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Manual Reference Pages  -  GRDIMAGE (1)


grdimage - Create grayshaded or colored image from a 2-D netCDF grid file


Grid File Formats
Imaging Grids With Nans


grdimage 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 ]


grdimage reads one 2-D grid file and produces a gray-shaded (or colored) map by plotting rectangles centered on each grid node and assigning 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 components directly (all must be in the 0-255 range). Optionally, illumination 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 rectangular 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 projected back onto the input grid after which a value is interpolated between the surrounding input grid values. By default bi-cubic interpolation 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 dimension, or min dimension, append h, +, or - to the width, respectively.
More details can be found in the psbasemap man pages.


-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 parallel)
-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)


-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)


-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)


-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)


-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)


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 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 interest 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 resolution.
-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 system].
-P Selects Portrait plotting mode [Default is Landscape, see gmtdefaults 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[][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 latitude). 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 delimiters 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 smoothing, 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 interpolation 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 antialiasing 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 geographical 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 (absolute 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).


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.


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 PostScript 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.


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 >

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 >

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 >

When GDAL support is built in: To create a sinusoidal projection of a remotely located Jessica Rabbit

grdimage -JI15c -Rd -Dr -P >


GMT(1), gmt2rgb(1), grdcontour(1), grdview(1), grdgradient(1), grdhisteq(1)
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GMT 4.5.14 GRDIMAGE (1) 1 Nov 2015

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