To predict the intersection of geologic structure with topography, you have to understand topography.
|1. Contour the elevation data shown.
2. Interpolate between nearby points. Also locate elevations along streams and valley bottoms.
3. Connect the most obvious segments of contours first.
4. In areas of sparse data, you may have to draw contours approximately.
In each pair of maps below the left map is incorrect and the right map is correct. There may be other ways to contour the data that are equally correct, however.
The map on the left is of no use to anybody. Beginners often draw contours through each elevation point. Don't! Use only even contour intervals and interpolate between points to locate the contours.
|Water flows downhill. A contour can only cross a valley if the valley floor is at that elevation. This map shows the stream flowing over a hill!|
|Slopes must be consistent across a contour. This error is especially common around saddles.|
|Do not end, cross or branch contours. These errors tend to occur when the mapper mismatches contours when joining them up, or cannot figure out how to connect sections of contours.|
|Do not interpolate across valleys (a) or between very distant points (b), or any time you suspect the slope is not approximately constant (c).|
|Depressions are relatively rare. Do not draw them unless you have real evidence that they exist. Although both maps are possible, the map on the right is much more plausible.|
|Do not make the contours any more complex than the evidence requires. There is no evidence for any of the small detail on the left map.|
|Draw V's wherever contours cross streams, and make sure they point the right way! Both maps are consistent with the data but the right map is much more likely to occur in nature.|
|Do not vary contour spacing or curvature unless you have positive reason to do so. This problem usually stems from poor hand-eye coordination while drawing. It's better to depart a bit from perfect interpolation than to have contours with irregular spacing or curvature.|