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GRDGRADIENT(1) Generic Mapping Tools GRDGRADIENT(1)
NAME
grdgradient - Compute directional derivative or gradient from 2-D grid
file representing z(x,y)
SYNOPSIS
grdgradient in_grdfile -Gout_grdfile [ -Aazim[/azim2] ] [ -D[c][o][n] ]
[ -E[s|p]azim/elev[/ambient/diffuse/specular/shine] ] [ -Lflag ] [ -M ]
[ -N[e][t][amp][/sigma[/offset]] ] [ -Sslopefile ] [ -V ]
DESCRIPTION
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 grid file from which to compute directional derivative.
(See GRID FILE FORMATS below).
-G Name of the output grid file for the directional derivative.
(See GRID FILE FORMATS below).
OPTIONS
No space between the option flag and the associated arguments.
-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 up-slope 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).
-E 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.)
-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 grid file with scalar magnitudes of gradient
vectors. Requires -D but makes -G optional.
-V Selects verbose mode, which will send progress reports to stderr
[Default runs "silently"].
HINTS
If you don't know what OPT(N) options to use to make an intensity file
for grdimage or grdview, a good first try is -Ne 0.6.
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.
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 2- or 4-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. When
reading grids, the format is generally automatically recognized. If
not, the same suffix can be added to input grid file names. See
grdreformat(1) and Section 4.17 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 grdreformat(1) and Section
4.18 of the GMT Technical Reference and Cookbook for more information,
particularly on how to read splices of 3-, 4-, or 5-dimensional grids.
EXAMPLES
To make a file for illuminating the data in geoid.grd using exp-
normalized gradients imitating light sources in the north and west
directions:
grdgradient geoid.grd -A 0/270 -G gradients.grd -Ne0.6 -V
To find the azimuth orientations of seafloor fabric in the file
topo.grd:
grdgradient topo.grd -Dno -G azimuths.grd -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(1), gmtdefaults(1), grdhisteq(1), grdimage(1), grdview(1),
grdvector(1)
GMT 4.5.14 1 Nov 2015 GRDGRADIENT(1)