The last independent measurement we can make on a topographic map is that of direction .
Direction is an interesting variable because it can be either uni directional or bi directional.
When making orientation measurements, be sure to specify whether it is uni- or bidirectional. By convention, all directions are given in terms of variance from True North. We have several choices for datums on our map. Each of the coordinate systems we have used for location purposes is somehow related to a north-south axis. The sides of the map are each aligned north-south, as are the grid tics along the margins and in the body of the map.