GRDIMAGE(1) Generic Mapping Tools GRDIMAGE(1)NAMEgrdimage - Create grayshaded or colored image from a 2-D netCDF grid
file
SYNOPSISgrdimage grd_z | grd_r grd_g grd_b -Ccptfile [ -D[r] ] -Jparameters [
-B[p|s]parameters ] [ -Ei|dpi ] [ -G[f|b]color ] [ -Iintensfile ] [ -K
] [ -M ] [ -N ] [ -O ] [ -P ] [ -Q ] [ -Rwest/east/south/north[r] ] [
-S[-]b|c|l|n[/threshold] ] [ -T ] [ -U[just/dx/dy/][c|label] ] [ -V ] [
-X[a|c|r][x-shift[u]] ] [ -Y[a|c|r][y-shift[u]] ] [ -ccopies ] [
-f[i|o]colinfo ] [ -r ]
DESCRIPTIONgrdimage reads one 2-D grid file and produces a gray-shaded (or col‐
ored) map by plotting rectangles centered on each grid node and assign‐
ing them a gray-shade (or color) based on the z-value. Alternatively,
grdimage reads three 2-D grid files with the red, green, and blue com‐
ponents directly (all must be in the 0-255 range). Optionally, illumi‐
nation may be added by providing a file with intensities in the (-1,+1)
range. Values outside this range will be clipped. Such intensity
files can be created from the grid using grdgradient and, optionally,
modified by grdmath or grdhisteq.
When using map projections, the grid is first resampled on a new rec‐
tangular grid with the same dimensions. Higher resolution images can be
obtained by using the -E option. To obtain the resampled value (and
hence shade or color) of each map pixel, its location is inversely pro‐
jected back onto the input grid after which a value is interpolated
between the surrounding input grid values. By default bi-cubic interpo‐
lation is used. Aliasing is avoided by also forward projecting the
input grid nodes. If two or more nodes are projected onto the same
pixel, their average will dominate in the calculation of the pixel
value. Interpolation and aliasing is controlled with the -S option.
The -R option can be used to select a map region larger or smaller than
that implied by the extent of the grid.
A (color) PostScript file is output.
grd_z | grd_r grd_g grd_b
2-D gridded data set (or red, green, blue grids) to be imaged
(See GRID FILE FORMATS below.)
-C name of the color palette table (for grd_z only).
-J Selects the map projection. Scale is UNIT/degree, 1:xxxxx, or
width in UNIT (upper case modifier). UNIT is cm, inch, or m,
depending on the MEASURE_UNIT setting in .gmtdefaults4, but this
can be overridden on the command line by appending c, i, or m to
the scale/width value. When central meridian is optional,
default is center of longitude range on -R option. Default
standard parallel is the equator. For map height, max dimen‐
sion, or min dimension, append h, +, or - to the width, respec‐
tively.
More details can be found in the psbasemap man pages.
CYLINDRICAL PROJECTIONS:
-Jclon0/lat0/scale (Cassini)
-Jcyl_stere/[lon0/[lat0/]]scale (Cylindrical Stereographic)
-Jj[lon0/]scale (Miller)
-Jm[lon0/[lat0/]]scale (Mercator)
-Jmlon0/lat0/scale (Mercator - Give meridian and standard paral‐
lel)
-Jo[a]lon0/lat0/azimuth/scale (Oblique Mercator - point and
azimuth)
-Jo[b]lon0/lat0/lon1/lat1/scale (Oblique Mercator - two points)
-Joclon0/lat0/lonp/latp/scale (Oblique Mercator - point and
pole)
-Jq[lon0/[lat0/]]scale (Cylindrical Equidistant)
-Jtlon0/[lat0/]scale (TM - Transverse Mercator)
-Juzone/scale (UTM - Universal Transverse Mercator)
-Jy[lon0/[lat0/]]scale (Cylindrical Equal-Area)
CONIC PROJECTIONS:
-Jblon0/lat0/lat1/lat2/scale (Albers)
-Jdlon0/lat0/lat1/lat2/scale (Conic Equidistant)
-Jllon0/lat0/lat1/lat2/scale (Lambert Conic Conformal)
-Jpoly/[lon0/[lat0/]]scale ((American) Polyconic)
AZIMUTHAL PROJECTIONS:
-Jalon0/lat0[/horizon]/scale (Lambert Azimuthal Equal-Area)
-Jelon0/lat0[/horizon]/scale (Azimuthal Equidistant)
-Jflon0/lat0[/horizon]/scale (Gnomonic)
-Jglon0/lat0[/horizon]/scale (Orthographic)
-Jglon0/lat0/altitude/azimuth/tilt/twist/Width/Height/scale
(General Perspective).
