[Qgis-user] Raster Calculator bug?

Fri Jul 31 06:37:32 PDT 2015

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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> >
>
>
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