TMS tile layers have vertical blank stripes in Swiss Grid projection (EPSG:21781)

[Shernott]

When the tiles of TMS tile layers are reprojected for the Swiss Grid projection (EPSG:21781), there seems to be a (small) (rounding?) error, as a result TMS tile layers have vertical blank stripes in Swiss Grid projection.

[stoecker]

There is no reprojection at all. You need to choose the projection matching the source, so for TMS it usually is Merkaartor or WGS84.

[Shernott]

Replying to stoecker:

There is no reprojection at all. You need to choose the projection matching the source, so for TMS it usually is Merkaartor or WGS84.

I don't know how you want to call what is going on there, and I agree that most TMS tiles look best in Merkaartor or WGS84 (their original projections). However, if you chose "Swiss Grid" projection, the TMS tiles are tilted and realigned and they are almost perfectly matching. But only almost, there remain vertical (and no horizontal) gaps of about 2-3 pixels between the tile columns, no matter what zoom level is selected. This is why I guess that there might be a rounding up/down error somewhere in the algorithm that turns and realigns the tiles.

[Shernott]

Ok, I think I see what is going on: The tiles are actually not tilted or turned, but they are drawn at the correct new offsets, which makes the tile columns look slightly tilted. Probably the Swiss Grid is close enough to the Merkaartor projection to make it almost work, but since the tiles are not scaled or tilted. the gaps occur.

Well, in that case it's not a bug report but a feature request, it would be nice to have a reprojection in order to be able to mix tile sources with different projections.

[stoecker]

Closed as duplicate of #6432.