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This option is to load grid(s) that have both the horizontal and
vertical velocities in the same file.
Can be tested with the following grid
https://github.com/rouault/sample_proj_gtiff_grids/blob/master/nkgrf03vel_realigned.tif
converted from the original .ct2 and .gtx with
```
gdal_translate nkgrf03vel_realigned.vrt nkgrf03vel_realigned.tif -co COMPRESS=DEFLATE -co PREDICTOR=3 -co BLOCKYSIZE=241 -co INTERLEAVE=BAND
```
where nkgrf03vel_realigned.vrt is
```
<VRTDataset rasterXSize="223" rasterYSize="241">
<SRS dataAxisToSRSAxisMapping="2,1">GEOGCRS["Unknown based on GRS80",
DATUM["Unknown based on GRS80",
ELLIPSOID["GRS 1980",6378137,298.257222101,
LENGTHUNIT["metre",1]]],
PRIMEM["Greenwich",0,
ANGLEUNIT["degree",0.0174532925199433]],
CS[ellipsoidal,2],
AXIS["geodetic latitude (Lat)",north,
ORDER[1],
ANGLEUNIT["degree",0.0174532925199433]],
AXIS["geodetic longitude (Lon)",east,
ORDER[2],
ANGLEUNIT["degree",0.0174532925199433]]]
</SRS>
<GeoTransform> 2.9166666666666670e+00, 1.6666666666666666e-01, 0.0000000000000000e+00, 7.3041666666666686e+01, 0.0000000000000000e+00, -8.3333333333333329e-02</GeoTransform>
<Metadata>
<MDI key="DESCRIPTION"></MDI>
<MDI key="area_of_use">Nordic and Baltic countries</MDI>
<MDI key="AREA_OR_POINT">Point</MDI>
<MDI key="TIFFTAG_COPYRIGHT">The Nordic Geodetic Commission. Creative Commons Attribution 4.0 https://creativecommons.org/licenses/by/4.0/</MDI>
<MDI key="TIFFTAG_DATETIME">2019:12:30 00:00:00</MDI>
<MDI key="TIFFTAG_IMAGEDESCRIPTION">Deformation model covering the Nordic and Baltic countries. Used in transformations between global reference frames and the local realisations of ETRS89 in the Nordic and Baltic countries</MDI>
<MDI key="TYPE">VELOCITY</MDI>
</Metadata>
<VRTRasterBand dataType="Float32" band="1">
<Description>east_velocity</Description>
<UnitType>mm/year</UnitType>
<SimpleSource>
<SourceFilename relativeToVRT="0">/home/even/proj/proj-datumgrid/europe/nkgrf03vel_realigned_xy.ct2</SourceFilename>
<SourceBand>2</SourceBand> <!-- GDAL exposes the first physical component of the file (longitude offset normally, here east velocity) as the second band -->
<SrcRect xOff="0" yOff="0" xSize="223" ySize="241" />
<DstRect xOff="0" yOff="0" xSize="223" ySize="241" />
</SimpleSource>
</VRTRasterBand>
<VRTRasterBand dataType="Float32" band="2">
<Description>north_velocity</Description>
<UnitType>mm/year</UnitType>
<SimpleSource>
<SourceFilename relativeToVRT="0">/home/even/proj/proj-datumgrid/europe/nkgrf03vel_realigned_xy.ct2</SourceFilename>
<SourceBand>1</SourceBand> <!-- and the second physical component (latitude offset normally, here north velocity) as the first band -->
<SrcRect xOff="0" yOff="0" xSize="223" ySize="241" />
<DstRect xOff="0" yOff="0" xSize="223" ySize="241" />
</SimpleSource>
</VRTRasterBand>
<VRTRasterBand dataType="Float32" band="3">
<Description>up_velocity</Description>
<UnitType>mm/year</UnitType>
<SimpleSource>
<SourceFilename relativeToVRT="0">/home/even/proj/proj-datumgrid/europe/nkgrf03vel_realigned_z.gtx</SourceFilename>
<SourceBand>1</SourceBand>
<SrcRect xOff="0" yOff="0" xSize="223" ySize="241" />
<DstRect xOff="0" yOff="0" xSize="223" ySize="241" />
</SimpleSource>
</VRTRasterBand>
</VRTDataset>
```
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