[udig-devel] accuracy of GetFeatureInfo

Tue Sep 5 10:42:27 PDT 2006

uDIG requests a 3x3 image. And then it requests the topleft pixel (X=1, 
Y=1).

I don't know the internals ofcourse, but should it not request the 
center pixel (X=2, Y=2)? At least that gives the correct results for my 
testcase :-)

Why else would it request 3x3, it might as well request 1x1 or 2x2 if it 
is using the topleft pixel.

Best regards,
Bart

Jody Garnett schreef:
> 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
>>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> _______________________________________________
>>>>>>> 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
>>>>>
>>>>>
>>>>
>>>>
>>>
>>>
>>> _______________________________________________
>>> 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
>
>
> _______________________________________________
> User-friendly Desktop Internet GIS (uDig)
> http://udig.refractions.net
> http://lists.refractions.net/mailman/listinfo/udig-devel
>
>

-- 
Bart van den Eijnden
OSGIS, Open Source GIS
http://www.osgis.nl