5 Rules of Contour Lines
1. Contour lines never intersect or cross one another
2. Contour lines curve in an upstream direction when they cross a stream valley.
3. Every contour line should eventually meet itself, either inside or outside the map area.
4. Where contours are spaced close together, the topography is steep; where they are far apart the slopes are gentler.
5. Hachured contours indicate a depression.
Gradient = Elevation Difference / Horizontal Difference