NAME
img2mercgrd - Extract region of img, preserving Mercator, save as grd
SYNOPSIS
img2mercgrd imgfile -Ggrdfile -Rwest/east/south/north -Ttype [ -Nnavg ]
[ -Sscale ] [ -V ] [ -mminutes ] [ -xmaxlon ] [ -yminlat/maxlat ]
DESCRIPTION
img2mercgrd reads an img format file and creates a grdfile. The Spheri-
cal Mercator projection of the img file is preserved, so that the
region -R set by the user is modified slightly; the modified region
corresponds to the edges of pixels [or groups of navg pixels]. The grd-
file header is set so that the x and y axis lengths represent distance
from the west and south edges of the image, measured in user default
units, with -Jm1 and the adjusted -R. By setting the gmtdefaults ELLIP-
SOID = Sphere, the user can make overlays with the adjusted -R so that
they match. See EXAMPLES below. The adjusted -R is also written in the
grdheader remark, so it can be found later. The -Ttype selects all data
or only data at constrained pixels, and can be used to create a grid of
1s and 0s indicating constraint locations. The output grd file is pixel
registered; it inherits this from the img file.
imgfile
An img format file such as the marine gravity or seafloor topog-
raphy fields estimated from satellite altimeter data by Sandwell
and Smith. If the user has set an environment variable
GMT_IMGDIR, then img2mercgrd will try to find imgfile in
$GMT_IMGDIR; else it will try to open imgfile directly.
-G grdfile is the name of the output grdfile.
-R west, east, south, and north specify the Region of interest. To
specify boundaries in degrees and minutes, use the dd:mm format.
-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; all more recent
files do not support this choice) 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
grd 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
-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 grd
-x 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].
-y Indicate minlat/maxlat as the latitude extent of the input img
file. All versions to date have used -72.006/72.006. [Default is
-72.006/72.006].
EXAMPLES
To extract data in the region -R-40/40/-70/-30 from world_grav.img.7.2,
run
img2mercgrd world_grav.img.7.2 -Gmerc_grav.grd -R-40/40/-70/-30 -T1 -V
Note that the -V option tells us that the range was adjusted to
-R-40/40/-70.0004681551/-29.9945810754. We can also use grdinfo to find
that the grd file header shows its region to be -R0/80/0/67.9666667
This is the range of x,y we will get from a Spherical Mercator projec-
tion using -R-40/40/-70.0004681551/-29.9945810754 and -Jm1. Thus, to
take ship.lonlatgrav and use it to sample the merc_grav.grd, we can do
this:
gmtset ELLIPSOID Sphere
mapproject -R-40/40/-70.0004681551/-29.9945810754 -Jm1 ship.lonlatgrav
| grdtrack -Gmerc_grav.grd | mapproject
-R-40/40/-70.0004681551/-29.9945810754 -Jm1 -I > ship.lonlatgravsat
It is recommended to use the above method of projecting and unproject-
ing the data in such an application, because then there is only one
interpolation step (in grdtrack). If one first tries to convert the grd
file to lon,lat and then sample it, there are two interpolation steps
(in conversion and in sampling).
To make a lon,lat grid from the above grid we can use
grdproject merc_grav.grd -R-40/40/-70.0004681551/-29.9945810754 -Jm1 -I
-F -D2m -Ggrav.grd
In some cases this will not be easy as the -R in the two coordinate
systems may not align well. When this happens, we can also use (in
fact, it may be always better to use)
grd2xyz merc_grav.grd | mapproject
-R-40/40/-70.0004681551/-29.994581075 -Jm1 -I | surface -R-40/40/-70/70
-I2m -Ggrav.grd
To make a Mercator map of the above region, suppose our .gmtdefaults4
MEASURE_UNIT is inch. Then since the above merc_grav.grd file is pro-
jected with -Jm1 it is 80 inches wide. We can make a map 8 inches wide
by using -Jx0.1 on any map programs applied to this grid (e.g., grdcon-
tour, grdimage, grdview), and then for overlays which work in lon,lat
(e.g., psxy, pscoast) we can use the above adjusted -R and -Jm0.1 to
get the two systems to match up.
img2mercgrd world_grav.img.7.2 -Gmerc_grav_2.grd -R-40/40/-70/-30 -T1
-N3 -V
This time we find the adjusted region is
-R-40/40/-70.023256525/-29.9368261101 and the output is 800 by 601 pix-
els, a better size for us. Now we can create an artificial illumination
file for this using grdgradient:
grdgradient merc_grav_2.grd -Gillum.grd -A0/270 -Ne0.6
and if we also have a cpt file called "grav.cpt" we can create a color
shaded relief map like this:
grdimage merc_grav_2.grd -Iillum.grd -Cgrav.cpt -Jx0.1 -K > map.ps
psbasemap -R-40/40/-70.023256525/-29.9368261101 -Jm0.1 -Ba10 -O >>
map.ps
Suppose you want to obtain only the constrained data values from an img
file, in lat/lon coordinates. Then run img2mercgrd with the -T2
option, use grd2xyz to dump the values, pipe through grep -v NaN to
eliminate NaNs, and pipe through mapproject with the inverse projection
as above.
SEE ALSO
gmt(GMTMANSECTION), grdproject(GMTMANSECTION), mapproject(GMTMANSEC-
TION)
GMT4.0 1 Oct 2004 IMG2MERCGRD(GMTMANSECTION)
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