The xyz2grd program, as I understand it, uses a rectangular grid by default on the sphere (as evidenced by the -I option and the x_inc and y_inc parameters). But in a global view, this type of grid is not optimal for the polar regions since the grid points get much closer together compared to the equator.
Suppose I want to use an equidistant grid on a sphere (for example a Fibonacci grid). This grid will not have regular latitude/longitude nodes. Is it possible to display such a grid using the xyz2grd and grdimage programs?
No, I mean that I have an ASCII file of (lat, lon, z) values, which are scattered (e.g. not a regular rectangular grid). I want to display this with some kind of interpolation between grid points. The standard grids used by GMT appear to require rectangular grids (with a fixed x and y increment). I don’t see any support for non-rectangular grids such as Fibonacci grids.
The are several tools to interpolate and create a grid from an ASCII file (e.g. surface, see the full list here). Note that xyz2grd is just to convert data table to a grid.
GMT only supports regular grids. The issue with high latitudes is normally solved by projecting the grids to some appropriate cartesian projection, or alternatively, projecting the lon,lat data to cartesian and grid it after.