[webmap-discuss] OGC and google style tiling

Rob Atkinson rob at socialchange.net.au
Thu Apr 27 18:07:28 EDT 2006

The issues below are the relatively simple ones compared to the issue I 
saw previously raised - are you going to constrain the map scales the 
client is allowed to ask for, and if so:
1) how to advertise the scales available
2) how to get the client to ask for the appropriate scale
3) how to handle intermediate scales

Possibly, modern desktop power would allow scaling of images to occur in 
the desktop, giving advantages of caching at the browser side. There is 
still going to be a contract between the client code to choose the 
scales and the server's capabilities, realised through some form of 
"getCapabilities" response.

The same contract would be required for the client to restrict - jump 
between available scales.

The alternative is to have scale-independent server side - i.e. have the 
cache itself generate tiles for intermediate scales seamlessly. This may 
be a large server load, quality issues would affect any "intermediate 
scale" solution, client or server

The other issue is whether the tiling strategy is one of partitioning or 
caching potentially overlapping requests.  Gazetteer driven solutions 
can afford to cache requests (tiled or not) since the users will go to 
specific areas repeatedly, but eventually a tile based solution would win.

The final issue affecting client design is knowing whether a service can 
be tiled. If a tiling proxy is used in front of all services, this may 
not be an issue, but you will be bound to a server. If not, you need to 
handle tiling strategy per layer. Maybe its possible to have this 
seamlessly - i.e. the client API justs accepts a layer handle and works 
it out, or maybe this needs to be specified. Some way of choosing the 
"aggregate behaviour" would be required if certain layers are restricted 
to scales and you want to restrict the client to these scales

Feels like a tractable problem, but it would be nice to see the APIs of 
the components defined at both server and client side.


dblasby at openplans.org wrote:

>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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