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grdgradient - Compute directional derivative or gradient from a grid


grdgradient in_grdfile out_grdfile [ azim[/azim2] ] [ [a][c][o][n] ] [
[s|p]azim/elev[/ambient/diffuse/specular/shine] ] [ flag ] [ [e][t][amp][/sigma[/offset]]
] [ region ] [ slopefile ] [ [level] ] [ -fg ] [ -n<flags> ]

Note: No space is allowed between the option flag and the associated arguments.


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


2-D grid file from which to compute directional derivative. (See GRID FILE FORMATS

Name of the output grid file for the directional derivative. (See GRID FILE FORMATS


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).

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 ).

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. azim
and elev are the azimuth and elevation of light vector. Optionally, supply ambient
diffuse specular shine which are parameters that control the reflectance properties
of the surface. Default values are: 0.55/0.6/0.4/10 To leave some of the values
untouched, specify = as the new value. For example -E60/30/=/0.5 sets the azim elev
and diffuse to 60, 30 and 0.5 and leaves the other reflectance parameters
untouched. Append s to use a simpler Lambertian algorithm. Note that with this form
you only have to provide the azimuth and elevation parameters. Append p to use 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).]

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.

-R[unit]xmin/xmax/ymin/ymax[r] (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.

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
use just -).

-+ 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 options, then

Print GMT version and exit.

Print full path to GMT share directory and exit.


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.


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.


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. To specify the precision, scale and offset, the user should
add the suffix =id[/scale/offset[/nan]], where id is a two-letter identifier of the grid
type and precision, and scale and offset are optional scale factor and offset to be
applied to all grid values, and nan is the value used to indicate missing data. In case
the two characters id is not provided, as in =/scale than a id=nf is assumed. When
reading grids, the format is generally automatically recognized. If not, the same suffix
can be added to input grid file names. See grdconvert and Section grid-file-format of the
GMT Technical Reference and Cookbook for more information.

When reading a netCDF file that contains multiple grids, GMT will read, by default, the
first 2-dimensional grid that can find in that file. To coax GMT into reading another
multi-dimensional variable in the grid file, append ?varname to the file name, where
varname is the name of the variable. Note that you may need to escape the special meaning
of ? in your shell program by putting a backslash in front of it, or by placing the
filename and suffix between quotes or double quotes. The ?varname suffix can also be used
for output grids to specify a variable name different from the default: "z". See
grdconvert and Sections modifiers-for-CF and grid-file-format of the GMT Technical
Reference and Cookbook for more information, particularly on how to read splices of 3-,
4-, or 5-dimensional grids.


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


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)

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