[OSGeo-Standards] WMTS different <TopLeftCorner> values

[Jason Lee]

Hi,
I am setting up a WMTS service with pre-built tiles in a non-Web Mercator
projection (osgb). It has zoom levels which are not always half on each
level, in particular zoom 7 below:-

 * zoom 6 @scale 1:100k,
 * zoom 7 @scale 1:75k,
 * zoom 8 @scale 1:50k
 * zoom 8 @scale 1:25k

I set the <TopLeftCorner> appropriately and things seem to look good when
viewed in QGIS, capabilities XML as follows:-

<TileMatrix>
<ows:Identifier>6</ows:Identifier>
<ScaleDenominator>100000</ScaleDenominator>
<TopLeftCorner>0 1433600</TopLeftCorner>
<TileWidth>256</TileWidth>
<TileHeight>256</TileHeight>
<MatrixWidth>98</MatrixWidth>
<MatrixHeight>200</MatrixHeight>
</TileMatrix>

<TileMatrix>
<ows:Identifier>7</ows:Identifier>
<ScaleDenominator>75000</ScaleDenominator>
<TopLeftCorner>0 1433600</TopLeftCorner>
<TileWidth>256</TileWidth>
<TileHeight>256</TileHeight>
<MatrixWidth>131</MatrixWidth>
<MatrixHeight>273</MatrixHeight>
</TileMatrix>

<TileMatrix>
<ows:Identifier>8</ows:Identifier>
<ScaleDenominator>50000</ScaleDenominator>
<TopLeftCorner>0 1433600</TopLeftCorner>
<TileWidth>256</TileWidth>
<TileHeight>256</TileHeight>
<MatrixWidth>196</MatrixWidth>
<MatrixHeight>426</MatrixHeight>
</TileMatrix>

...but I notice zoom 7 is displayed offset (and same in ArcGIS) and I think
I needed to recalculate the <TopLeftCorner> as I guess it's grid doesn't
"snap" into the grids of the other zoom levels given the same
<TopLeftCorner>.

However, when I change this value just for this TileMatrix, even giving it
a silly value like "<TopLeftCorner>0 0</TopLeftCorner>" it makes no
difference to how the grid aligns at this zoom level.

I found that you have to alter the BOTTOM-MOST TileMatrix (zoom 18) that
makes the grid shift - which is then fine for zoom 7 but ALL the other zoom
levels are offset.

I don't understand why the <TopLeftCorner> value change to zoom 7
TileMatrix does shift this grid alone??

Any help or advice much appreciated, thanks.

Jason
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