# (2) Examining DEMs using variable illumination

Our next animation uses a gridded topography for parts of Colorado (US); the file is distributed with the tutorial examples. Here, we want to use grdimage to generate a shaded-relief image sequence in which we sweep the illumination azimuth around the entire horizon. The resulting animation illustrates how changing the illumination azimuth can bring out subtle features (or artifacts) in the gridded data. The red arrow points in the direction of the illumination.

    #!/usr/bin/env bash
    #               GMT ANIMATION 02
    #
    # Purpose:      Make simple animated GIF of an illuminated DEM grid
    # GMT modules   math, makecpt, grdimage, plot, movie
    # Unix progs:   cat
    # Note:         Run with any argument to build movie; otherwise 1st frame is plotted only.

    if [ $# -eq 0 ]; then	# Just make master PostScript frame 0
    	opt="-Mps -Fnone"
    else	# Make animated GIF
    	opt="-A+l"
    fi
    # 1. Create files needed in the loop
    cat << EOF > pre.sh
    gmt begin
    	gmt math -T0/360/10 T 180 ADD = angles.txt
    	gmt makecpt -Crainbow -T500/4500 -H > main.cpt
    gmt end
    EOF
    # 2. Set up the main frame script
    cat << EOF > main.sh
    gmt begin
    	width=\$(gmt math -Q \${MOVIE_WIDTH} 0.5i SUB =)
    	gmt grdimage @tut_relief.nc -I+a\${MOVIE_COL0}+nt2 -JM\${width} -Cmain.cpt \
    		-BWSne -B1 -X0.35i -Y0.3i --FONT_ANNOT_PRIMARY=9p
    	gmt plot -Sc0.8i -Gwhite -Wthin <<< "256.25 35.6"
    	gmt plot -Sv0.1i+e -Gred -Wthick <<< "256.25 35.6 \${MOVIE_COL1} 0.37i"
    gmt end
    EOF
    # 3. Run the movie
    gmt movie main.sh -C3.5ix4.167ix72 -Nanim02 -Tangles.txt -Sbpre.sh -D6 -Zs $opt

As you can see, these sorts of animations are not terribly difficult to put together, especially since our vantage point is fixed. In the next example we will move the “camera” around and must therefore deal with how to frame perspective views.

Animation of a DEM using variable illumination.