1IMG2MERCGRD(1) Generic Mapping Tools IMG2MERCGRD(1)
2
6 img2mercgrd - Extract region of img, preserving Mercator, save as grd
7
9 img2mercgrd imgfile -Ggrdfile -Rwest/east/south/north[r] -Ttype [ -C ]
10 [ -Dminlat/maxlat ] [ -Nnavg ] [ -Sscale ] [ -V ] [ -Wmaxlon ] [ -mmin‐
11 utes ]
12
14 img2mercgrd reads an img format file and creates a grid file. The
15 Spherical Mercator projection of the img file is preserved, so that the
16 region -R set by the user is modified slightly; the modified region
17 corresponds to the edges of pixels [or groups of navg pixels]. The
18 grid file header is set so that the x and y axis lengths represent dis‐
19 tance from the west and south edges of the image, measured in user
20 default units, with -Jm1 and the adjusted -R. By setting the default
21 ELLIPSOID = Sphere, the user can make overlays with the adjusted -R so
22 that they match. See EXAMPLES below. The adjusted -R is also written
23 in the grdheader remark, so it can be found later. The -Ttype selects
24 all data or only data at constrained pixels, and can be used to create
25 a grid of 1s and 0s indicating constraint locations. The output grid
26 file is pixel registered; it inherits this from the img file.
27 imgfile
29 An img format file such as the marine gravity or seafloor topog‐
30 raphy fields estimated from satellite altimeter data by Sandwell
31 and Smith. If the user has set an environment variable
32 $GMT_IMGDIR, then img2mercgrd will try to find imgfile in
33 $GMT_IMGDIR; else it will try to open imgfile directly.
34 -G grdfile is the name of the output grid file.
36 -R west, east, south, and north specify the Region of interest, and
38 you may specify them in decimal degrees or in
39 [+-]dd:mm[:ss.xxx][W|E|S|N] format. Append r if lower left and
40 upper right map coordinates are given instead of w/e/s/n. The
41 two shorthands -Rg and -Rd stand for global domain (0/360 and
42 -180/+180 in longitude respectively, with -90/+90 in latitude).
43 Alternatively, specify the name of an existing grid file and the
44 -R settings (and grid spacing, if applicable) are copied from
45 the grid.
46 -T type handles the encoding of constraint information. type = 0
48 indicates that no such information is encoded in the img file
49 (used for pre-1995 versions of the gravity data; all more recent
50 files do not support this choice) and gets all data. type > 0
51 indicates that constraint information is encoded (1995 and later
52 (current) versions of the img files) so that one may produce a
53 grid file as follows: -T1 gets data values at all points, -T2
54 gets data values at constrained points and NaN at interpolated
55 points; -T3 gets 1 at constrained points and 0 at interpolated
56 points.
57
59 -C Set the x and y Mercator coordinates relative to projection cen‐
60 ter (lon = lat = 0) [Default is relative to lower left corner of
61 grid].
62 -D Use the extended latitude range -80.738/+80.738. Alternatively,
64 append minlat/maxlat as the latitude extent of the input img
65 file. [Default is -72.006/72.006].
66 -N Average the values in the input img pixels into navg by navg
68 squares, and create one output pixel for each such square. If
69 used with -T3 it will report an average constraint between 0 and
70 1. If used with -T2 the output will be average data value or
71 NaN according to whether average constraint is > 0.5. navg must
72 evenly divide into the dimensions of the imgfile in pixels.
73 [Default 1 does no averaging].
74 -S Multiply the img file values by scale before storing in grid
76 file. [Default is 1.0]. (img topo files are stored in (cor‐
77 rected) meters; gravity files in mGal*10; vertical deflection
78 files in microradians*10, vertical gravity gradient files in
79 Eotvos*10. Use -S0.1 for those files.)
80 -V Selects verbose mode, which will send progress reports to stderr
82 [Default runs "silently"]. Particularly recommended here, as it
83 is helpful to see how the coordinates are adjusted.
84 -m Indicate minutes as the width of an input img pixel in minutes
86 of longitude. [Default is 2.0].
87 -W Indicate maxlon as the maximum longitude extent of the input img
89 file. Versions since 1995 have had maxlon = 360.0, while some
90 earlier files had maxlon = 390.0. [Default is 360.0].
