[webmap-discuss] OGC and google style tiling

dblasby at openplans.org dblasby at openplans.org
Thu Apr 27 15:28:29 EDT 2006

I've been thinking of tiling rendering for WMS in general, and geoserver
in particular (especially with OpenLayers apparently coming out soon
and mapbuilder also adding similar functionality).

Paul gave a good account of the problems that I've been thinking about:
a) Edge Rendering Effects
b) cache HTTP headers
c) Get capabilities (ie. tell clients about how to make requests --
hopefully OGC will define this)
d) Labeling

I think a-c are actually fairly easy to handle with a "dumb" proxy WMS.
Take a quick look at the two images I attached.

a) Edge Rendering Effects
     This can be handled simply by intercepting an incoming WMS
request, expanding the request bounding box and image size and sending
the request down to the actual WMS server.  The image returned is then
"cookie cut" so its the proper size/extent of the original request.

     You'd have to tune the expansion size, but generally you'll want it
to be a few pixels wider than your "widest style."  For example, if
your roads are 12 pixels wide you'd want to expand the image by about
14 pixels in all directions (extra pixels are to account for

b) cache HTTP headers
      Geoserver allows you to set the cache expire headers, but you can
also have the WMS proxy mangle header for servers that dont allow this.

c) Get capabilities
      At some time in the future, the WMS 'get capabilities' document
will contain the grid information the server wants the client to use
for tiling.  Once this is defined, the proxy WMS can augment the real
server's get capabilities so that clients make "proper" requests.

Unfortunately, I dont see a good solution to labeling issues (except
making requests for very large images) - anyone have any ideas?


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