[gdal-dev] Is it possible to write custom CRS WKTs within a GeoTiff?

[Scott Henderson via gdal-dev]

Hello gdal-dev,

I’ve looked around for an answer to this one to no avail and would appreciate 
any insight from those more familiar with the GeoTiff format. I’d like to be 
able to store a custom WKT *inside GeoTiff metadata* rather than in an external 
PAM file. I’ve seen some solutions to add custom entries in the PROJ database 
to accomplish this, but is that really the only way?

Here is a motivating example, using a WKT that specifies a specific WGS84 
realization for a given UTM zone:
```
INPUT=https://opentopography.s3.sdsc.edu/raster/COP30/COP30_hh/Copernicus_DSM_10_N38_00_W109_00_DEM.tif
OUTPUT=/tmp/Copernicus_DSM_10_N38_00_W109_00_DEM_UTM.tif

CPL_DEBUG=ON \
 GDAL_DISABLE_READDIR_ON_OPEN=EMPTY_DIR \
 gdalwarp -co GEOTIFF_VERSION=1.1 -overwrite -dstnodata 0 -r bilinear -tap -tr 
30.0 30.0 -s_srs EPSG:7661 -t_srs utm12N_wgs1150.wkt $INPUT $OUTPUT
```

Here is the wkt, I’d like for it not to end up in a sidecar file where I might 
loose it, but embedded in the GTiff:
```
PROJCRS["WGS84 (G1150) / UTM zone 12N",
    BASEGEOGCRS["WGS 84 (G1150)",
        DYNAMIC[
            FRAMEEPOCH[2001]],
        DATUM["World Geodetic System 1984 (G1150)",
            ELLIPSOID["WGS 84",6378137,298.257223563,
                LENGTHUNIT["metre",1]]],
        PRIMEM["Greenwich",0,
            ANGLEUNIT["degree",0.0174532925199433]],
        ID["EPSG",7661]],
    CONVERSION["UTM zone 12N",
        METHOD["Transverse Mercator",
            ID["EPSG",9807]],
        PARAMETER["Latitude of natural origin",0,
            ANGLEUNIT["degree",0.0174532925199433],
            ID["EPSG",8801]],
        PARAMETER["Longitude of natural origin",-111,
            ANGLEUNIT["degree",0.0174532925199433],
            ID["EPSG",8802]],
        PARAMETER["Scale factor at natural origin",0.9996,
            SCALEUNIT["unity",1],
            ID["EPSG",8805]],
        PARAMETER["False easting",500000,
            LENGTHUNIT["metre",1],
            ID["EPSG",8806]],
        PARAMETER["False northing",0,
            LENGTHUNIT["metre",1],
            ID["EPSG",8807]],
        ID["EPSG",16012]],
    CS[Cartesian,3],
        AXIS["(E)",east,
            ORDER[1],
            LENGTHUNIT["metre",1,
                ID["EPSG",9001]]],
        AXIS["(N)",north,
            ORDER[2],
            LENGTHUNIT["metre",1,
                ID["EPSG",9001]]],
        AXIS["ellipsoidal height (h)",up,
            ORDER[3],
            LENGTHUNIT["metre",1,
                ID["EPSG",9001]]],
    USAGE[
        SCOPE["unknown"],
        AREA["Between 114°W and 108°W, northern hemisphere between equator and 
84°N, onshore and offshore. Canada - Alberta; Northwest Territories (NWT); 
Nunavut; Saskatchewan. Mexico. United States (USA)."],
        BBOX[0,-114,84,-108]],
    REMARK["Custom WKT2 combining UTM_12N with WGS84 (G1150)”]]
```

Cheers,
Scott
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