Slope Between Contours

This formula converts the vertical change between two contour lines and the horizontal map spacing between them into a percentage slope. It works because slope is essentially rise divided by run, and multiplying by 100 expresses that ratio as a percent. If contour lines are close together, the ground is steep; if they are far apart, the ground is gentle.

Use this when reading a topographic map to estimate how steep a hillside, road alignment, drainage path, or cut-and-fill area is. It is especially useful in surveying, geomatics, and basic terrain analysis when you know the contour interval and the map distance between adjacent contours.

• Using the map scale distance incorrectly instead of the actual horizontal ground distance
• Forgetting to multiply by 100, which leaves the answer as a ratio instead of a percent
• Mixing up contour interval with contour spacing on the map