the accuracy of the map.