(2) Image presentations

As our second example we will demonstrate how to make color images from gridded data sets (again, we will defer the actual making of grid files to later examples). We will use grdraster to extract 2-D grid files of bathymetry and Geosat geoid heights and put the two images on the same page. The region of interest is the Hawaiian islands, and due to the oblique trend of the island chain we prefer to rotate our geographical data sets using an oblique Mercator projection defined by the hotspot pole at (68W, 69N). We choose the point (190, 25.5) to be the center of our projection (e.g., the local origin), and we want to image a rectangular region defined by the longitudes and latitudes of the lower left and upper right corner of region. In our case we choose (160, 20) and (220, 30) as the corners. We use grdimage to make the illustration:

# Purpose:	Make two color images based gridded data
# GMT modules:	gmtset, grd2cpt, grdimage, makecpt, psscale, pstext
# Unix progs:	rm
gmt makecpt -Crainbow -T-2/14/2 > g.cpt
gmt grdimage HI_geoid2.nc -R160/20/220/30+r -JOc190/25.5/292/69/4.5i -E50 -K -P \
            -B10 -Cg.cpt -X1.5i -Y1.25i > $ps
gmt psscale -Cg.cpt -DJRM+o0.6i/0+e+mc -R -J -O -K -Bx2+lGEOID -By+lm >> $ps
gmt grd2cpt HI_topo2.nc -Crelief -Z > t.cpt
gmt grdimage HI_topo2.nc -I+a0 -R -J -B+t"H@#awaiian@# T@#opo and @#G@#eoid@#" \
            -B10 -E50 -O -K -Ct.cpt -Y4.5i --MAP_TITLE_OFFSET=0.5i >> $ps
gmt psscale -Ct.cpt -DJRM+o0.6i/0+mc -R -J -O -K -I0.3 -Bx2+lTOPO -By+lkm >> $ps
gmt pstext -R0/8.5/0/11 -Jx1i -F+f30p,Helvetica-Bold+jCB -O -N -Y-4.5i >> $ps << END
-0.4 7.5 a)
-0.4 3.0 b)
rm -f ?.cpt gmt.conf

The first step extracts the 2-D data sets from the local data base using grdraster that may be adapted to reflect the nature of your data base format. It automatically figures out the required extent of the region given the two corners points and the projection. The extreme meridians and parallels enclosing the oblique region is -R159:50/220:10/3:10/47:35. This is the area extracted by grdraster. For your convenience we have commented out those lines and provided the two extracted files so you do not need grdraster to try this example. By using the embedded grid file format mechanism we saved the topography using kilometers as the data unit. We now have two grid files with bathymetry and geoid heights, respectively. We use makecpt to generate a linear color palette file geoid.cpt for the geoid and use grd2cpt to get a histogram-equalized CPT topo.cpt for the topography data. To emphasize the structures in the data we calculate the slopes in the north-south direction using grdgradient; these will be used to modulate the color image. Next we run grdimage to create a color-code image of the Geosat geoid heights, and draw a color legend to the right of the image with psscale. Similarly, we run grdimage but specify -Y4.5i to plot above the previous image. Adding scale and label the two plots a) and b) completes the illustration.


Color images from gridded data.