DEM sources & datums

The terrain behind an absolute DSM comes from a public or user-supplied DEM (geo/dem.py).

Source (`dem_source`)	Vertical datum	Resolution	Access
`copernicus30` (default)	EGM2008 geoid	~30 m	AWS open data, no login
`srtmgl1`	EGM96 geoid	~30 m	OpenTopography API key
`nasadem`	EGM96 geoid	~30 m	OpenTopography API key
`terrain_tiles`	EGM96 (mixed sources)	varies by zoom	AWS Terrarium tiles
`cartodem`	WGS84 ellipsoid	user file	local GeoTIFF via `--dem`

Datum handling

flowchart LR
  A[DEM tile<br/>native datum] --> B[Reproject to<br/>scene CRS]
  B --> C[PROJ geoid grids<br/>EGM96 ⇄ EGM2008 ⇄ ellipsoid]
  C --> D[Fit / anchor terrain]
  D --> E[DSM GeoTIFF<br/>compound CRS]

Heights are converted between EGM96, EGM2008 and the WGS84 ellipsoid with PROJ geoid grids. The difference between the two geoids is small but not zero, and the geoid–ellipsoid separation can reach tens of metres, so mixing datums is a common source of large absolute errors. Reference data uploaded for validation can declare its datum (EGM96, EGM2008 or WGS84), and it is converted before scoring.

Offline use

Fetched DEM tiles are cached. To prepare a demo machine with no network:

python -m geo.dem --prime scene.tif      # fills the DEM cache for the scene's footprint