[gdal-dev] Masked VRT
Simon Wright
Simon.Wright at jbarisk.com
Mon Feb 24 11:22:29 PST 2025
Dear subscribers
I'm exploring the use of a VRT to mosaic raster tiles while also applying a mask layer using separate mask tiles.
Below is a VRT I created to test this approach:
<VRTDataset rasterXSize="90" rasterYSize="90">
<SRS dataAxisToSRSAxisMapping="2,1">GEOGCS["WGS 84",DATUM["WGS_1984",SPHEROID["WGS 84",6378137,298.257223563,AUTHORITY["EPSG","7030"]],AUTHORITY["EPSG","6326"]],PRIMEM["Greenwich",0,AUTHORITY["EPSG","8901"]],UNIT["degree",0.0174532925199433,AUTHORITY["EPSG","9122"]],AXIS["Latitude",NORTH],AXIS["Longitude",EAST],AUTHORITY["EPSG","4326"]]</SRS>
<GeoTransform> 0.0000000000000000e+00, 1.0000000000000000e+00, 0.0000000000000000e+00, 9.0000000000000000e+01, 0.0000000000000000e+00, -1.0000000000000000e+00</GeoTransform>
<VRTRasterBand dataType="Float32" band="1">
<NoDataValue>-99</NoDataValue>
<ColorInterp>Gray</ColorInterp>
<SimpleSource resampling="nearest">
<SourceFilename relativeToVRT="1">tile_00.tif</SourceFilename>
<SourceBand>1</SourceBand>
<SourceProperties RasterXSize="30" RasterYSize="30" DataType="Float32" BlockXSize="30" BlockYSize="30" />
<SrcRect xOff="0" yOff="0" xSize="30" ySize="30" />
<DstRect xOff="0" yOff="60" xSize="30" ySize="30" />
<NODATA>-99</NODATA>
</SimpleSource>
<SimpleSource resampling="nearest">
<SourceFilename relativeToVRT="1">tile_01.tif</SourceFilename>
<SourceBand>1</SourceBand>
<SourceProperties RasterXSize="30" RasterYSize="30" DataType="Float32" BlockXSize="30" BlockYSize="30" />
<SrcRect xOff="0" yOff="0" xSize="30" ySize="30" />
<DstRect xOff="0" yOff="30" xSize="30" ySize="30" />
<NODATA>-99</NODATA>
</SimpleSource>
<SimpleSource resampling="nearest">
<SourceFilename relativeToVRT="1">tile_02.tif</SourceFilename>
<SourceBand>1</SourceBand>
<SourceProperties RasterXSize="30" RasterYSize="30" DataType="Float32" BlockXSize="30" BlockYSize="30" />
<SrcRect xOff="0" yOff="0" xSize="30" ySize="30" />
<DstRect xOff="0" yOff="0" xSize="30" ySize="30" />
<NODATA>-99</NODATA>
</SimpleSource>
<SimpleSource resampling="nearest">
<SourceFilename relativeToVRT="1">tile_10.tif</SourceFilename>
<SourceBand>1</SourceBand>
<SourceProperties RasterXSize="30" RasterYSize="30" DataType="Float32" BlockXSize="30" BlockYSize="30" />
<SrcRect xOff="0" yOff="0" xSize="30" ySize="30" />
<DstRect xOff="30" yOff="60" xSize="30" ySize="30" />
<NODATA>-99</NODATA>
</SimpleSource>
<SimpleSource resampling="nearest">
<SourceFilename relativeToVRT="1">tile_11.tif</SourceFilename>
<SourceBand>1</SourceBand>
<SourceProperties RasterXSize="30" RasterYSize="30" DataType="Float32" BlockXSize="30" BlockYSize="30" />
<SrcRect xOff="0" yOff="0" xSize="30" ySize="30" />
<DstRect xOff="30" yOff="30" xSize="30" ySize="30" />
<NODATA>-99</NODATA>
</SimpleSource>
<SimpleSource resampling="nearest">
<SourceFilename relativeToVRT="1">tile_12.tif</SourceFilename>
<SourceBand>1</SourceBand>
<SourceProperties RasterXSize="30" RasterYSize="30" DataType="Float32" BlockXSize="30" BlockYSize="30" />
<SrcRect xOff="0" yOff="0" xSize="30" ySize="30" />
