IMG2MERCGRD(1) Generic Mapping Tools IMG2MERCGRD(1)NAMEimg2mercgrd - Extract region of img, preserving Mercator, save as grd
SYNOPSISimg2mercgrd imgfile -Ggrdfile -Rwest/east/south/north[r] -Ttype [ -C ]
[ -D[minlat/maxlat] ] [ -Nnavg ] [ -Sscale ] [ -V ] [ -Wmaxlon ] [
-mminutes ]
DESCRIPTIONimg2mercgrd reads an img format file and creates a grid file. The
Spherical 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
grid file header is set so that the x and y axis lengths represent dis‐
tance from the west and south edges of the image, measured in user
default units, with -Jm 1 and the adjusted -R. By setting the default
ELLIPSOID = 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 grid
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 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; 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
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 cen‐
ter (lon = lat = 0) [Default is relative to lower left corner of
grid].
-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].
-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]. (img topo files are stored in (cor‐
rected) meters; gravity files in mGal*10; vertical deflection
files in microradians*10, vertical gravity gradient files in
Eotvos*10. Use -S 0.1 for those files.)
-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,
run
img2mercgrd world_grav.img.7.2 -G merc_grav.grd -R-40/40/-70/-30 -T 1
-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 grid file header shows its region to be -R
0/80/0/67.9666667 This is the range of x,y we will get from a Spheri‐
cal Mercator projection using -R-40/40/-70.0004681551/-29.9945810754
and -Jm 1. 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 -Jm 1 ship.lonlatgrav
| grdtrack -G merc_grav.grd | mapproject
-R-40/40/-70.0004681551/-29.9945810754 -Jm 1 -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
grid 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 -Jm 1
-I -F -D 2m -G grav.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 -Jm 1 -I | surface
-R-40/40/-70/70 -I 2m -G grav.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 -Jm 1 it is 80 inches wide. We can make a map 8 inches
wide by using -Jx 0.1 on any map programs applied to this grid (e.g.,
grdcontour, grdimage, grdview), and then for overlays which work in
lon,lat (e.g., psxy, pscoast) we can use the above adjusted -R and -Jm
0.1 to get the two systems to match up.
However, we can be smarter than this. Realizing that the input img
file had pixels 2.0 minutes wide (or checking the nx and ny with
grdinfo merc_grav.grd) we realize that merc_grav.grd used the full res‐
olution of the img file and it has 2400 by 2039 pixels, and at 8 inches
wide this is 300 pixels per inch. We decide we don't need that many
and we will be satisfied with 100 pixels per inch, so we want to aver‐
age the data into 3 by 3 squares. (If we want a contour plot we will
probably choose to average the data much more (e.g., 6 by 6) to get
smooth contours.) Since 2039 isn't divisible by 3 we will get a dif‐
ferent adjusted OPT(R) this time:
img2mercgrd world_grav.img.7.2 -G merc_grav_2.grd -R-40/40/-70/-30 -T 1
-N 3 -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 illumina‐
tion file for this using grdgradient:
grdgradient merc_grav_2.grd -G illum.grd -A 0/270 -N e0.6
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 -I illum.grd -C grav.cpt -Jx 0.1 -K > map.ps
psbasemap -R-40/40/-70.023256525/-29.9368261101 -Jm 0.1 -B a10 -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 -T 2
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 ALSOGMT(1), grdproject(1), mapproject(1)GMT 4.5.14 1 Nov 2015 IMG2MERCGRD(1)
