Topography file ordering¶
See also
To compute the cell-averaged topography value in each computational cell, the default approach in GeoClaw is to use the finest topography available at each point in the cell. Since each topofile (DEM) is on a rectangular grid with regular spacing dx and dy, the resolutions are compared by computing the cell areas dx*dy. If two or more topofiles overlap a point in the cell then the finer one is used.
This usually works fine but may not always be what is wanted. In particular there may be two DEMs at essentially the same resolution, but one is a more recent and hence a better dataset to use (but perhaps covers a smaller area, so the older DEM is also needed outside this area). In this case we want to guarantee that the newer one is used where available. Even if the resolutions are essentially identical, dx*dy maybe be slightly different (perhaps at the roundoff level) and so it could be that the older one is viewed as “finer” and would be used preferentially. Even if dx*dy is exactly the same, it is not obvious which one would be used in GeoClaw. (The one listed later in rundata.topo_data.topofiles would be considered finer in this case.)
Starting in v5.13.0, there is a new setrun.py parameter rundata.topo_data.override_order that is set to False by default to produce the default behavior described above.
If setrun.py specifies:
rundata.topo_data.override_order = True
then the preference order is no longer based on the computed resolution. Instead it is assumed that the files in rundata.topo_data.topofiles are ordered from least preferable to most preferable (so normally you would list the coarsest DEM first, going down to the finest DEM, in order to reproduce the default behavior). This allows precise specification of the order of preference.
