SRS handling questions

[Martin Schulze]

Moin!

Silke Reimer wrote:
> > Yes.  They should.  However, if they aren't, and the list of SRS is
> > larger, it's likely to cause problems since there won't be a bounding
> > box associated to an SRS.  Hence, I tend to ignore the <SRS> tags
> > except for the root layer, and generate the list of SRS for child
> > layers from their bounding box elements.  Except if you disagree.
> 
> Sorry to reopen the discussion again, but I had the idea that the
> perhaps the SRS attribute in the BoundingBox element is not meant to
> define the existence of a new SRS but does only assign the
> BoundingBox to a special SRS. Perhaps a SRS should even not be

true.

> appear in an BoundingBox element if it is not part of the SRS

*shrug*  I don't remember the specs drawing any connection between the
<SRS> element and the <BoundingBox> elements.

> element. If there is no BoundingBox for in SRS the BoundingBox would
> just be the LatLonBoundingBox projected to the SRS.

There's nothing like this mentioned in the specs, as far as I can see.

> Does the spec write something about this relation? At least this
> would make sense in my opinion. As a follow-up the list of SRSs
> should not be derived from the bounding box elements of a layer, but
> only from the SRS element.

As far as I have understood the use of Bounding Boxes and SRS, each
layer is available in multiple SRS.  The map server will render the
requested layer clipping using the specified projection.  From the
clients point of view, the layer is available in a number of SRSs,
EPSG:4326 being the one used for the LatLonBoundingBox clipping, and
others (or the same) for the other BoundingBox clippings.

I'll quote the specs a bit:

    6.5.5

    [..] OGC Web Services are not required to support all possible
    SRSes, but shall advertise in their Capabilities XML those
    projections which they do offer and shall accept requests for all
    advertised projections.

    If a request contains an SRS not offered by a particular server,
    the server shall throw a Service Exception (code="InvalidSRS").


    7.1.4.5.7

    [..] Each BoundingBox states the bounding rectangle of the map data
    in a particular spatial reference system; the attribute SRS
    indicates which SRS applies.

    [..] A Layer may have multiple Boundingbox elements, but each one
    shall state a different SRS.

    A Layer inherits any Bounding Box values defined by its parents.

    [..] A single Layer element shall not contain more than one
    BoundingBox for the same SRS.

    [..] A server which has the ability to transform data to different
    SRSes may choose not to provide an explicit BoundingBox for every
    possible SRS available for each Layer.

    The server should provide BoundingBox information for a tleast the
    native SRS of the Layer.

My interpretation is that a layer is available for the SRS it
announces in the <BoundingBox> elements (and llbbox) with the
respective minimum enclosing rectangle as specified.

When you request a map to be rendered by the map server, you'll have
to supply the SRS you want to receive it rendered in.  Using an SRS
which wasn't announced, will most probably result in an exception
instead of an image.

Hence, I believe that the SRS denoted in the <BoundingBox> elements
build the authoritative list of SRSes the map can be rendered in.

Regards,

	Joey

-- 
We all know Linux is great... it does infinite loops in 5 seconds.
        -- Linus Torvalds