NAME¶
img2grd - Extract region of img in Mercator or geographic form
SYNOPSIS¶
img2grd imgfile -Ggrdfile
-Rwest/
east/
south/
north[
r]
-T type [
-C ] [
-D[
minlat/maxlat] ] [
-E ] [
-L ] [
-M ] [
-Nnavg ] [
-Sscale ] [
-V ] [
-Wmaxlon ] [
-mminutes ]
DESCRIPTION¶
img2grd is a front-end to
img2mercgrd which reads an img format
file and creates a grid file. The
-M option dictates whether or not the
Spherical Mercator projection of the img file is preserved.
- imgfile
- An img format file such as the marine gravity or seafloor topography
fields estimated from satellite altimeter data by Sandwell and Smith. If
the user has set an environment variable $GMT_DATADIR, then
img2mercgrd will try to find imgfile in $GMT_DATADIR;
else it will try to open imgfile directly.
- -G
- grdfile is the name of the output grid file.
- -R
- west, east, south, and north specify the Region of interest,
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 latitude).
Alternatively, specify the name of an existing grid file and the -R
settings (and grid spacing, if applicable) are copied from the grid.
- -T
- type handles the encoding of constraint information. type =
0 indicates that no such information is encoded in the img file (used for
pre-1995 versions of the gravity data) and gets all data. type >
0 indicates that constraint information is encoded (1995 and later
(current) versions of the img files) so that one may produce a grid file
as follows: -T1 gets data values at all points,
-T2 gets data values at constrained points and NaN at
interpolated points; -T3 gets 1 at constrained points and 0
at interpolated points.
OPTIONS¶
- -C
- Set the x and y Mercator coordinates relative to projection center
[Default is relative to lower left corner of grid]. Requires
-M.
- -D
- Use the extended latitude range -80.738/+80.738. Alternatively, append
minlat/maxlat as the latitude extent of the input img file.
[Default is -72.006/72.006].
- -E
- Can be used when -M is not set to force the final grid to have the
exact same region as requested with -R. By default, the final
region is a direct projection of the original Mercator region and will
typically extend slightly beyond the requested latitude range, and
furthermore the grid increment in latitude does not match the longitude
increment. However, the extra resampling introduces small interpolation
errors and should only be used if the output grid must match the requested
region and have x_inc = y_inc. In this case the region set by -R
must be given in multiples of the increment (.e.g, -R
0/45/45/72).
- -L
- With no other arguments, list all *.img files found in the directory
pointed to by $GMT_DATADIR, or the current directory if not
defined. Ignored if other options are present on the command line.
- -M
- Output a Spherical Mercator grid [Default is a geographic lon/lat
grid].
- -N
- Average the values in the input img pixels into navg by navg
squares, and create one output pixel for each such square. If used with
-T3 it will report an average constraint between 0 and 1. If
used with -T2 the output will be average data value or NaN
according to whether average constraint is > 0.5. navg must
evenly divide into the dimensions of the imgfile in pixels. [Default
1 does no averaging].
- -S
- Multiply the img file values by scale before storing in grid file.
[Default is 1.0]. For recent img files: img topo files are stored in
(corrected) meters [ -S 1]; free-air gravity files in mGal*10 [
-S 0.1 to get mGal]; vertical deflection files in microradians*10 [
-S 0.1 to get microradians], vertical gravity gradient files in
Eotvos*50 [ -S 0.02 to get Eotvos, or -S 0.002 to get
mGal/km]).
- -V
- Selects verbose mode, which will send progress reports to stderr [Default
runs "silently"]. Particularly recommended here, as it is
helpful to see how the coordinates are adjusted.
- -m
- Indicate minutes as the width of an input img pixel in minutes of
longitude. [Default is 2.0].
- -W
- Indicate maxlon as the maximum longitude extent of the input img
file. Versions since 1995 have had maxlon = 360.0, while some
earlier files had maxlon = 390.0. [Default is 360.0].
EXAMPLES¶
To extract data in the region
-R-40/40/-70/-30 from
world_grav.img.7.2 and preserve the Mercator gridding:
img2grd world_grav.img.7.2
-G merc_grav.grd
-R-40/40/-70/-30
-M -T 1
-V
Without the
-M option the same command will yield a geographic grid.
SEE ALSO¶
GMT(1),
img2mercgrd(1)