[Qgis-user] Raster Calculator bug?

Fri Jul 31 07:23:05 PDT 2015

Ok,
if I apply this approach

(“m at 1" < 238 AND “m at 1" > 213 AND “m at 2" < 123 AND “m at 2" > 98 AND “m at 3" < 
125 AND “m at 3” > 99)

it is perfectly working either. Except that QGis is not cleanly 
extrapolating the legend!
After that I applied pseudocolor and 1 channel with min=0.1 and max=1 
and do classify (only 1 class!). Et Voila, there is indeed the desired 
value in that result layer.

cheers

Stefan

Am 31.07.2015 um 15:37 schrieb Stefan Kiefer:
> Hi Nick,
> back to office I was eager to try by myself. Actually it seems that 
> the result of multiple AND or multiple layers - I didn't check this by 
> now - results in values slightly lower 1 (e.g. 0.9995 in my case). And 
> therefore maybe rounded to "0". What I have done is the folowing:
>
> (1/("pm2-5-europe-2001-2010 at 1">240 AND 
> "pm2-5-europe-2001-2010 at 1"<250))*(1/("pm2-5-europe-2001-2010 at 2">139 
> AND "pm2-5-europe-2001-2010 at 2"<145)) * 
> (1/("pm2-5-europe-2001-2010 at 3">80 AND "pm2-5-europe-2001-2010 at 3"<85))
>
> which results in a perfektly fitting mask of my pseudo demand.
>
> Mayby you could verfy this with your data
>
>
> cheers
>
>
> Stefan
>
> Am 31.07.2015 um 08:48 schrieb Stefan Kiefer:
>> Hi Nick,
>> you are absolutely right. My thought was, that you get A layer with 
>> distinct values to identify the road. For a mask you are on the right 
>> way, and I either don't understand the behaviour except that you 
>> operate over three layers, which of course should work.
>> Have you tryed to generate a composit of the three layers and mask 
>> the single values resulting for road structures? (it's more or less 
>> what I expected from my first approach.)
>> cheers
>> Stefan
>>
>> > Nick Papadonis <npapadonis at gmail.com> hat am 31. Juli 2015 um 08:31 
>> geschrieben:
>> >
>> >
>> > Hi Stefan,
>> >
>> > It’s my understanding black has a value of 0 in the resulting layer.
>> >
>> > I tried this and it results in similar image to step (a) and also 
>> includes other colors at lower intensities mixed in with the red. The 
>> red has the highest intensity in the greyscale. I’m looking to create 
>> a binary image with just the colors of red in the palette I choose 
>> and using this trace vectors over the paths.
>> >
>> > Thanks,
>> > Nick
>> >
>> > > On Jul 31, 2015, at 2:04 AM, Stefan Kiefer <st_kiefer at web.de> wrote:
>> > >
>> > > Hi Nick,
>> > > I believe it is black bcause you always get a value of "1". 
>> Unfortunately I can not verify this, because I have no QGis by this 
>> moment. Most propably you wanted to calculate:
>> > >
>> > > (“m at 1" < 238 AND “m at 1" > 213 AND “m at 2" < 123 AND “m at 2" > 98 AND 
>> “m at 3" < 125 AND “m at 3” > 99) * ((“m at 1" < 238 AND “m at 1" > 210) * "m at 1") 
>> + ((“m at 2" < 123 AND “m at 2" > 94) * “m at 2") + ((“m at 3" < 130 AND “m at 3" > 
>> 98) *“m at 3"))
>> > >
>> > > cheers
>> > >
>> > > Stefan
>> > >
>> > > > Nick Papadonis <npapadonis at gmail.com> hat am 31. Juli 2015 um 
>> 07:49 geschrieben:
>> > > >
>> > > >
>> > > > One more comment. The resulting layer histogram is showing the 
>> pixel range spread over frequency in floating point values. Is the 
>> raster calculator performing floating point math with potential 
>> rounding error?
>> > > >
>> > > > I found it also interesting that the following expression 
>> resulted in a layer, which when inspected for band values, has 
>> integer values of 2 and 3. 3 being the value I want for the red route.
