grdgradient - Compute directional derivative or gradient from 2-D grd file representing z(x,y) grdgradient in _ grdfile -G out _ grdfile [ -A azim [/ azim2 ] ] [ -D [ c ][ o ][ n ] ] [ -L flag ] [ -M ] [ -N [ e ][ t ][ amp ][/ sigma [/ offset ]] ] [ -S slopefile ] [ -V ] grdgradient may be used to compute the directional derivative in a given direction ( -A ), or the direction ( -S ) [and the magnitude ( -D )] of the vector gradient of the data. ... If amp is not given, default amp = 1. ...