grdgradient(1gmt) on Linux
The text ad03e85b2874, as Debian 5.0 (Lenny) shipped it.
Compute directional derivative or gradient from 2-D grid file representing z(x,y)
Flags it lists
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-A -
-D -
-E -
-G -
-L -
-M -
-N -
-S -
-V