-Jslon0/lat0[/horizon]/scale (General Stereographic)
MISCELLANEOUS PROJECTIONS:
-Jh[lon0/]scale (Hammer)
-Ji[lon0/]scale (Sinusoidal)
-Jkf[lon0/]scale (Eckert IV)
-Jk[s][lon0/]scale (Eckert VI)
-Jn[lon0/]scale (Robinson)
-Jr[lon0/]scale (Winkel Tripel)
-Jv[lon0/]scale (Van der Grinten)
-Jw[lon0/]scale (Mollweide)
NON-GEOGRAPHICAL PROJECTIONS:
-Jp[a]scale[/origin][r|z] (Polar coordinates (theta,r))
-Jxx-scale[d|l|ppow|t|T][/y-scale[d|l|ppow|t|T]] (Linear, log,
and power scaling)
OPTIONS
No space between the option flag and the associated arguments.
-B Sets map boundary annotation and tickmark intervals; see the
psbasemap man page for all the details.
-D Specifies that the grid supplied is an image file to be read via
GDAL. Obviously this option will work only with GMT versions
built with GDAL support. The image can be indexed or true color
(RGB) and can be an URL of a remotely located file. That is -D
http://www.somewhere.com/image.jpg is a valid file syntax.
Note, however, that to use it this way you must not be blocked
by a proxy. If you are, chances are good that it can work by
setting the environmental variable http_proxy with the value
'your_proxy:port' Append r to use the region specified by -R to
apply to the image. For example, if you have used -Rd then the
image will be assigned the limits of a global domain. The inter‐
est of this mode is that you can project a raw image (an image
without referencing coordinates).
-E Sets the resolution of the projected grid that will be created
if a map projection other than Linear or Mercator was selected.
By default, the projected grid will be of the same size (rows
and columns) as the input file. Specify i to use the PostScript
image operator to interpolate the image at the device resolu‐
tion.
-G This option only applies when the resulting image otherwise
would consist of only two colors: black (0) and white (255). If
so, this option will instead use the image as a transparent mask
and paint the mask (or its inverse, with -Gb) with the given
color combination. (See SPECIFYING COLOR below).
-I Gives the name of a grid file with intensities in the (-1,+1)
range. [Default is no illumination].
-K More PostScript code will be appended later [Default terminates
the plot system].
-M Force conversion to monochrome image using the (television) YIQ
transformation. Cannot be used with -Q.
-N Do not clip the image at the map boundary (only relevant for
non-rectangular maps).
-O Selects Overlay plot mode [Default initializes a new plot sys‐
tem].
-P Selects Portrait plotting mode [Default is Landscape, see gmtde‐
faults to change this].
-Q Make grid nodes with z = NaN transparent, using the colormasking
feature in PostScript Level 3 (the PS device must support PS
Level 3).
-R xmin, xmax, ymin, and ymax specify the Region of interest. For
geographic regions, these limits correspond to west, east,
south, and north and you may specify them in decimal degrees or
in [+-]dd:mm[:ss.xxx][W|E|S|N] format. Append r if lower left
and upper right map coordinates are given instead of w/e/s/n.
The two shorthands -Rg and -Rd stand for global domain (0/360
and -180/+180 in longitude respectively, with -90/+90 in lati‐
tude). Alternatively, specify the name of an existing grid file
and the -R settings (and grid spacing, if applicable) are copied
from the grid. For calendar time coordinates you may either
give (a) relative time (relative to the selected TIME_EPOCH and
in the selected TIME_UNIT; append t to -JX|x), or (b) absolute
time of the form [date]T[clock] (append T to -JX|x). At least
one of date and clock must be present; the T is always required.
The date string must be of the form [-]yyyy[-mm[-dd]] (Gregorian
calendar) or yyyy[-Www[-d]] (ISO week calendar), while the clock
string must be of the form hh:mm:ss[.xxx]. The use of delim‐
iters and their type and positions must be exactly as indicated
(however, input, output and plot formats are customizable; see
gmtdefaults). You may ask for a larger w/e/s/n region to have
more room between the image and the axes. A smaller region than
specified in the grid file will result in a subset of the grid
[Default is the region given by the grid file].
