[gdal-dev] Masked VRT

Daniel Baston dbaston at gmail.com
Tue Feb 25 16:22:48 PST 2025 

Simon,

Setting the NoData value of your mask band to 255 instead of 0 should cause
pixels not covered by a mask tile to be considered valid.

Dan

On Mon, Feb 24, 2025 at 2:37 PM Simon Wright via gdal-dev <
gdal-dev at lists.osgeo.org> wrote:

> 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*
>
>
>
> --
>
>
>
> e: ​​​​
>
> Simon.Wright at jbarisk.com
>
> d:
>
> +44 (0)1756 587258
>
> t:
>
> +44 (0)1756 799919 <+44%20(0)1756%20799919>
>
> www.jbarisk.com
>
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>
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>
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