1IMG2GRD(1) Generic Mapping Tools IMG2GRD(1)
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6 img2grd - Extract region of img in Mercator or geographic form
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9 img2grd imgfile -Ggrdfile -Rwest/east/south/north[r] -Ttype [ -C ] [
10 -Dminlat/maxlat ] [ -E ] [ -L ] [ -M ] [ -Nnavg ] [ -Sscale ] [ -V ] [
11 -Wmaxlon ] [ -mminutes ]
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14 img2grd is a front-end to img2mercgrd which reads an img format file
15 and creates a grid file. The -M option dictates whether or not the
16 Spherical Mercator projection of the img file is preserved.
17 imgfile
19 An img format file such as the marine gravity or seafloor topog‐
20 raphy fields estimated from satellite altimeter data by Sandwell
21 and Smith. If the user has set an environment variable
22 $GMT_IMGDIR, then img2mercgrd will try to find imgfile in
23 $GMT_IMGDIR; else it will try to open imgfile directly.
24 -G grdfile is the name of the output grid file.
26 -R west, east, south, and north specify the Region of interest, and
28 you may specify them in decimal degrees or in
29 [+-]dd:mm[:ss.xxx][W|E|S|N] format. Append r if lower left and
30 upper right map coordinates are given instead of w/e/s/n. The
31 two shorthands -Rg and -Rd stand for global domain (0/360 and
32 -180/+180 in longitude respectively, with -90/+90 in latitude).
33 -T type handles the encoding of constraint information. type = 0
35 indicates that no such information is encoded in the img file
36 (used for pre-1995 versions of the gravity data) and gets all
37 data. type > 0 indicates that constraint information is encoded
38 (1995 and later (current) versions of the img files) so that one
39 may produce a grid file as follows: -T1 gets data values at all
40 points, -T2 gets data values at constrained points and NaN at
41 interpolated points; -T3 gets 1 at constrained points and 0 at
42 interpolated points.
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45 -C Set the x and y Mercator coordinates relative to projection cen‐
46 ter [Default is relative to lower left corner of grid].
47 Requires -M.
48 -D Use the extended latitude range -80.738/+80.738. Alternatively,
50 append minlat/maxlat as the latitude extent of the input img
51 file. [Default is -72.006/72.006]. -E Can be used when -M is
52 not set to force the final grid to have the exact same region as
53 requested with -R. By default, the final region is a direct
54 projection of the original Mercator region and will typically
55 extend slightly beyond the requested latitude range, and futher‐
56 more the grid increment in latitude does not match the longitude
57 increment. However, the extra resampling introduces small
58 interpolation errors and should only be used if the output grid
59 must match the requested region and have x_inc = y_inc. In this
60 case the region set by -R must be given in multiples of the
61 increment (.e.g, -R0/45/45/72).
62 -L With no other arguments, list all *.img files found in the
64 directory pointed to by $GMT_IMGDIR, or the current directory if
65 not defined. Ignored if other options are present on the com‐
66 mand line.
67 -M Output a Spherical Mercator grid [Default is a geographic
69 lon/lat grid].
70 -N Average the values in the input img pixels into navg by navg
72 squares, and create one output pixel for each such square. If
73 used with -T3 it will report an average constraint between 0 and
74 1. If used with -T2 the output will be average data value or
75 NaN according to whether average constraint is > 0.5. navg must
76 evenly divide into the dimensions of the imgfile in pixels.
77 [Default 1 does no averaging].
78 -S Multiply the img file values by scale before storing in grid
80 file. [Default is 1.0]. (img topo files are stored in (cor‐
81 rected) meters; gravity files in mGal*10; vertical deflection
82 files in microradians*10, vertical gravity gradient files in
83 Eotvos*10.)
84 -V Selects verbose mode, which will send progress reports to stderr
86 [Default runs "silently"]. Particularly recommended here, as it
87 is helpful to see how the coordinates are adjusted.
88 -m Indicate minutes as the width of an input img pixel in minutes
90 of longitude. [Default is 2.0].
91 -W Indicate maxlon as the maximum longitude extent of the input img
93 file. Versions since 1995 have had maxlon = 360.0, while some
94 earlier files had maxlon = 390.0. [Default is 360.0].
95
97 To extract data in the region -R-40/40/-70/-30 from world_grav.img.7.2
98 and preserve the Mercator gridding:
99 img2grd world_grav.img.7.2 -Gmerc_grav.grd -R-40/40/-70/-30 -M -T1 -V
101 Without the -M option the same command will yield a geographic grid.
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105 GMT(1), img2mercgrd(1)
106GMT 4.3.1 15 May 2008 IMG2GRD(1)