>> > > >
>> > > > a) ((“m at 1" < 238 AND “m at 1" > 210) * 1) + ((“m at 2" < 123 AND 
>> “m at 2" > 94) * 1) + ((“m at 3" < 130 AND “m at 3" > 98) * 1)
>> > > >
>> > > > I then change the expression to only use values 2 and greater 
>> and this shows properly:
>> > > > b) ((“m at 1" < 238 AND “m at 1" > 210) * 1) + ((“m at 2" < 123 AND 
>> “m at 2" > 94) * 1) + ((“m at 3" < 130 AND “m at 3" > 98) * 1) > 2
>> > > >
>> > > > I then changed the expression to ensure all three values are 
>> obtained and it results in a black image of 0’s. I was expecting only 
>> the red route to appear as it resulted in value of 3 in step (a).
>> > > >
>> > > > ((“m at 1" < 238 AND “m at 1" > 210) * 1) + ((“m at 2" < 123 AND “m at 2" > 
>> 94) * 1) + ((“m at 3" < 130 AND “m at 3" > 98) * 1) > 2.1
>> > > > ((“m at 1" < 238 AND “m at 1" > 210) * 1) + ((“m at 2" < 123 AND “m at 2" > 
>> 94) * 1) + ((“m at 3" < 130 AND “m at 3" > 98) * 1) >= 3
>> > > >
>> > > > I’m wondering how much testing the Raster Calculator has gone 
>> through and if there is a possible bug here. Perhaps something to do 
>> with floating point?
>> > > >
>> > > > Thanks again
>> > > >
>> > > > > On Jul 31, 2015, at 12:39 AM, Nick Papadonis 
>> <npapadonis at gmail.com> wrote:
>> > > > >
>> > > > > Folks,
>> > > > >
>> > > > > I’m using QGIS 10.1. The following expressions result in a 
>> black raster of 0’s, when I expected only red pixels to appears in 
>> the binary image indicating routes on a map:
>> > > > >
>> > > > > a) (“m at 1" < 238 AND “m at 1" > 213 AND “m at 2" < 123 AND “m at 2" > 
>> 98 AND “m at 3" < 125 AND “m at 3” > 99) * 1
>> > > > > b) ((“m at 1" < 238 AND “m at 1" > 210) * 1) * ((“m at 2" < 123 AND 
>> “m at 2" > 94) * 1) * ((“m at 3" < 130 AND “m at 3" > 98) * 1)
>> > > > >
>> > > > > I then tried the following individual expressions for each 
>> band as separate steps (sanity check) and they work to cover the 
>> pixels in range:
>> > > > > c) (“m at 1" < 238 AND “m at 1" > 213) * 1
>> > > > > d) (“m at 2" < 123 AND “m at 2" > 98) * 1
>> > > > > e) (“m at 3" < 125 AND “m at 3” > 99) * 1
>> > > > >
>> > > > > I then tried the following expression which appears to create 
>> a proper greyscale image focusing on the red pixels. I replaced the 
>> multiplication with addition to see what was happening:
>> > > > > f) ((“m at 1" < 238 AND “m at 1" > 210) * 1) + ((“m at 2" < 123 AND 
>> “m at 2" > 94) * 1) + ((“m at 3" < 130 AND “m at 3" > 98) * 1)
>> > > > >
>> > > > > The resulting raster has a Min = 0 and Max = 1.998. I was 
>> expecting it to be Min = 0 and Max = 3. The value of 3 would indicate 
>> all 3 bands were positive on color match. I then go to the layer 
>> properties and load calculate min/max again and it is Min = 0 and Max 
>> = 3. I tried to change the min/max settings on they layer and these 
>> settings will not stay set. The layer goes back to Max = 1.998. 
>> What’s even more odd is the max being a floating point number. I 
>> suspect that may be part of the issue. Anyone know why this is the 
>> case for integer band values? Has anyone successfully used the Raster 
>> Calculator to perform this sort of work before?
>> > > > >
>> > > > > Thanks again,
>> > > > > Nick
>> > > >
>> > > > _______________________________________________
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>> > > > Qgis-user at lists.osgeo.org
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>> >
>>
>>
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