Planar Area Calculations
Planar area is computed by projecting the cut and fill portions of the surface onto a plane and calculating the area of the projection.
Positive Planar Area
Positive Planar Area represents the projection of the cut (map areas where the upper surface is above the lower surface) onto a horizontal plane.
Negative Planar Area
Negative Planar Area represents the projection of the fill (map areas where the upper surface is below the lower surface) onto a horizontal plane.

This is a contour map showing the relation between positive and negative planar area on a map. The lower surface is defined by a plane where Z = 75. 