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Is there an existing method or best practice for calculating Geoid height for a lat/lon location in PostGIS? I don't believe there is an "ST_" function.

I know that PostGIS makes use of GeographicLib for some calculations already and that GeographicLib has methods for calculating Geoid height.

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Calculating, No. Looking up, Yes.

Pre-calculated data is available for the various Geoid models (EGM84, EGM96, EGM2008) in raster format. Here's an easy to use source:

https://sourceforge.net/projects/geographiclib/files/geoids-distrib/ or https://geographiclib.sourceforge.io/html/geoid.html#geoidinst

The raster files can be imported into your db using raster2pgsql. For example:

raster2pgsql -s 4326 -I -C -M egm2008-1.tif -F -t 100x100 myschema.geoid_egm2008 | psql -h localhost -p 5432 -U user -d db

And then querying:

SELECT ST_Value(rast, ST_SetSRID(ST_MakePoint(mod(-75.49393 + 360,360), 39.96002),4326))::numeric * 0.003 - 108
FROM myschema.geoid_egm2008
WHERE ST_Intersects(rast, ST_SetSRID(ST_MakePoint(mod(-75.49393 + 360,360), 39.96002),4326));
--Should be ~ -33.9943

Note the scaling & offsetting spec:

   # Offset -108
   # Scale 0.003
   # Origin 90N 0E

Explanation here: https://geographiclib.sourceforge.io/html/geoid.html#geoidformat

See here and here for further information on raster2pgsql tile size optimization and querying raster data.

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