[udig-devel] accuracy of GetFeatureInfo

[Jody Garnett]

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
>>
>>
>
>