[WMS] Support SRS=NONE (and CRS:1 in WMS 1.2)

[dmorissette]

Frank Warmerdam wrote:
---
Have you ever encountered anything about EPSG:NONE being a valid way of
describing a coordinate system not related to the earth?  I see a PCI
server is using this convention and I am wondering if MapServer should
somehow be made to support it or not.  A clients application copies
the EPSG strings into new mapfiles when using the MapServer WMS client
support. 


Tom Kralidis wrote:
> I say support it.  EPSG:NONE is good for images which are not
> necessarily
> spatial.  Like old maps, charts, etc.  The PCI software allows one to
> output
> projects as WMS layers (map, legend, chart, other info) in one bundle.
> Not
> really for fusion w/ other data, yet a bundled approach.
> 

Ed McNierny wrote:
>Frank & Tom -
>> 
>> Does either EPSG or OGC have anything to say about this?  The 
>> support is a good in concept, but I don't want to start 
>> propagating a particular specification simply because we 
>> discovered one vendor using it.
>> 

Tom wrote:

This is noted in the WMS 1.1.1 specification (section 6.5.5.3).

Ed McNierney wrote:
> 
> Thanks for the reference!  The specification seems to say that the
> correct implementation is "SRS=NONE", not "SRS=EPSG:NONE", which makes
> perfect sense (the keyword NONE is not case-sensitive), and that this
> value *shall* be used for data whose spatial reference is undefined.
> The EPSG: prefix should be reserved for valid EPSG codes, and it sounds
> like (a) MapServer should support the SRS=NONE parameter and (b) PCI
> should fix their software to comply with the spec.
>

[dmorissette]

Perhaps MapServer should try to support SRS=NONE, but we need to think about the
way to support this in a mapfile and internally.  For instance the spec also
states that 
 "Clients should not attempt to overlay information whose SRS=none 
  with other information."
This restriction is important otherwise the behavior is undefined if you request
a geographical layer using SRS=NONE or if you try to overlay two layers that
don't have a SRS.

BTW, in WMS 1.2, SRS=NONE is replaced by 'CRS:1' which is the image (pixel)
coordinate system.

[fwarmerdam]

*** Bug 484 has been marked as a duplicate of this bug. ***

[dmorissette]

Jan Hartmann wrote:
>
> Daniel, what is the problem with overlaying two layers without an SRS? I am
working with unreferenced historical maps with superimposed architectural
drawings digitized from the maps.  I would like to publish
> these as WMS maps in a few months, but for now everything needs to be
geo-referenced. This gives some distortion as the maps are not exactly to scale.
MapServer can overlay non-referenced images without problem, so why could its
WMS version not do the same?
>


If you know that the two layers with SRS=NONE represent the same area and can
overlap then there is no problem (which seems to be the case with your
historical maps).  But just putting SRS=NONE in a capabilities document doesn't
tell the client that it is safe to combine two layers.  OTOH when two layers
have SRS=EPSG:4326 then it is clear that it's OK to draw them on top of each other.

We could let MapServer overlay images with SRS=NONE and let the users deal with
the unpredictable results I guess.

BTW, should we allow zooming on layers with SRS=NONE?  Does this assume that the
BBOX coordinates are in pixels in this case?  I think this behavior is clarified
with the CRS:1 that replaces SRS=NONE in WMS 1.2, I would have to re-read that
part of the spec before we implement anything.

[assefa]

What is the status on this ? Should it be set to FUTURE.

[dmorissette]

I've set this to FUTURE since attempting to handle this could lead to
side-effects or new bugs, so it's too late for 4.4.

[fwarmerdam]

I concur that we should leave it for post 4.4. 

I would also note that SRS:NONE support is likely to interact badly with
the need for a projection on layers and maps in our current WMS support, 
for the nonsquare support for instance.