Grassgis/arcgis and nip2.
You can generate “splat” maps from GIS programs by printing out slope values in a 0 to 1 range.
You can generate high detail 1:1 by running a gaussian blur filter over a stretched out version of a 30m DTM, which is maybe 20% accuracy vs real world and possibly lowers mountain ranges.
But if you start that way, youd need several TB just to store the heightmap data locally. You’d never be able to publish/release something that requires more than 100gb to run.
Which is where the streaming in part becomes important…