GRDPROJECT(1) Generic Mapping Tools GRDPROJECT(1)NAMEgrdproject - Forward and Inverse map transformation of 2-D grid files
SYNOPSISgrdproject in_grdfile -Gout_grdfile -Jparameters [ -A[k|m|n|i|c|p] ] [
-C[dx/dy] ] [ -Dxinc[unit][=|+][/yinc[unit][=|+]] ] [ -Edpi ] [ -F ] [
-I ] [ -Mc|i|m|p ] [ -Nnx/ny ] [ -Rwest/east/south/north[r] ] [
-S[-]b|c|l|n[/threshold] ] [ -V ]
DESCRIPTIONgrdproject will do one of two things depending whether -I has been set.
If set, it will transform a gridded data set from a rectangular coordi‐
nate system onto a geographical system by resampling the surface at the
new nodes. If not set, it will project a geographical gridded data set
onto a rectangular grid. To obtain the value at each new node, 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 pro‐
jected onto the same new node, their average will dominate in the cal‐
culation of the new node value. Interpolation and aliasing is con‐
trolled with the -S option. The new node spacing may be determined in
one of several ways by specifying the grid spacing, number of nodes, or
resolution. Nodes not constrained by input data are set to NaN.
The -R option can be used to select a map region larger or smaller than
that implied by the extent of the grid file.
in_grdfile
2-D binary grid file to be transformed. (See GRID FILE FORMATS
below.)
-G Specify the name of the output grid file. (See GRID FILE FOR‐
MATS below.)
-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.
-A Force 1:1 scaling, i.e., output (or input, see -I) data are in
actual projected meters. To specify other units, append k (km),
m (mile),n (nautical mile), i (inch), c (cm), or p (points).
Without -A, the output (or input, see -I) are in the units spec‐
ified by MEASURE_UNIT (but see -M).
-C Let projected coordinates be relative to projection center
[Default is relative to lower left corner]. Optionally, add
offsets in the projected units to be added (or subtracted when
-I is set) to (from) the projected coordinates, such as false
eastings and northings for particular projection zones [0/0].
-D x_inc [and optionally y_inc] is the grid spacing. Optionally,
append a suffix modifier. Geographical (degrees) coordinates:
Append m to indicate arc minutes or c to indicate arc seconds.
If one of the units e, k, i, or n is appended instead, the
increment is assumed to be given in meter, km, miles, or nauti‐
cal miles, respectively, and will be converted to the equivalent
degrees longitude at the middle latitude of the region (the con‐
version depends on ELLIPSOID). If /y_inc is given but set to 0
it will be reset equal to x_inc; otherwise it will be converted
to degrees latitude. All coordinates: If = is appended then the
corresponding max x (east) or y (north) may be slightly adjusted
to fit exactly the given increment [by default the increment may
be adjusted slightly to fit the given domain]. Finally, instead
of giving an increment you may specify the number of nodes
desired by appending + to the supplied integer argument; the
increment is then recalculated from the number of nodes and the
domain. The resulting increment value depends on whether you
have selected a gridline-registered or pixel-registered grid;
see Appendix B for details. Note: if -Rgrdfile is used then
grid spacing has already been initialized; use -D to override
the values.
-E Set the resolution for the new grid in dots per inch.
-F Toggle between pixel and gridline node registration [Default is
same as input].
-I Do the Inverse transformation, from rectangular to geographical.
-M Append c, i, or m to indicate that cm, inch, or meter should be
the projected measure unit [Default is set by MEASURE_UNIT in
.gmtdefaults4]. Cannot be used with -A.
-N Set the number of grid nodes in the new grid.
-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 to project only a subset of the grid
by specifying a smaller input w/e/s/n region [Default is the
region given by the grid file]. Special case for the UTM pro‐
jection: If -C and -I are used and -R is not given then the
region is set to coincide with the given UTM zone so as to pre‐
serve the full ellipsoidal solution (See RESTRICTIONS for more
information).
-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].
-V Selects verbose mode, which will send progress reports to stderr
[Default runs "silently"].
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 facili‐
tates 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 transform the geographical grid dbdb5.grd onto a pixel Mercator grid
at 300 dpi, run
grdproject dbdb5.grd -R 20/50/12/25 -Jm 0.25i -E 300 -F-G
dbdb5_merc.grd
To inversely transform the file topo_tm.grd back onto a geographical
grid, use
grdproject topo_tm.grd -R-80/-70/20/40 -Jt-75/1:500000 -I -D 5m -V-G
topo.grd
This assumes, of course, that the coordinates in topo_tm.grd were cre‐
ated with the same projection parameters.
To inversely transform the file topo_utm.grd (which is in UTM meters)
back to a geographical grid we specify a one-to-one mapping with meter
as the measure unit:
grdproject topo_utm.grd -R 203/205/60/65 -Ju 5/1:1 -I -Mm -G topo.grd
-V
RESTRICTIONS
The boundaries of a projected (rectangular) data set will not necessar‐
ily give rectangular geographical boundaries (Mercator is one excep‐
tion). In those cases some nodes may be unconstrained (set to NaN).
To get a full grid back, your input grid may have to cover a larger
area than you are interested in.
For some projections, a spherical solution may be used despite the user
having selected an ellipsoid. This occurs when the users -R setting
implies a region that exceeds the domain in which the ellipsoidal
series expansions are valid. These are the conditions: (1) Lambert
Conformal Conic (-JL) and Albers Equal-Area (-JB) will use the spheri‐
cal solution when the map scale exceeds 1.0E7. (2) Transverse Mercator
(-JT) and UTM (-JU) will will use the spherical solution when either
the west or east boundary given in -R is more than 10 degrees from the
central meridian, and (3) same for Cassini (-JC) but with a limit of
only 4 degrees.
SEE ALSOGMT(1), gmtdefaults(1), mapproject(1)GMT 4.5.14 1 Nov 2015 GRDPROJECT(1)
