[Qgis-user] nodata and raster calc

[Havard Tveite]

nodata + 1 = nodata
nodata * 1 = nodata
nodata AND 1 = nodata
nodata OR 1 = 1

nodata + 0 = nodata
nodata * 0 = nodata
nodata AND 0 = 0
nodata OR 0 = nodata

Håvard

On 11. feb. 2019 09:47, st_kiefer at web.de wrote:
> I absolutely agree.
> 
> For the Discussion. nodata is a valuable information, and is getting lost when dealing as desired value. Although the OR part needs more Discussion. For consistency that has to be nodata as well.  What about nodata OR 0? In my opinion nodata always needs a discrete treatment to avoid generating fallcious information.
> 
> Cheers
> 
> Stefan
> 
>> Havard Tveite <havard.tveite at nmbu.no> hat am 11. Februar 2019 um 09:38 geschrieben:
>>
>>
>> nodata + 1 = nodata
>> nodata * 1 = nodata
>> nodata AND 1 = nodata
>> nodata OR 1 = 1
>>
>> Håvard
>>
>> On 11. feb. 2019 03:11, Nyall Dawson wrote:
>>> Hey all,
>>>
>>> I'm seeking some feedback on whether our current handling of nodata
>>> pixels in the raster calculator is correct, or (my suspicion) annoying
>>> and limiting.
>>>
>>> Currently, we treat nodata pixels the same as a "NULL" value in SQL --
>>> so ANY function or operation with nodata as an input results in a
>>> nodata output.
>>>
>>> This means:
>>>
>>> - nodata + 1 = nodata
>>> - nodata * 1 = nodata
>>> - nodata AND 1 = nodata
>>> - nodata OR 1 = nodata
>>>
>>> To me this is a mistake. I don't think we should treat nodata the same
>>> as NULL, and I think for rasters then "nodata OR 1" should return 1.
>>>
>>> The end result of the current behaviour is that ANY nodata input
>>> blocks processing of that pixel. So you can't do something like take
>>> two raster files, and create an output showing where either input has
>>> a 1 value (which I would expect raster1 at 1 OR raster2 at 1 to do).
>>>
>>> Otherwise, we need to have some concept of "coalesce" in the raster
>>> calc. So then at least you could do:
>>>
>>>       coalesce(raster1 at 1, 0) OR coalesce(raster at 1, 0) ....
>>>
>>>
>>> Thoughts?
>>>
>>> Nyall
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