Whether or not aspects and slopes are calculated is governed by the control parameter sdi. By default, slopes and aspects are not calculated.
If slopes and aspects are calculated, then Natgrid allocates memory for storing this information internally so that it will be available to the procedures that retrieve aspect and slope values ( NNGETASPECTS, NNGETSLOPES, NNGETASPECTD, NNGETSLOPED, c_nngetaspects, c_nngetslopes, c_nngetaspectd, and c_nngetsloped). The memory for the slope and aspect data is reallocated whenever one of the gridding procedures ( NATGRIDS, NATGRIDD, c_natgrids, or c_natgridd) is called and sdi is set to indicate aspects and slopes are to be calculated.
The control parameter rad determines whether aspect and slope values are returned in degrees or radians. The default is in degrees.
Example 3 illustrates computing aspect and slopes. A vector plot of the aspects and a contour plot of the slopes are supplied in the example.