[gdal-dev] How does WMTS driver detect DataWindow?

[Rahkonen Jukka (MML)]

Hi,

While playing with gdal_translate and WMST my colleague found that GDAL was picking a wrong DataWindow with


gdal_translate "WMTS:http://xx.yy.fi/foo/service?REQUEST=GetCapabilities&SERVICE=WMTS,layer=bar" bar.xml -of WMTS



After some thinking we started to believe that the reason may be in the wrong WGS84BoundingBox that this WMTS service puts into GetCapabilities

<ows:WGS84BoundingBox>

<ows:LowerCorner>-6.38153862706 55.6179644952</ows:LowerCorner>

<ows:UpperCorner>60.3815386271 75.5825702342</ows:UpperCorner>

</ows:WGS84BoundingBox>



I checked with gdaltransform that longitude -6.38153862706 converts into the value that was used as the DataWindow UpperLeftX value:

<UpperLeftX>-1553364.000000402</UpperLeftX>



However, the TopLeftCorner of the TileMatrix as it is defined in the GetCapabilities has a much smaller X value: -548576.000000



<TileMatrix>

<ows:Identifier>0</ows:Identifier>

<ScaleDenominator>29257142.85714285820722579956</ScaleDenominator>

<TopLeftCorner>-548576.000000 8388608.000000</TopLeftCorner>

<TileWidth>256</TileWidth>

<TileHeight>256</TileHeight>

<MatrixWidth>1</MatrixWidth>

<MatrixHeight>1</MatrixHeight>

</TileMatrix>



Probably because of the wrong UpperLeftX value was gdal_translate seems to think that the leftmost tile in this layer has a negative TileX value

<TileX>-3924</TileX>



By reading the WMTS standard the negative index values are not valid in WMTS:

TileCol: Column index of tile matrix

Non negative integer type value between 0 and MatrixWidth-1 of this tile matrix defined in the ServiceMetadata document



BoundingBoxes does not seem to be mandatory in WMTS because they can be computed from other, mandatory parameters as it is written in the notes of Table 14 - Parts of TileMatrix data structure



"NOTE 7 The bounding box of a tile matrix is not supplied explicitly because it can be calculated from topLeftCorner, tileWidth, tileHeight and scaleDenominator."



It feels that gdal_translate should never write a DataWindow that is bigger than the extents of the TileMatrix.   Perhaps BoundingBoxes, if they exist, could be used for limiting the maximum extent, like the TileMatrixSetLimits if they are found from the GetCapabilities. Or is there something special in WMTS that makes it much more comfortable for GDAL to trust in BoundingBoxes? Like the fact that different tile levels in WMTS may have different bounding boxes?



-Jukka Rahkonen-

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