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7.15 Gridding, contouring, and masking of unconstrained areas

This example (Figure 7.15) demonstrates some off the different ways one can use to grid data in GMT, and how to deal with unconstrained areas. We first convert a large ASCII file to binary with gmtconvert since the binary file will read and process much faster. Our lower left plot illustrates the results of gridding using a nearest neighbor technique (nearneighbor) which is a local method: No output is given where there are no data. Next (lower right), we use a minimum curvature technique (surface) which is a global method. Hence, the contours cover the entire map although the data are only available for portions of the area (indicated by the gray areas plotted using psmask). The top left scenario illustrates how we can create a clip path (using psmask) based on the data coverage to eliminate contours outside the constrained area. Finally (top right) we simply employ pscoast to overlay gray land masses to cover up the unwanted contours, and end by plotting a star at the deepest point on the map with psxy. This point was extracted from the grid files using grdinfo.

    #!/bin/sh
    #               GMT EXAMPLE 15
    #
    # Purpose:      Gridding and clipping when data are missing
    # GMT progs:    blockmedian, gmtconvert, grdclip, grdcontour, grdinfo, minmax
    # GMT progs:    nearneighbor, pscoast, psmask, pstext, surface
    # Unix progs:   awk, echo, rm
    #
    ps=example_15.ps
    gmtconvert ship.xyz -bo > ship.b
    #
    region=`minmax ship.b -I1 -bi3`
    nearneighbor $region -I10m -S40k -Gship.nc ship.b -bi3
    info=`grdinfo -C -M ship.nc`
    grdcontour ship.nc -JM3i -P -B2WSne -C250 -A1000 -G2i -K -U"Example 15 in Cookbook" > $ps
    #
    blockmedian $region -I10m ship.b -bi3 -bo > ship_10m.b
    surface $region -I10m ship_10m.b -Gship.nc -bi3
    psmask $region -I10m ship.b -J -O -K -T -Glightgray -bi3 -X3.6i >> $ps
    grdcontour ship.nc -J -B2WSne -C250 -L-8000/0 -A1000 -G2i -O -K >> $ps
    #
    psmask $region -I10m ship_10m.b -bi3 -J -B2WSne -O -K -X-3.6i -Y3.75i >> $ps
    grdcontour ship.nc -J -C250 -A1000 -L-8000/0 -G2i -O -K >> $ps
    psmask -C -O -K >> $ps
    #
    grdclip ship.nc -Sa-1/NaN -Gship_clipped.nc
    grdcontour ship_clipped.nc -J -B2WSne -C250 -A1000 -L-8000/0 -G2i -O -K -X3.6i >> $ps
    pscoast $region -J -O -K -Ggray -Wthinnest >> $ps
    echo $info | $AWK '{print $12,$13}' | psxy -R -J -O -K -Sa0.15i -Wthick >> $ps
    echo "-0.3 3.6 24 0 1 CB Gridding with missing data" | pstext -R0/3/0/4 -Jx1i -O -N >> $ps
    rm -f ship.b ship_10m.b ship.nc ship_clipped.nc .gmt*

Figure 7.15: Gridding, contouring, and masking of data.

Next: 7.16 Gridding of data, Up: 7. Creating GMT Graphics Previous: 7.14 Gridding of data Contents Index