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grdgradient - Compute directional derivative or gradient from 2-D grd file representing z(x,y)
grdgradient in_grdfile -Gout_grdfile [ -Aazim[/azim2] ] [ -D[c][o][n] ] [ -Lflag ] [ -M ] [ -N[e][t][amp][/sigma[/offset]] ] [ -Sslopefile ] [ -V ]
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). in_grdfile 2-D grd file from which to compute directional derivative. -G Name 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. -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 calculated 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 geographical 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 normalizes 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"].
If 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.
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 -V To find the azimuth orientations of seafloor fabric in the file topo.grd, try grdgradient topo.grd -Snao -Gazimuths.grd -V