Provided by: gmt-common_5.4.3+dfsg-1_all 
NAME
grdgradient - Compute directional derivative or gradient from a grid
SYNOPSIS
grdgradient in_grdfile -Gout_grdfile [ -Aazim[/azim2] ] [ -D[a][c][o][n] ] [
-E[m|s|p]azim/elev[+aambient][+ddiffuse][+pspecular][+sshine] ] [ -Lflag ] [
-N[e|t][amp][+ssigma][+ooffset] ] [ -Rregion ] [ -Sslopefile ] [ -V[level] ] [ -fg ] [
-nflags ]
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 to find 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).
OPTIONAL ARGUMENTS
-Aazim[/azim2]
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). Finally, if azim is a file it
must be a grid of the same domain, spacing and registration as in_grdfile and we
will update the azimuth at each output node when computing the directional
derivatives.
-D[a][c][o][n]
Find the direction of the positive (up-slope) gradient of the data. To instead
find the aspect (the down-slope direction), use -Da. By default, 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 local strikes of the surface ).
-E[m|s|p]azim/elev[+aambient][+ddiffuse][+pspecular][+sshine]
Compute Lambertian radiance appropriate to use with grdimage and grdview. The
Lambertian Reflection assumes an ideal surface that reflects all the light that
strikes it and the surface appears equally bright from all viewing directions.
Here, azim and elev are the azimuth and elevation of the light vector. Optionally,
supply ambient [0.55], diffuse [0.6], specular [0.4], or shine [10], which are
parameters that control the reflectance properties of the surface. Default values
are given in the brackets. Use -Es for a simpler Lambertian algorithm. Note that
with this form you only have to provide azimuth and elevation. Alternatively, use
-Ep for the Peucker piecewise linear approximation (simpler but faster algorithm;
in this case the azim and elev are hardwired to 315 and 45 degrees. This means that
even if you provide other values they will be ignored.)
-Lflag 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).]
-N[e|t][amp][+ssigma][+ooffset]
Normalization. [Default is 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.
-Rxmin/xmax/ymin/ymax[+r][+uunit] (more …)
Specify the region of interest. Using the -R option will select a subsection of
in_grdfile grid. If this subsection exceeds the boundaries of the grid, only the
common region will be extracted.
-Sslopefile
Name of output grid file with scalar magnitudes of gradient vectors. Requires -D
but makes -G optional.
-V[level] (more …)
Select verbosity level [c].
-fg Geographic grids (dimensions of longitude, latitude) will be converted to meters
via a “Flat Earth” approximation using the current ellipsoid parameters.
-n[b|c|l|n][+a][+bBC][+c][+tthreshold] (more …)
Select interpolation mode for grids.
-^ or just -
Print a short message about the syntax of the command, then exits (NOTE: on Windows
just use -).
-+ or just +
Print an extensive usage (help) message, including the explanation of any
module-specific option (but not the GMT common options), then exits.
-? or no arguments
Print a complete usage (help) message, including the explanation of all options,
then exits.
GRID DISTANCE UNITS
If the grid does not have meter as the horizontal unit, append +uunit to the input file
name to convert from the specified unit to meter. If your grid is geographic, convert
distances to meters by supplying -fg instead.
HINTS
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.
Usually 255 shades are more than enough for visualization purposes. You can save 75% disk
space by appending =nb/a to the output filename out_grdfile.
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.
GRID FILE FORMATS
By default GMT writes out grid as single precision floats in a COARDS-complaint netCDF
file format. However, GMT is able to produce grid files in many other commonly used grid
file formats and also facilitates so called “packing” of grids, writing out floating point
data as 1- or 2-byte integers. (more …)
EXAMPLES
To make a file for illuminating the data in geoid.nc using exp- normalized gradients in
the range [-0.6,0.6] imitating light sources in the north and west directions:
gmt grdgradient geoid.nc -A0/270 -Ggradients.nc=nb/a -Ne0.6 -V
To find the azimuth orientations of seafloor fabric in the file topo.nc:
gmt grdgradient topo.nc -Dno -Gazimuths.nc -V
REFERENCES
Horn, B.K.P., Hill-Shading and the Reflectance Map, Proceedings of the IEEE, Vol. 69, No.
1, January 1981, pp. 14-47. (http://people.csail.mit.edu/bkph/papers/Hill-Shading.pdf)
SEE ALSO
gmt, gmt.conf grdhisteq, grdinfo, grdmath, grdimage, grdview, grdvector
COPYRIGHT
2018, P. Wessel, W. H. F. Smith, R. Scharroo, J. Luis, and F. Wobbe