[mapserver-users] MapCache and the GoogleCRS84Quad TileMatrixSet

[thomas bonfort]

Armin,
I don't have much to add to what you have already found out. I personally
consider the GoogleCRS84Quad grid as defined by the spec to be broken, and
have not implemented its support as it would require some specific hacks in
the code (as the extent for level 0 needs to be hardcoded for this specific
grid, instead of it being derived from grid properties as is the case with
all other grid definitions).

I suppose that support for this could be added if you really need it badly.
The impact is not negligible as all places where an extent is derived from
a tile coordinate and vice-versa would need to be updated to support this
illogical quirk.

FWIW, I believe I had brought this up as a change request to the wmts
working group, but this was ignored as I am not a member.

best regards,

thomas


On 9 April 2013 20:57, Armin Burger <armin.burger at gmx.net> wrote:

> Dear all
>
> I have a question regarding the GoogleCRS84Quad TileMatrixSet defined in
> MapCache. MapCache specifies this in the docs with the first level [0] as
> pixel resolution 0.70312500 deg, and an extent of
>   <extent>-180 -90 180 90</extent>
>
> and returns the GoogleCRS84Quad TileMatrixSet in the Capabilities response
> like
>
> <TileMatrixSet>
>   <Identifier>WGS84</Identifier>
>   <Title>GoogleCRS84Quad</Title>
>   <SupportedCRS>urn:ogc:def:crs:**EPSG:6.3:4326</SupportedCRS>
>   <BoundingBox crs="urn:ogc:def:crs:EPSG:6.3:**4326">
>     <LowerCorner>-180.000000 -90.000000</LowerCorner>
>     <UpperCorner>180.000000 90.000000</UpperCorner>
>   </BoundingBox>
>
> <WellKnownScaleSet>urn:ogc:**def:wkss:OGC:1.0:**GoogleCRS84Quad</**
> WellKnownScaleSet>
>   <TileMatrix>
>     <Identifier>0</Identifier>
>     <ScaleDenominator>279541132.**01435887813568115234</**
> ScaleDenominator>
>     <TopLeftCorner>90.000000 -180.000000</TopLeftCorner>
>     <TileWidth>256</TileWidth>
>     <TileHeight>256</TileHeight>
>     <MatrixWidth>2</MatrixWidth>
>     <MatrixHeight>1</MatrixHeight>
>   </TileMatrix>
>   .....
> </TileMatrixSet>
>
> This looks clear and logical, but the OGC specs at
>   http://portal.opengeospatial.**org/files/?artifact_id=35326<http://portal.opengeospatial.org/files/?artifact_id=35326>
> define this TileMatrixSet differently as
>
> "E.3 GoogleCRS84Quad (urn:ogc:def:wkss:OGC:1.0:**GoogleCRS84Quad)
> This well-known scale set has been defined to allow quadtree pyramids in
> CRS84. Level 0 allows representing the whole world in a single 256x256
> pixels (where the first 64 and last 64 lines of the tile are left blank).
> The next level represents the whole world in 2x2
> tiles of 256x256 pixels and so on in powers of 2. Scale denominator is
> only accurate near the equator."
>
> The level 0 in the OGC specs uses a resolution of 1.40625000.
>
> The tile 0/0/0 in MapCache is not a single tile as in the OGC specs but
> the first out of 2 tiles for this level, correctly defined with MatrixWidth
> 2 and MatrixHeight 1 (level 1 has 4x2 tiles). For me the OGC specs are not
> logical, because the TopLeft corner would correspond to 180 -180 degrees
> which makes no sense for me, but somehow they are regarded as the default
> specs...
>
> So the XML definition of the OGC specs should probably result in something
> like
>
>   <TileMatrix>
>     <Identifier>0</Identifier>
>     <ScaleDenominator>559082264.**0287178</ScaleDenominator>
>     <TopLeftCorner>180.000000 -180.000000</TopLeftCorner>
>     <TileWidth>256</TileWidth>
>     <TileHeight>256</TileHeight>
>     <MatrixWidth>1</MatrixWidth>
>     <MatrixHeight>1</MatrixHeight>
>   </TileMatrix>
>
> Unfortunately the OGC doc does not provide this matrix set's full
> definition in XML to check this.
>
>
> So I'm wondering which is the correct definition? And any idea how to deal
> with this discrepancy?
>
> Thanks
> Armin
>
>
>
>
>
>
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