img2mercgrd - Extract region of img, preserving Mercator, save as grd

img2mercgrd imgfile -Ggrdfile -Rwest/east/south/north -Ttype [ -Nnavg ] [ -Sscale ] [ -V ] [ -mminutes ] [ -xmaxlon ] [ -yminlat/maxlat ]

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; 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.

-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. Use -S0.1 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].

-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].

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 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 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 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.

gmt(1gmt), grdproject(1gmt), mapproject(1gmt)