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

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

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.

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 cor- rect sense for shading the illumination of an image (seegrdim-ageandgrdview) 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 direc- tions 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 orienta- tion 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 vec- tors. Requires-D.-VSelects verbose mode, which will send progress reports to stderr [Default runs "silently"].

If you donâ€™t know what-Noptions to use to make an intensity file forgrdimageorgrdview, 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.

To make a file for illuminating the data in geoid.grd using exp-normal- ized gradients imitating light sources in the north and west direc- tions:grdgradientgeoid.grd-A0/270-Ggradients.grd-Ne0.6-VTo find the azimuth orientations of seafloor fabric in the file topo.grd:grdgradienttopo.grd-Snao-Gazimuths.grd-V

gmt(l),gmtdefaults(l),grdhisteq(l),grdimage(l),grdview(l),grdvec-tor(l) GMT4.0 1 Oct 2004 GRDGRADIENT(l)

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