Add support for GDAL VRT allowing client side regridding

[danlooo]

Currently, we have an OSM slippy tiles server to integrate DGGS data in other tools such as leaflet or QGIS. This is server side rendering. However, one might also want to have client side regridding, e.g. to put the data on storage only clouds that don't provide computing ressources. One solution would be GDAL virtual rasters (VRT).

However, this VRT file results on error No transform is available between PROJ-based coordinate operation and Custom CRS: GEOGCRS["unknown",DATUM["Unknown based on WGS 84 e…. in qgis and error ERROR 1: PROJ: proj_crs_get_coordinate_system: Object is not a SingleCRS in gdal. It seems that we can't support VRT for DGGS data.

VRT file:

<VRTDataset rasterXSize="64" rasterYSize="32">
  <GeoTransform> -5.0000000000000000e-01, 1.0000000000000000e+00, 0.0000000000000000e+00,
    -5.0000000000000000e-01, 0.0000000000000000e+00, 1.0000000000000000e+00</GeoTransform>
  <SRS>
    CONVERSION["PROJ-based coordinate operation",
    METHOD["PROJ-based operation method: +proj=isea"]]
  </SRS>
  <Metadata>
    <MDI key="actual_range">{144,222.75}</MDI>
    <MDI key="dggs_dggsrs">ISEA4D.Penta</MDI>
    <MDI key="dggs_resolution">5</MDI>
    <MDI key="missing_value">1.00000000000000005e+32</MDI>
    <MDI key="name">Gray</MDI>
  </Metadata>
  <VRTRasterBand dataType="Float64" band="1">
    <NoDataValue>1e+32</NoDataValue>
    <SimpleSource>
      <SourceFilename relativeToVRT="0">
        ZARR:"/vsicurl/https://s3.bgc-jena.mpg.de:9000/dggs/natural-earth.dggs.zarr":/Gray:0</SourceFilename>
      <SourceBand>1</SourceBand>
      <SourceProperties RasterXSize="64" RasterYSize="32" DataType="Float64" BlockXSize="64"
        BlockYSize="32" />
      <SrcRect xOff="0" yOff="0" xSize="64" ySize="32" />
      <DstRect xOff="0" yOff="0" xSize="64" ySize="32" />
    </SimpleSource>
  </VRTRasterBand>
</VRTDataset>

tested on qgis:

QGIS version	3.42.2-Münster
QGIS code branch	Release 3.42
Libraries
Qt version	5.15.15
Python version	3.12.10
GDAL version	3.10.3
PROJ version	9.6.0
EPSG Registry database version	v12.004 (2025-03-02)
GEOS version	3.13.1-CAPI-1.19.2
SQLite version	3.49.1
PDAL version	2.8.4
PostgreSQL client version	17.5
SpatiaLite version	5.1.0
QWT version	6.1.5
QScintilla2 version	2.14.1
OS version	KDE Flatpak runtime
Active Python plugins
processing	2.12.99
MetaSearch	0.3.6
db_manager	0.1.20

[danlooo]

The issue is that a proj transformation does not have a datum. Might be possible with a workaround to create a single crs with datum see OSGeo/gdal#10537.