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,
-R0/45/45/72).
- -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 [ -S1]; free-air gravity files in
mGal*10 [ -S0.1 to get mGal]; vertical deflection files in
microradians*10 [ -S0.1 to get microradians], vertical gravity
gradient files in Eotvos*50 [ -S0.02 to get Eotvos, or
-S0.002 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
-Gmerc_grav.grd
-R-40/40/-70/-30
-M -T1
-V
Without the
-M option the same command will yield a geographic grid.
SEE ALSO¶
GMT(1),
img2mercgrd(1)