[udig-devel] accuracy of GetFeatureInfo

[Jody Garnett]

In that case we will need to ask Richard to implement a different plugin 
for the InfoTool for each renderer he creates. Not fun but possible. I 
did think he ended up making a 2x2 reference image and selecting the 
first pixel (after having a few WMS fail on a 1x1 image reference image.
Jody
>
> 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
>>>>>>>
>>>>>>
>>>>>>
>>>>>> _______________________________________________
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>>>>>> http://udig.refractions.net
>>>>>> http://lists.refractions.net/mailman/listinfo/udig-devel
>>>>>>
>>>>>>
>>>>>
>>>>>
>>>>
>>>>
>>>> _______________________________________________
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>>>> http://udig.refractions.net
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>>>>
>>>>
>>>
>>>
>>
>>
>> _______________________________________________
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>> http://udig.refractions.net
>> http://lists.refractions.net/mailman/listinfo/udig-devel
>
> _______________________________________________
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> http://udig.refractions.net
> http://lists.refractions.net/mailman/listinfo/udig-devel