[udig-devel] accuracy of GetFeatureInfo

Jesse Eichar jeichar at refractions.net
Tue Sep 5 10:48:38 PDT 2006 

Richard is looking into this and has made the same observation.  He's  
on the job and says he'll have it fixed by friday.  Thanks Rich.

Jesse

On 5-Sep-06, at 10:42 AM, Bart van den Eijnden (OSGIS) wrote:

> 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=188907.50415179 
>>>>>>> 788,391802.21024004533,192453.4125990071,395348.1186872545&SRS=E 
>>>>>>> PSG: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
>
> _______________________________________________
> User-friendly Desktop Internet GIS (uDig)
> http://udig.refractions.net
> http://lists.refractions.net/mailman/listinfo/udig-devel

More information about the udig-devel mailing list