Query: grdgradient
OS: debian
Section: 1gmt
Format: Original Unix Latex Style Formatted with HTML and a Horizontal Scroll Bar
GRDGRADIENT(l) GRDGRADIENT(l)NAMEgrdgradient - Compute directional derivative or gradient from 2-D grd file representing z(x,y)SYNOPSISgrdgradient in_grdfile -Gout_grdfile [ -Aazim[/azim2] ] [ -D[c][o][n] ] [ -Lflag ] [ -M ] [ -N[e][t][amp][/sigma[/offset]] ] [ -Sslopefile ] [ -V ]DESCRIPTIONgrdgradient 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). in_grdfile 2-D grd file from which to compute directional derivative. -G Name of the output grdfile for the directional derivative.OPTIONSNo space between the option flag and the associated arguments. Use upper case for the option flags and lower case for modifiers. -A Azimuthal direction for a directional derivative; azim is 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 the azim direction, the correct sense for shading the illumination of an image (see grdimage and grdview) by a light source above the x,y plane shining from the azim direction. Optionally, supply two azimuths, -Aazim/azim2, in which case the gradients in each of these directions are calcu- lated and the one larger in magnitude is retained; this is useful for illuminating data with two directions of lineated structures, e.g. -A0/270 illuminates from the north (top) and west (left). -D Find the direction of the gradient of the data. By default, the directions are measured clockwise from north, as azim in -A above. Append c to use conventional cartesian angles measured counterclockwise from the positive x (east) direction. Append o to report orientations (0-180) rather than directions (0-360). Append n to add 90 degrees to all angles (e.g., to give orientation of lineated features). -L Boundary condition flag may be x or y or xy indicating data is periodic in range of x or y or both, or flag may be g indicating geo- graphical conditions (x and y are lon and lat). [Default uses "natural" conditions (second partial derivative normal to edge is zero).] -M By default the units of grdgradient are 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 of grdgradient are in z_units/meter. -N Normalization. [Default: no normalization.] The actual gradients g are offset and scaled to produce normalized gradients gn with a maximum output magnitude of amp. If amp is not given, default amp = 1. If offset is not given, it is set to the average of g. -N yields gn = amp * (g - offset)/max(abs(g - offset)). -Ne normalizes using a cumulative Laplace distribution yielding gn = amp * (1.0 - exp(sqrt(2) * (g - offset)/sigma)) where sigma is estimated using the L1 norm of (g - offset) if it is not given. -Nt normal- izes using a cumulative Cauchy distribution yielding gn = (2 * amp / PI) * atan( (g - offset)/sigma) where sigma is estimated using the L2 norm of (g - offset) if it is not given. -S Name of output grdfile with scalar magnitudes of gradient vectors. Requires -D. -V Selects verbose mode, which will send progress reports to stderr [Default runs "silently"].HINTSIf you don't know what -N options to use to make an intensity file for ' grdimage or grdview, 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 -N option and supply the same value of sigma and offset to grdgradient for each map. A good guess is offset = 0 and sigma found by grdinfo -L2 or -L1 applied to an unnormalized gradient grd. If you simply need the x- or y-derivatives of the grid, use grdmath.EXAMPLESTo make a file for illuminating the data in geoid.grd using exp-normalized gradients imitating light sources in the north and west direc- tions, do grdgradient geoid.grd -A0/270 -Ggradients.grd -Ne0.6 -V To find the azimuth orientations of seafloor fabric in the file topo.grd, try grdgradient topo.grd -Snao -Gazimuths.grd -VSEE ALSOgmt(1gmt), gmtdefaults(1gmt), grdhisteq(1gmt), grdimage(1gmt), grdview(1gmt), grdvector(1gmt) 1 Jan 2004 GRDGRADIENT(l)