Vertical Coordinate System

[gislab-augsburg]

Hi there,

The AWS terrain tiles come in horizontal coordinate system WGS84 Pseudo Mercator (EPSG:3857). I did not find explicit information about their vertical datum and vertical coordinate reference system. The elevation data in most of the US are derived from NED or 3DEP data, which are in the horizontal coordinate system NAD83 and referenced to the vertical system NAVD88. As far as I understand, the Mapzen Terrain files (e.g. NED13) are still in NAD83 horizontal and NAVD88 vertical.

Now my question: Are the AWS terrain tiles on the S3-Buckets already projected to the vertical Datum/ System WGS84 (as usually associated with WGS84 Pseudo Mercator) or are the elevation values still referenced to the NAVD88 system?

Thank you very much in advance for some help! Cheers, Martin

[pchilds]

The SRTM dataset uses EGM96 as listed in the GDAL metadata field in the geotiff files. Mapzen geotiffs have a GDALMetadata field but no information on the vertical coordinate system used. The tie point also has a zero value too (at least where I've looked) so no offset there. It lists rio_creation_kwds which is RasterIO. RasterIO makes no mention of vertical coordinate systems in its documentation https://rasterio.readthedocs.io/_/downloads/en/stable/pdf/ and only takes a single CRS id. I suspect that means it has just been fused in a VCS agnostic manner. It is possible they harmonised it before passing it to rasterIO (the vertical offset in https://github.com/tilezen/joerd/blob/0b86765156d0612d837548c2cf70376c43b3405c/joerd/source/greatlakes.py#L21 for the great lakes area seems to indicate so - the values are against LWD and refer to IGLD the great lakes datum) but as to what, that information has not been preserved.