Line of greatest slope
In topography, the line of greatest slope is a curve following the steepest slope. It is always orthogonal to the contour lines. Mathematically it is determined by the gradient of the height, taken as a potential field, so the lines of greatest slope are analogous to lines of force.
If inertial forces and terrain roughness are set side, a ball rolling down a slope, or water flowing down, will follow the line of greatest slope.
The line of greatest slope has practical significance in map reading. In the terrain it often is better visible than the contour line, but knowing that it is orthogonal to the contour line, one can deduce the direction of the contour line from the line of greatest slope. The direction of the contour line is found on the topographic map.