elevation unit for geotif files not detected correctly

[adamgeo]

Gdal always reads the elevation unit of the geotiff files as meter even if the elevation unit is in feet and causes scale up in height. Sample data is attached.

[Jukka Rahkonen]

How do you use GDAL for reading the elevation?

[Even Rouault]

If you explicitly set the GTIFF_REPORT_COMPD_CS=YES configuration option, you'll get a COMPD_CS reported with vertical units = US survey foot

$ GTIFF_REPORT_COMPD_CS=YES gdalinfo Elevation_feet.tif
Driver: GTiff/GeoTIFF
Files: Elevation_feet.tif
Size is 509, 356
Coordinate System is:
COMPD_CS["unknown",
    PROJCS["GCS_WGS_1984",
        GEOGCS["WGS 84",
            DATUM["WGS_1984",
                SPHEROID["WGS 84",6378137,298.2572235604902,
                    AUTHORITY["EPSG","7030"]],
                AUTHORITY["EPSG","6326"]],
            PRIMEM["Greenwich",0],
            UNIT["degree",0.0174532925199433],
            AUTHORITY["EPSG","4326"]],
        PROJECTION["Transverse_Mercator"],
        PARAMETER["latitude_of_origin",0],
        PARAMETER["central_meridian",-153],
        PARAMETER["scale_factor",0.9996],
        PARAMETER["false_easting",1640416.666666667],
        PARAMETER["false_northing",0],
        UNIT["US survey foot",0.3048006096012192,
            AUTHORITY["EPSG","9003"]]],
    VERT_CS["unknown",
        VERT_DATUM["unknown",2005],
        UNIT["US survey foot",0.3048006096012192,
            AUTHORITY["EPSG","9003"]],
        AXIS["Up",UP]]]
Origin = (649119.524002539925277,7223949.479605697095394)
Pixel Size = (2.457814315667396,-2.457814315667140)
Metadata:
  AREA_OR_POINT=Point
Image Structure Metadata:
  COMPRESSION=LZW
  INTERLEAVE=BAND
Corner Coordinates:
Upper Left  (  649119.524, 7223949.480) (155d53' 7.84"W, 19d53'22.88"N)
Lower Left  (  649119.524, 7223074.498) (155d53' 7.68"W, 19d53'14.21"N)
Upper Right (  650370.551, 7223949.480) (155d52'54.74"W, 19d53'23.09"N)
Lower Right (  650370.551, 7223074.498) (155d52'54.58"W, 19d53'14.42"N)
Center      (  649745.038, 7223511.989) (155d53' 1.21"W, 19d53'18.65"N)
Band 1 Block=509x1 Type=Float32, ColorInterp=Gray
  NoData Value=-32767

We could perhaps use that to feed that into the more easy to use GDALRasterBand::GetUnits() interface.

[adamgeo]

Thanks, the config option was helpful and helped a lot!

[Even Rouault]

In 37085:

GTiff: make GetUnitType() use VERT_CS unit as a fallback (fixes #6675)