[mapserver-users] WMS Layer Ordering

[Paul Ramsey]

Rob Atkinson wrote:
> 
> The WMS has no concept of "absolute" cartographic ordering
> relationships precisely because it is designed to share data from
> multiple sources. Typically a client would use an external
> configuration to select which layers from which servers appear in what
> order.

This seems a little retrograde, to say the least. It almost completely
forstalls the usefulness of a WMS with more than a handful of
non-interfering layers. It is nice to provide clients with the ability
to 'reorder' their layers, but it is also nice to provide them with a
cartographically sensible default view. 
 
> FYI some proposed future capabilities schemas allow for multiple
> presentation hierarchies which would allow for this use case.

Am I completely out-to-lunch in thinking that people will use the
capabilities order as an implied layer order? Hell, it is the easiest
thing to do, foreach(<Layer>){add(layerWidget)} it is happening right
now through computational inertia...

> Regards
> 
> Rob Atkinson
> 
> Daniel Morissette wrote:
> 
> > Paul Ramsey wrote:
> >
> >> What is the algorithm which determines the relationship between
> >> layer
> >> ordering in the map file and the layer ordering in the WMS
> >> capabilities?
> >> They do not appear to match up 1:1.
> >>
> > Hi Paul,
> > Unless you have uncovered a bug, which is always possible, then the
> > order of the layers in the capabilities should be:
> > 1- First all layers that do NOT belong to any group are listed in
> > the
> > order that they appear in the mapfile
> > 2- Then the layers that belong to a group are listed by group.  The
> > order of the groups should be based on the order of first of
> > occurence
> > of each group when scanning the layers in the mapfile.
> > If this is not the behavior that you witnessed then please file a
> > bug
> > about this with a sample mapfile and I'll have a look.
> >
> >> This is not an unimportant problem,
> >> since the layer order passed in the GetMap request seems to be
> >> respected
> >> by the renderer (so if I say LAYERS=roads,lakes my water features
> >> will
> >> be rendered on top of my roads features). So if I read my layers
> >> out of
> >> GetCapabilities, and then apply those layers in a GetMap in
> >> exactly the
> >> same order I get them, the result may be quite different from the
> >> result
> >> if I did the same query using the mapserver interface instead of
> >> the WMS
> >> interface.
> >>
> > I agree that if you have some layers with a group value and some
> > without
> > group values then following the order of layers in the capabilities
> > could lead to cartographically invalid results.
> > Suggestions of better ways to handle this are welcome.
> > Thanks,
> > Daniel
> >