91
93 To extract data in the region -R-40/40/-70/-30 from world_grav.img.7.2,
94 run
95 img2mercgrd world_grav.img.7.2 -Gmerc_grav.grd -R-40/40/-70/-30 -T1 -V
97 Note that the -V option tells us that the range was adjusted to
99 -R-40/40/-70.0004681551/-29.9945810754. We can also use grdinfo to
100 find that the grid file header shows its region to be
101 -R0/80/0/67.9666667 This is the range of x,y we will get from a
102 Spherical Mercator projection using
103 -R-40/40/-70.0004681551/-29.9945810754 and -Jm1. Thus, to take
104 ship.lonlatgrav and use it to sample the merc_grav.grd, we can do this:
105 gmtset ELLIPSOID Sphere
107 mapproject -R-40/40/-70.0004681551/-29.9945810754 -Jm1 ship.lonlatgrav
108 | grdtrack -Gmerc_grav.grd | mapproject
109 -R-40/40/-70.0004681551/-29.9945810754 -Jm1 -I > ship.lonlatgravsat
110 It is recommended to use the above method of projecting and unproject‐
112 ing the data in such an application, because then there is only one
113 interpolation step (in grdtrack). If one first tries to convert the
114 grid file to lon,lat and then sample it, there are two interpolation
115 steps (in conversion and in sampling).
116 To make a lon,lat grid from the above grid we can use
118 grdproject merc_grav.grd -R-40/40/-70.0004681551/-29.9945810754 -Jm1 -I
120 -F -D2m -Ggrav.grd
121 In some cases this will not be easy as the -R in the two coordinate
123 systems may not align well. When this happens, we can also use (in
124 fact, it may be always better to use)
125 grd2xyz merc_grav.grd | mapproject
127 -R-40/40/-70.0004681551/-29.994581075 -Jm1 -I | surface -R-40/40/-70/70
128 -I2m -Ggrav.grd
129 To make a Mercator map of the above region, suppose our .gmtdefaults4
131 MEASURE_UNIT is inch. Then since the above merc_grav.grd file is pro‐
132 jected with -Jm1 it is 80 inches wide. We can make a map 8 inches wide
133 by using -Jx0.1 on any map programs applied to this grid (e.g., grdcon‐
134 tour, grdimage, grdview), and then for overlays which work in lon,lat
135 (e.g., psxy, pscoast) we can use the above adjusted -R and -Jm0.1 to
136 get the two systems to match up.
137 However, we can be smarter than this. Realizing that the input img
139 file had pixels 2.0 minutes wide (or checking the nx and ny with
140 grdinfo merc_grav.grd) we realize that merc_grav.grd used the full res‐
141 olution of the img file and it has 2400 by 2039 pixels, and at 8 inches
142 wide this is 300 pixels per inch. We decide we don't need that many
143 and we will be satisfied with 100 pixels per inch, so we want to aver‐
144 age the data into 3 by 3 squares. (If we want a contour plot we will
145 probably choose to average the data much more (e.g. 6 by 6) to get
146 smooth contours.) Since 2039 isn't divisible by 3 we will get a dif‐
147 ferent adjusted OPT(R) this time:
148 img2mercgrd world_grav.img.7.2 -Gmerc_grav_2.grd -R-40/40/-70/-30 -T1
150 -N3 -V
151 This time we find the adjusted region is
153 -R-40/40/-70.023256525/-29.9368261101 and the output is 800 by 601 pix‐
154 els, a better size for us. Now we can create an artificial illumina‐
155 tion file for this using grdgradient:
156 grdgradient merc_grav_2.grd -Gillum.grd -A0/270 -Ne0.6
158 and if we also have a cpt file called "grav.cpt" we can create a color
160 shaded relief map like this:
161 grdimage merc_grav_2.grd -Iillum.grd -Cgrav.cpt -Jx0.1 -K > map.ps
163 psbasemap -R-40/40/-70.023256525/-29.9368261101 -Jm0.1 -Ba10 -O >>
164 map.ps
165 Suppose you want to obtain only the constrained data values from an img
167 file, in lat/lon coordinates. Then run img2mercgrd with the -T2
168 option, use grd2xyz to dump the values, pipe through grep -v NaN to
169 eliminate NaNs, and pipe through mapproject with the inverse projection
170 as above.
171
173 GMT(1), grdproject(1), mapproject(1)
174GMT 4.5.6 10 Mar 2011 IMG2MERCGRD(1)