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
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 grdfile 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 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 grd file is pixel registered; it inherits this
from the img file.
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_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) 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 file.
[Default is 1.0]. (img topo files are stored in (corrected)
meters; gravity files in mGal*10; vertical deflection files in
microradians*10, vertical gravity gradient files in Eotvos*10.)
-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]
-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
try
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
projection 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
unprojecting 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 won't be easy as the -R in the two coordinate
systems won't 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 gmtdefault
MEASURE_UNIT is inch. Then since the above merc_grav.grd file is
projected 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.,
grdcontour, 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.
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
resolution 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
average 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
different adjusted -R this time:
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 pixels, 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, grdproject, mapproject
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