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**NAME**

grdgradient - Compute directional derivative or gradient from 2-D grd file representing z(x,y)

**SYNOPSIS**

grdgradientin_grdfile-Gout_grdfile[-Aazim[/azim2] ] [-D[c][o][n] ] [-Lflag] [-M] [-N[e][t][amp][/sigma[/offset]] ] [-Sslopefile] [-V]

**DESCRIPTION**

grdgradientmay 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).in_grdfile2-D grd file from which to compute directional derivative.-GName of the output grdfile for the directional derivative.

**OPTIONS**

No space between the option flag and the associated arguments. Use upper case for the option flags and lower case for modifiers.-AAzimuthal direction for a directional derivative;azimis the angle in the x,y plane measured in degrees positive clockwise from north (the +y direction) toward east (the +x direction). The negative of the directional derivative, -[dz/dx*sin(azim) + dz/dy*cos(azim)], is found; negation yields positive values when the slope of z(x,y) is downhill in theazimdirection, the correct sense for shading the illumination of an image (seegrdimageandgrdview) by a light source above the x,y plane shining from theazimdirection. Optionally, supply two azimuths,-Aazim/azim2, in which case the gradients in each of these directions are calculated and the one larger in magnitude is retained; this is useful for illuminating data with two directions of lineated structures, e.g.-A0/270illuminates from the north (top) and west (left).-DFind the direction of the gradient of the data. By default, the directions are measured clockwise from north, asazimin-Aabove. Appendcto use conventional cartesian angles measured counterclockwise from the positive x (east) direction. Appendoto report orientations (0-180) rather than directions (0-360). Appendnto add 90 degrees to all angles (e.g., to give orientation of lineated features).-LBoundary conditionflagmay bexoryorxyindicating data is periodic in range of x or y or both, orflagmay begindicating geographical conditions (x and y are lon and lat). [Default uses "natural" conditions (second partial derivative normal to edge is zero).]-MBy default the units ofgrdgradientare in units_of_z/units_of_dx_and_dy. However, the user may choose this option to convert dx,dy in degrees of longitude,latitude into meters, so that the units ofgrdgradientare in z_units/meter.-NNormalization. [Default: no normalization.] The actual gradientsgare offset and scaled to produce normalized gradientsgnwith a maximum output magnitude ofamp. Ifampis not given, defaultamp= 1. Ifoffsetis not given, it is set to the average ofg.-Nyieldsgn=amp* (g-offset)/max(abs(g-offset)).-Nenormalizes using a cumulative Laplace distribution yieldinggn=amp* (1.0 - exp(sqrt(2) * (g-offset)/sigma)) wheresigmais estimated using the L1 norm of (g-offset) if it is not given.-Ntnormalizes using a cumulative Cauchy distribution yieldinggn= (2 *amp/ PI) * atan( (g-offset)/sigma) wheresigmais estimated using the L2 norm of (g-offset) if it is not given.-SName of output grdfile with scalar magnitudes of gradient vectors. Requires-D.-VSelects verbose mode, which will send progress reports to stderr [Default runs "silently"].

**HINTS**

If you don't know what-Noptions to use to make an intensity file for 'grdimageorgrdview, a good first try is-Ne0.6. If you want to make several illuminated maps of subregions of a large data set, and you need the illumination effects to be consistent across all the maps, use the-Noption and supply the same value ofsigmaandoffsettogrdgradientfor each map. A good guess isoffset= 0 andsigmafound bygrdinfo-L2or-L1applied to an unnormalized gradient grd. If you simply need thex- ory-derivatives of the grid, usegrdmath.

**EXAMPLES**

To make a file for illuminating the data in geoid.grd using exp-normalized gradients imitating light sources in the north and west directions, do grdgradient geoid.grd-A0/270-Ggradients.grd-Ne0.6-VTo find the azimuth orientations of seafloor fabric in the file topo.grd, try grdgradient topo.grd-Snao-Gazimuths.grd-V

**SEE** **ALSO**

gmt(1gmt),gmtdefaults(1gmt),grdhisteq(1gmt),grdimage(1gmt),grdview(1gmt),grdvector(1gmt) 1 Jan 2004 GRDGRADIENT(l)