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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