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7.24 Data selection based on geospatial criteria

Although we are not seismologists, we have yet another example involving seismicity. We use seismicity data for the Australia/New Zealand region to demonstrate how we can extract subsets of data using geospatial criteria. In particular, we wish to plot the epicenters given in the file oz_quakes.d as red or green circles. Green circles should only be used for epicenters that satisfy the following three criteria:

1. They are located in the ocean and not on land
2. They are within 3000 km of Hobart
3. They are more than 1000 km away from the International Dateline

All remaining earthquakes should be plotted in red. Rather that doing the selection process twice we simply plot all quakes as red circles and then replot those that pass our criteria. Most of the work here is done by gmtselect; the rest is carried out by the usual pscoast and psxy workhorses. Note for our purposes the Dateline is just a line along the 180° meridian.

    #!/bin/sh
    #               GMT EXAMPLE 24
    #
    # Purpose:      Extract subsets of data based on geospatial criteria
    # GMT progs:    gmtselect, pscoast, psxy, minmax
    # Unix progs:   echo, cat, awk
    #
    # Highlight oceanic earthquakes within 3000 km of Hobart and > 1000 km from dateline
    ps=example_24.ps
    echo "147:13 -42:48 3000 Hobart" > point.d
    cat << END > dateline.d
    > Our proxy for the dateline
    180     0
    180     -90
    END
    R=`minmax -I10 oz_quakes.d`
    pscoast $R -JM9i -K -Gtan -Sdarkblue -Wthin,white -Dl -A500 -Ba20f10g10WeSn \
            -U"Example 24 in Cookbook" > $ps
    psxy -R -J -O -K oz_quakes.d -Sc0.05i -Gred >> $ps
    gmtselect oz_quakes.d -L1000/dateline.d -Nk/s -C3000/point.d -fg -R -Il \
            | psxy -R -JM -O -K -Sc0.05i -Ggreen >> $ps
    $AWK '{print $1, $2, 0, 2*$3, 2*$3}' point.d | psxy -R -J -O -K -SE -Wfat,white >> $ps
    $AWK '{print $1, $2, 14, 0, 1, "LT", $4}' point.d | pstext -R -J -O -K -Gwhite -D0.1i/-0.1i >> $ps
    psxy -R -J -O -K point.d -Wfat,white -S+0.2i >> $ps
    psxy -R -J -O -m dateline.d -Wfat,white -A >> $ps
    rm -f point.d dateline.d .gmt*

Figure 7.24: Data selection based on geospatial criteria.

The script produces the plot in Figure 7.24. Note that the horizontal distance from the dateline seems to increase as we go south; however that is just the projected distance (Mercator distortion) and not the actual distance which remains constant at 1000 km.

Next: 7.25 Global distribution of Up: 7. Creating GMT Graphics Previous: 7.23 All great-circle paths Contents Index