code: def get_geoid_offset(ds, geoid_srs=egm08_srs):
      
      ds_srs = get_ds_srs(ds)
      c = get_center(ds)
      x, y, z = cT_helper(c[0], c[1], 0.0, ds_srs, geoid_srs)
      return z
docs: Return offset for center of ds
      
      Offset is added to input (presumably WGS84 HAE) to get to geoid
      
      Note: requires vertical offset grids in proj share dir - see earlier note