[OpenLayers-Users] Geoserver GWC layers plotting offset at some zoom levels.

[Andrea Aime]

On Tue, Jul 7, 2015 at 6:21 AM, Phil Scadden <p.scadden at gns.cri.nz> wrote:

> I have GWC tile caching turned on for a layer in geoserver 2.7.1.1 (direct
> integration turned on), then at some zoom levels, it plots displaced from
> where it should be. Turn GWC off for the layer, refresh browser, and it
> plots okay. This when calling as WMS layer from Openlayer 2.13-1 master. I
> remember this being an issue back in 2012 and issue
> https://github.com/openlayers/openlayers/issues/688 is still open. I am
> not doing any pre-seeding. Problem only occurs at some zoom levels.
> A request looks like:
>
>
> http://myserver/wms?LAYERS=mylayer&TILED=true&TRANSPARENT=true&SERVICE=WMS&VERSION=1.1.1&REQUEST=GetMap&STYLES=&FORMAT=image%2Fpng&SRS=EPSG:3857&BBOX=18628621.034844,-6521221.593125,18785164.06875,-6364678.5592188&WIDTH=256&HEIGHT=256
>
> Note the EPSG:3857 - the tileset in GWC is 900913 but I am guessing the
> equivalence is handled.
>
> Is this still the maxextent playing badly?
>

That is still your starting point, if the maxentent of the server and
client are not aligning, some requests will fall
off the grid.

If I may share my 2 cents, the arrangement of asking tiles thought the WMS
protocol is not the best one, the client
should go for the WMTS capabilities, check the gridset definitions, abide
to them, and then make x/y/z requests.

The WMS with direct integration was made under the notion of "most clients
will ask data along well known gridsets
anyways, so let's try to use the cache when possible", that is,
opportunistic speedup.
The thing is, GWC has a bit of tolerance, if you ask for a tile that's
slightly off the actual cached tile extents, it will
force it to the cached ones... which might be the reason of your
mis-alignment

In case of doubts, try the GWC internal preview, it's using WMS but with
well aligned gridsets, do you also get
the misalignments there?

Cheers
Andrea

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Ing. Andrea Aime
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