.. _example_29: (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 :doc:`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) data set we can use to demonstrate spherical surface gridding. Since :doc:`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 :doc:`grdimage ` and :doc:`grdcontour ` and a color scale is placed between them. .. literalinclude:: /_verbatim/example_29.txt :language: bash .. figure:: /_images/example_29.* :width: 500 px :align: center Gridding of spherical surface data using Green's function splines.