[udig-devel] accuracy of GetFeatureInfo

[Paul Ramsey]

I think the problem with this approach, as now demonstrated, is that  
it doesn't work :)  It might take something a little more  
"arbitrary", like a pretend image of "about 300 pixels in size" with  
the click point in the "middle of that image" to get a correct- 
seeming answer.  Sending back a one-pixel map with a click "on the  
pixel" is begging to hit corner cases in the WMS server side code.

On 5-Sep-06, at 12:29 AM, Jody Garnett wrote:

> Ah, interesting question ... because of a difference between what  
> we display and what we requested from the WMS.
> - often we will have to make several requests and tile content onto  
> the screen in WMS happy chunks,
> - or we will need to make slightly larger requests and resample  
> them onto the screen (as mentioned previously).
>
> It was just easier to back project a pixel and use it to make a  
> request in the correct location once, rather then trying to reverse  
> several rendering process es to exactly match the original request  
> (aka the approach we used for uDig 0.8).
>
> Cheers,
> Jody
>> Hi Jody,
>>
>> why did you end up changing the BBOX and width/height client-side?  
>> Why not leave it up to the WMS server like Gaia does?
>>
>> Best regards,
>> Bart
>>
>> Jody Garnett schreef:
>>> Indeed, we eventually figured out that WMS getInfo is considered  
>>> a check in the center of a single pixel.
>>>
>>> Thinking,
>>>
>>> Richard for a while we forced the renderers to leave us a hook so  
>>> we could make our info request, is there any way we can ask the  
>>> WMS renderers to leave some hints on the layer blackboard about  
>>> the image rendered? The same plugin making the rendering  
>>> contribution can provide the hint ...
>>>
>>> Jody
>>>> Hi Jody,
>>>>
>>>> the projections I am trying are both supported by the WMS.
>>>>
>>>> Unfortunately I have no Java hosting, but it's a simple  
>>>> shapefile which can be set up with Geoserver. I am using 1.4.
>>>>
>>>> The shapefile can be found here (200 Kb, native projection EPSG: 
>>>> 28992, uDIG set to RD / New projection):
>>>>
>>>> http://www.osgis.nl/lausanne/nl-prov.zip
>>>>
>>>> An example request which uDIG is fyring off is (this request is  
>>>> really strange since the BBOX is not the BBOX of the map image  
>>>> shown anymore! and X and Y are always 1):
>>>>
>>>> /geoserver/wms?Y=1&X=1&SERVICE=WMS&INFO_FORMAT=text/ 
>>>> html&LAYERS=ogctestbed:nl-prov&FORMAT=image/ 
>>>> png&HEIGHT=3&REQUEST=GetFeatureInfo&WIDTH=3&BBOX=188907.50415179788 
>>>> ,391802.21024004533,192453.4125990071,395348.1186872545&SRS=EPSG: 
>>>> 28992&QUERY_LAYERS=ogctestbed%3Anl-prov&VERSION=1.1.1 HTTP/1.1"  
>>>> 200 447
>>>>
>>>> Thanks in advance.
>>>>
>>>> Best regards,
>>>> Bart
>>>>
>>>> Jody Garnett schreef:
>>>>> Good question, it makes a direct call when able to make a  
>>>>> direct rendering of the WMS image (ie udig is using a  
>>>>> coordinate system supported by the WMS). In rare cases it may  
>>>>> have to resample the image onto the screen (making up for lack  
>>>>> of WMS projections), we could check that we back project the  
>>>>> query into the coordinate space of the original image... but as  
>>>>> I recall I wrote this one and went to some trouble to get it  
>>>>> right.
>>>>>
>>>>> Can you point us at a service illustrating the problem, perhaps  
>>>>> in a jira report.
>>>>> Jody
>>>>>> Hi list,
>>>>>>
>>>>>> how accurate is uDIG with respect to GetFeatureInfo?
>>>>>>
>>>>>> I have tried it against 2 WMS's, Geoserver and Deegree, and at  
>>>>>> the borders of provinces, a lot of times the wrong element is  
>>>>>> returned. Ofcourse this could also be due to the WMS's.
>>>>>>
>>>>>> So I checked Gaia, and using Gaia it is very accurate for  
>>>>>> Geoserver, so I am assuming it is a uDIG problem.
>>>>>>
>>>>>> Any input appreciated.
>>>>>>
>>>>>> Best regards,
>>>>>> Bart
>>>>>>
>>>>>
>>>>>
>>>>> _______________________________________________
>>>>> User-friendly Desktop Internet GIS (uDig)
>>>>> http://udig.refractions.net
>>>>> http://lists.refractions.net/mailman/listinfo/udig-devel
>>>>>
>>>>>
>>>>
>>>>
>>>
>>>
>>> _______________________________________________
>>> User-friendly Desktop Internet GIS (uDig)
>>> http://udig.refractions.net
>>> http://lists.refractions.net/mailman/listinfo/udig-devel
>>>
>>>
>>
>>
>
>
> _______________________________________________
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