We have a number of functions to deal with horizontal surfaces, which are defined as GeoData or CartData structures that have 3 dimensional flat coordinate array. Flat implies that the resolution is n by m by 1, which is required to correctly display them in paraview.
The most straightforward surface is the topography (which you can obtain with import_topo).
Surfaces can be added and subtracted.
drape_on_topo
fit_surface_to_points
above_surface
below_surface
interpolate_data_surface
is_surface
remove_NaN_surface!