# (29) Gridding spherical surface data using splines

Finally, we demonstrate how gridding on a spherical surface can be accomplished using Green’s functions of surface splines, with or without tension. Global gridding does not work particularly well in Cartesian coordinates hence the chosen approach. We use greenspline to produce a crude topography grid for Mars based on radii estimates from the Mariner 9 and Viking Orbiter spacecrafts. This data comes from Smith and Zuber [Science, 1996] and is used here as a small (N = 370) remote data set we can use to demonstrate spherical surface gridding. Since greenspline must solve a N by N matrix system your system memory may impose limits on how large data sets you can handle; also note that the spherical surface spline in tension is particularly slow to compute.

Our script must first estimate the ellipsoidal shape of Mars from the parameters given by Smith and Zuber so that we can remove this reference surface from the gridded radii. We run the gridding twice: First with no tension using Parker‘s [1990] method and then with tension using the Wessel and Becker [2008] method. The grids are then imaged with grdimage and grdcontour and a color bar is placed between them.

    #!/usr/bin/env bash
    #       GMT EXAMPLE 29
    #
    # Purpose:  Illustrates spherical surface gridding with Green's function of splines
    # GMT modules:  makecpt, grdcontour, grdimage, grdmath greenspline, colorbar, text
    # Unix progs:   rm
    #
    gmt begin ex29
    	# This example uses 370 radio occultation data for Mars to grid the topography.
    	# Data and information from Smith, D. E., and M. T. Zuber (1996), The shape of
    	# Mars and the topographic signature of the hemispheric dichotomy, Science, 271, 184-187.

    	# Make Mars PROJ_ELLIPSOID given their three best-fitting axes:
    	a=3399.472
    	b=3394.329
    	c=3376.502
    	gmt grdmath -Rg -I4 -r X COSD $a DIV DUP MUL X SIND $b DIV DUP MUL ADD Y COSD DUP MUL MUL Y \
    			SIND $c DIV DUP MUL ADD SQRT INV = PROJ_ELLIPSOID.nc

    	#  Do both Parker and Wessel/Becker solutions (tension = 0.9975)
    	gmt greenspline -RPROJ_ELLIPSOID.nc @mars370.txt -Z4 -Sp -Gmars.nc
    	gmt greenspline -RPROJ_ELLIPSOID.nc @mars370.txt -Z4 -Sq0.9975 -Gmars2.nc
    	# Scale to km and remove PROJ_ELLIPSOID
    	gmt grdmath mars.nc  1000 DIV PROJ_ELLIPSOID.nc SUB = mars.nc
    	gmt grdmath mars2.nc 1000 DIV PROJ_ELLIPSOID.nc SUB = mars2.nc

    	gmt set FONT_TAG 14p,Helvetica-Bold
    	gmt subplot begin 2x1 -Fs18c/0 -Rg -JH0/18c -B30g30 -BWsne -A -M0.6c
    		gmt subplot set 0
    		gmt grdimage mars.nc -I+ne0.75+a45 --FONT_ANNOT_PRIMARY=12p
    		gmt grdcontour mars.nc -C1 -A5 -Glz+/z-
    		gmt plot -Sc0.1c -Gblack @mars370.txt
    		gmt colorbar -DJBC+o0/0.4c+w6i/0.25c -I --FONT_ANNOT_PRIMARY=12p -Bx2f1 -By+lkm

    		gmt subplot set 1
    		gmt makecpt -Crainbow -T-7/15
    		gmt grdimage mars2.nc -I+ne0.75+a45 --FONT_ANNOT_PRIMARY=12p
    		gmt grdcontour mars2.nc -C1 -A5 -Glz+/z-
    		gmt plot -Sc0.1c -Gblack @mars370.txt
    	gmt subplot end
    	rm -f mars.nc mars2.nc PROJ_ELLIPSOID.nc
    gmt end show

Gridding of spherical surface data using Green’s function splines.