The ACR rugosity index is defined as the contoured (real) surface area divided by the area of the surface orthogonally projected onto a plane of best fit (POBF), where the POBF is a function (linear interpolation) of the boundary data only. Using a POBF, instead of an arbitrary horizontal geometric plane, results in an important advantage of the ACR rugosity index: unlike most rugosity indices ACR rugosity is not confounded by slope.
The ACR rugosity index is calculated by dividing the actual surface area into the projected area onto a plane that accurately represents its boundaries, achieved through linear interpolation based on boundary data. This approach provides an essential distinction from other measures, which are often compromised by slope.