[Mapserver-dev] EPSG:NONE

Jan Hartmann jhart at frw.uva.nl
Thu Nov 20 07:54:14 EST 2003

Daniel Morissette wrote:
> 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.

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?

As to overlaying two layers with different SRS's, I'm not sure if the 
results are "undefined". You can perfectly well combine coordinates from 
different base systems; it just doesn't make sense, as you get maps with 
wildly divergent parts. That is what people do who try to display a 
latlong map in a geographic projection and "don't see anything" as a 
result. It's not impossible, you just don't get what you expected. I 
guess that is why the spec says you should not attempt it: you can do it 
  but it is not wise.

As I said, I would like very much this SRS=NONE capability. I refrained 
from asking for it, since I thought it was outside the WMS specs, but 
now I know it is in 1.1.1 (thanks Tom), I would like to add  my €0.02 
request for it.


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