-S Select the interpolation mode by adding b for B-spline smooth‐
ing, c for bicubic interpolation, l for bilinear interpolation,
or n for nearest-neighbor value (for example to plot categorical
data). Optionally, prepend - to switch off antialiasing. Add
/threshold to control how close to nodes with NaNs the interpo‐
lation will go. A threshold of 1.0 requires all (4 or 16) nodes
involved in interpolation to be non-NaN. 0.5 will interpolate
about half way from a non-NaN value; 0.1 will go about 90% of
the way, etc. [Default is bicubic interpolation with antialias‐
ing and a threshold of 0.5].
-T This option has become OBSOLETE. Use grdview -T instead. Use
-Sn to plot near-neighbor values only (use -E to increase the
resolution). Use -Sn -Q to obtain something similar to the old
option -Ts. The option -To is no longer supported.
-U Draw Unix System time stamp on plot. By adding just/dx/dy/, the
user may specify the justification of the stamp and where the
stamp should fall on the page relative to lower left corner of
the plot. For example, BL/0/0 will align the lower left corner
of the time stamp with the lower left corner of the plot.
Optionally, append a label, or c (which will plot the command
string.). The GMT parameters UNIX_TIME, UNIX_TIME_POS, and
UNIX_TIME_FORMAT can affect the appearance; see the gmtdefaults
man page for details. The time string will be in the locale set
by the environment variable TZ (generally local time).
-V Selects verbose mode, which will send progress reports to stderr
[Default runs "silently"].
-X -Y Shift plot origin relative to the current origin by (x-shift,y-
shift) and optionally append the length unit (c, i, m, p). You
can prepend a to shift the origin back to the original position
after plotting, or prepend r [Default] to reset the current
origin to the new location. If -O is used then the default (x-
shift,y-shift) is (0,0), otherwise it is (r1i, r1i) or (r2.5c,
r2.5c). Alternatively, give c to align the center coordinate (x
or y) of the plot with the center of the page based on current
page size.
-c Specifies the number of plot copies. [Default is 1].
-f Special formatting of input and/or output columns (time or geo‐
graphical data). Specify i or o to make this apply only to
input or output [Default applies to both]. Give one or more
columns (or column ranges) separated by commas. Append T (abso‐
lute calendar time), t (relative time in chosen TIME_UNIT since
TIME_EPOCH), x (longitude), y (latitude), or f (floating point)
to each column or column range item. Shorthand -f[i|o]g means
-f[i|o]0x,1y (geographic coordinates).
GRID FILE FORMATS
GMT is able to recognize many of the commonly used grid file formats,
as well as the precision, scale and offset of the values contained in
the grid file. When GMT needs a little help with that, you can 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. 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. 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.
IMAGING GRIDS WITH NANS
Be aware that if your input grid contains patches of NaNs, these
patches can become larger as a consequence of the resampling that must
take place with most map projections. Because grdimage uses the Post‐
Script colorimage operator, for most non-linear projections we must
resample your grid onto an equidistant rectangular lattice. If you
find that the NaN areas are not treated adequately, consider (a) use a
linear projection, or (b) use grdview -Ts instead.
EXAMPLES
To gray-shade the file hawaii_grav.grd with shades given in shades.cpt
on a Lambert map at 1.5 cm/degree along the standard parallels 18 and
24, and using 1 degree tickmarks:
grdimage hawaii_grav.grd -Jl 18/24/1.5c -C shades.cpt -B 1 >
hawaii_grav_image.ps
To create an illuminated color PostScript plot of the gridded data set
image.grd, using the intensities provided by the file intens.grd, and
color levels in the file colors.cpt, with linear scaling at 10 inch/x-
unit, tickmarks every 5 units:
grdimage image.grd -Jx 10i -C colors.cpt -I intens.grd -B 5 > image.ps
To create an false color PostScript plot from the three grid files
red.grd, green.grd, and blue.grd, with linear scaling at 10 inch/x-
unit, tickmarks every 5 units:
grdimage red.grd green.grd blue.grd -Jx 10i -B 5 > rgbimage.ps
When GDAL support is built in: To create a sinusoidal projection of a
remotely located Jessica Rabbit
grdimage-JI15c-Rd-Dr http://larryfire.files.word‐
press.com/2009/07/untooned_jessicarabbit.jpg -P > jess.ps
SEE ALSOGMT(1), gmt2rgb(1), grdcontour(1), grdview(1), grdgradient(1), grdhis‐
teq(1)GMT 4.5.14 1 Nov 2015 GRDIMAGE(1)
