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_DATADIR, then img2mercgrd will try to find imgfile in
23 $GMT_DATADIR; 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 Alternatively, specify the name of an existing grid file and the
34 -R settings (and grid spacing, if applicable) are copied from
35 the grid.
36 -T type handles the encoding of constraint information. type = 0
38 indicates that no such information is encoded in the img file
39 (used for pre-1995 versions of the gravity data) and gets all
40 data. type > 0 indicates that constraint information is encoded
41 (1995 and later (current) versions of the img files) so that one
42 may produce a grid file as follows: -T1 gets data values at all
43 points, -T2 gets data values at constrained points and NaN at
44 interpolated points; -T3 gets 1 at constrained points and 0 at
45 interpolated points.
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48 -C Set the x and y Mercator coordinates relative to projection cen‐
49 ter [Default is relative to lower left corner of grid].
50 Requires -M.
51 -D Use the extended latitude range -80.738/+80.738. Alternatively,
53 append minlat/maxlat as the latitude extent of the input img
54 file. [Default is -72.006/72.006].
55 -E Can be used when -M is not set to force the final grid to have
57 the exact same region as requested with -R. By default, the
58 final region is a direct projection of the original Mercator
59 region and will typically extend slightly beyond the requested
60 latitude range, and furthermore the grid increment in latitude
61 does not match the longitude increment. However, the extra
62 resampling introduces small interpolation errors and should only
63 be used if the output grid must match the requested region and
64 have x_inc = y_inc. In this case the region set by -R must be
65 given in multiples of the increment (.e.g, -R0/45/45/72).
66 -L With no other arguments, list all *.img files found in the
68 directory pointed to by $GMT_DATADIR, or the current directory
69 if not defined. Ignored if other options are present on the
70 command line.
71 -M Output a Spherical Mercator grid [Default is a geographic
73 lon/lat grid].
74 -N Average the values in the input img pixels into navg by navg
76 squares, and create one output pixel for each such square. If
77 used with -T3 it will report an average constraint between 0 and
78 1. If used with -T2 the output will be average data value or
79 NaN according to whether average constraint is > 0.5. navg must
80 evenly divide into the dimensions of the imgfile in pixels.
81 [Default 1 does no averaging].
82 -S Multiply the img file values by scale before storing in grid
84 file. [Default is 1.0]. For recent img files: img topo files
85 are stored in (corrected) meters [-S1]; free-air gravity files
86 in mGal*10 [-S0.1 to get mGal]; vertical deflection files in
87 microradians*10 [-S0.1 to get microradians], vertical gravity
88 gradient files in Eotvos*50 [-S0.02 to get Eotvos, or -S0.002 to
89 get mGal/km]).
90 -V Selects verbose mode, which will send progress reports to stderr
92 [Default runs "silently"]. Particularly recommended here, as it
93 is helpful to see how the coordinates are adjusted.
94 -m Indicate minutes as the width of an input img pixel in minutes
96 of longitude. [Default is 2.0].
97 -W Indicate maxlon as the maximum longitude extent of the input img
99 file. Versions since 1995 have had maxlon = 360.0, while some
100 earlier files had maxlon = 390.0. [Default is 360.0].
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103 To extract data in the region -R-40/40/-70/-30 from world_grav.img.7.2
104 and preserve the Mercator gridding:
105 img2grd world_grav.img.7.2 -Gmerc_grav.grd -R-40/40/-70/-30 -M -T1 -V
107 Without the -M option the same command will yield a geographic grid.
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111 GMT(1), img2mercgrd(1)
112GMT 4.5.6 10 Mar 2011 IMG2GRD(1)