Adjust flying height. Selecting a lens and film. Used where relief is negligible or data have no vertical dimension. Topographic maps show the configuration of the land surface as real world elevations. Elevation depends on defining a zero or base level.
Most maps use a constant interval.
Requires a base or reference line. Direction is defined as angular deviations from or along this base line. Eeven southern hemisphere maps usually put north at the top. The direction to the north pole of the axis of earth rotation. A line from any point on the earth to the north pole is called a true north base line.