from http://gmt.soest.hawaii.edu/doc/5.2.1/grdgradient.html

grdgradient

grdgradient - Compute directional derivative or gradient from a grid

Synopsis

grdgradient in_grdfile -Gout_grdfile [ -Aazim[/azim2] ] [ -D[a][c][o][n] ] [ -E[s|p]azim/elev[/ambient/diffuse/specular/shine] ] [ -Lflag ] [ -N[e][t][amp][/sigma[/offset]] ] [ -Rregion ] [ -Sslopefile ] [ -V[level] ] [ -fg ] [ -n<flags> ]

Note: No space is allowed between the option flag and the associated arguments.

Description

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.

Estimated values in the first/last row/column of output depend on boundary conditions (see -L).

Required Arguments

in_grdfile

2-D grid file from which to compute directional derivative. (See GRID FILE FORMATS below).

-Gout_grdfile

Name of the output grid file for the directional derivative. (See GRID FILE FORMATS below).

==============================================================

http://blog.163.com/jey_df/blog/static/1825501612011429104237863/

(2) grdgradient HI_topo2.grd –A0 –Nt –GHI_topo2_int.grd

Grdgradient制作梯度信息的网格文件，得到某一个方向的方向导数，输入文件为HI_topo2.grd，设置方向导数的方向为0°，-N表示归一化，后面的t表示一种归一化算法，输出的文件为HI_topo2_int.grd.

================================

example02
gmt grdgradient HI_topo2.nc -A0 -Nt -GHI_topo2_int.nc

example04
gmt grdgradient HI_geoid4.nc -A0 -Gg_intens.nc -Nt0.75 -fg
gmt grdgradient HI_topo4.nc -A0 -Gt_intens.nc -Nt0.75 -fg
example_17
gmt grdgradient india_geoid.nc -Nt1 -A45 -Gindia_geoid_i.nc
example_18
gmt grdgradient AK_gulf_grav.nc -Nt1 -A45 -GAK_gulf_grav_i.nc
example_27
gmt grdgradient tasman_grav.nc -Nt1 -A45 -Gtasman_grav_i.nc
example_28
gmt grdgradient Kilauea.utm.nc -Nt1 -A45 -GKilauea.utm_i.nc
example_29
gmt grdgradient mars2.nc -fg -Ne0.75 -A45 -Gmars2_i.nc

example_32
gmt grdgradient topo.nc -A0/270 -Gillum.nc -Ne0.6

example_33
gmt grdgradient spac.nc -A15 -Ne0.75 -Gspac_int.nc
gmt grdimage spac.nc -Ispac_int.nc -Cz.cpt -JM6i -P -Baf -K -Xc --FORMAT_GEO_MAP=dddF > %ps%

example_34
gmt grdgradient FR+IT.nc -A15 -Ne0.75 -GFR+IT_int.nc

example_37
set G=grav.V18.par.surf.1km.sq
gmt grdgradient %G%.nc -A0 -Nt1 -G%G%_int.nc