GetFeatureInfo and scale-dependent layers

[Bart van den Eijnden]

Hi list,

I could not find anything in the WMS spec about what a WMS should do if it
receives a GetFeatureInfo request for a layer which is not visible at the
scale of the map.

Should it return the feature info anyway even though the layer is not
visible, or not?

For instance I am using a group of layers, i.e. layers=A,B,C where A, B
and C are all the same data but represented at different scales. So e.g.
at 1:100000 A becomes invisible and B becomes visible.

But when I do a GetFeatureInfo request on UMN Mapserver in this case for
query_layers=A,B,C I get back 3 responses, but I would hope for only 1.
Since in the current situation I as a WMS client have to filter out the
right one from the response based on the scales of the layers, otherwise I
would be presenting information (e.g. height) which is not valid/excact at
the current map scale.

Best regards,
Bart

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[Keith Pomakis]

bartvde@... wrote:

> I could not find anything in the WMS spec about what a WMS should do if it
> receives a GetFeatureInfo request for a layer which is not visible at the
> scale of the map.
>
> Should it return the feature info anyway even though the layer is not
> visible, or not?
>
> For instance I am using a group of layers, i.e. layers=A,B,C where A, B
> and C are all the same data but represented at different scales. So e.g.
> at 1:100000 A becomes invisible and B becomes visible.
>
> But when I do a GetFeatureInfo request on UMN Mapserver in this case for
> query_layers=A,B,C I get back 3 responses, but I would hope for only 1.
> Since in the current situation I as a WMS client have to filter out the
> right one from the response based on the scales of the layers, otherwise I
> would be presenting information (e.g. height) which is not valid/excact at
> the current map scale.

All the WMS specification says about this is:

    The actual semantics of how a WMS decides what to return more
    information about, or what exactly to return, are left up to the
    WMS provider.

However, the spirit of the WMS GetFeatureInfo request, at least in my
opinion, is as you say.  It should endeavor to only return information
on visible features.  (Perhaps this should be stated more explicitly
in the specification.)  The current scale may affect this, of course,
as may filters in the selected style.

---
Keith Pomakis <pomakis@...>
Senior Software Developer, CubeWerx Inc.
15 rue Gamelin, Gatineau, Québec J8Y 3B5
Phone: [phone] x202, FAX: [phone]
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[Allan Doyle-3]

On Feb 27, 2009, at 12:03 PM, Keith Pomakis wrote:

> bartvde@... wrote:
>> I could not find anything in the WMS spec about what a WMS should  
>> do if it
>> receives a GetFeatureInfo request for a layer which is not visible  
>> at the
>> scale of the map.
>>
>> Should it return the feature info anyway even though the layer is not
>> visible, or not?
>>
>> For instance I am using a group of layers, i.e. layers=A,B,C where  
>> A, B
>> and C are all the same data but represented at different scales. So  
>> e.g.
>> at 1:100000 A becomes invisible and B becomes visible.
>>
>> But when I do a GetFeatureInfo request on UMN Mapserver in this  
>> case for
>> query_layers=A,B,C I get back 3 responses, but I would hope for  
>> only 1.
>> Since in the current situation I as a WMS client have to filter out  
>> the
>> right one from the response based on the scales of the layers,  
>> otherwise I
>> would be presenting information (e.g. height) which is not valid/
>> excact at
>> the current map scale.
>
> All the WMS specification says about this is:
>
>    The actual semantics of how a WMS decides what to return more
>    information about, or what exactly to return, are left up to the
>    WMS provider.
>
> However, the spirit of the WMS GetFeatureInfo request, at least in my
> opinion, is as you say.  It should endeavor to only return information
> on visible features.  (Perhaps this should be stated more explicitly
> in the specification.)  The current scale may affect this, of course,
> as may filters in the selected style.

This is a reasonable thing to do, but consider a mode where a user can  
click on a point, request GetFeatureInfo, and the returned information  
is a list of all the server's available layers that have data at that  
point. This one of the parts of the spec we left deliberately vague so  
that people could do interesting stuff with it.
>

--
Allan Doyle
Director of Technology
MIT Museum | http://web.mit.edu/museum | +1.617.452.2111



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