d calculates surface area
of each cell by using the height of the cell and its 8 surrounding cells
to create a surface. For border cells, the cells outside of the DEM are
assumed to be of equal height as the border cells. This means that
border cells may have lower surface area estimates. If the ratio of
border cells to total cells inside a DEM is high, there may be an
underestimation of surface area, which is the case when values inside
<code>lvec</code> are too close to the extent. For example, if
<code>l