<DstRect xOff="30" yOff="0" xSize="30" ySize="30" />
<NODATA>-99</NODATA>
</SimpleSource>
<SimpleSource resampling="nearest">
<SourceFilename relativeToVRT="1">tile_20.tif</SourceFilename>
<SourceBand>1</SourceBand>
<SourceProperties RasterXSize="30" RasterYSize="30" DataType="Float32" BlockXSize="30" BlockYSize="30" />
<SrcRect xOff="0" yOff="0" xSize="30" ySize="30" />
<DstRect xOff="60" yOff="60" xSize="30" ySize="30" />
<NODATA>-99</NODATA>
</SimpleSource>
<SimpleSource resampling="nearest">
<SourceFilename relativeToVRT="1">tile_21.tif</SourceFilename>
<SourceBand>1</SourceBand>
<SourceProperties RasterXSize="30" RasterYSize="30" DataType="Float32" BlockXSize="30" BlockYSize="30" />
<SrcRect xOff="0" yOff="0" xSize="30" ySize="30" />
<DstRect xOff="60" yOff="30" xSize="30" ySize="30" />
<NODATA>-99</NODATA>
</SimpleSource>
<SimpleSource resampling="nearest">
<SourceFilename relativeToVRT="1">tile_22.tif</SourceFilename>
<SourceBand>1</SourceBand>
<SourceProperties RasterXSize="30" RasterYSize="30" DataType="Float32" BlockXSize="30" BlockYSize="30" />
<SrcRect xOff="0" yOff="0" xSize="30" ySize="30" />
<DstRect xOff="60" yOff="0" xSize="30" ySize="30" />
<NODATA>-99</NODATA>
</SimpleSource>
</VRTRasterBand>
<MaskBand>
<VRTRasterBand dataType="Byte">
<NoDataValue>0</NoDataValue>
<SimpleSource>
<SourceFilename relativeToVRT="1">mask_m11.tif</SourceFilename>
<SourceBand>1</SourceBand>
<SrcRect xOff="0" yOff="0" xSize="30" ySize="30" />
<DstRect xOff="30" yOff="30" xSize="30" ySize="30" />
</SimpleSource>
<SimpleSource>
<SourceFilename relativeToVRT="1">mask_m12.tif</SourceFilename>
<SourceBand>1</SourceBand>
<SrcRect xOff="0" yOff="0" xSize="30" ySize="30" />
<DstRect xOff="30" yOff="0" xSize="30" ySize="30" />
</SimpleSource>
<SimpleSource>
<SourceFilename relativeToVRT="1">mask_m21.tif</SourceFilename>
<SourceBand>1</SourceBand>
<SrcRect xOff="0" yOff="0" xSize="30" ySize="30" />
<DstRect xOff="60" yOff="30" xSize="30" ySize="30" />
</SimpleSource>
<SimpleSource>
<SourceFilename relativeToVRT="1">mask_m22.tif</SourceFilename>
<SourceBand>1</SourceBand>
<SrcRect xOff="0" yOff="0" xSize="30" ySize="30" />
<DstRect xOff="60" yOff="0" xSize="30" ySize="30" />
</SimpleSource>
</VRTRasterBand>
</MaskBand>
</VRTDataset>
This VRT consists of nine 30 × 30 pixel raster tiles arranged in a 3 × 3 grid, with a mask layer composed of four tiles.
When I display the VRT in QGIS, only those raster tiles that intersect the mask tiles are visible. Similarly, when querying the masked VRT using a Python script using a GDAL dataset, a GDAL band, and a NumPy masked array, I retrieve data only from tiles that overlap the mask tiles.
My question is: Do I need to provide mask tiles covering the entire extent of the raster mosaic, or is there an alternative approach that allows me to use mask tiles only where I want to mask specific areas (e.g., to reduce redundancy)?
I’d appreciate any guidance or advice.
Many thanks in advance of any guidance or advice any can provide, best wishes, Simon.
Simon Wright
GISMO Scrum Master and RML Head of Software
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