Hi Stefan,
> On Jul 31, 2015, at 9:37 AM, Stefan Kiefer <[email protected]> wrote:
>
> 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:
>
Thanks! I did try your suggestion and it provided me the results I was seeking
of only showing the red routes:
(1/("map@1" < 238 AND "map@1" > 210)) + (1/("map@2" < 123 AND "map@2" > 94)) +
(1/("map@3" < 130 AND "map@3" > 98))
Perhaps my math is fuzzy. Why would the divisor resolve this situation whereas
multiplier in following has problems?
((“m@1" < 238 AND “m@1" > 210) * 1) * ((“m@2" < 123 AND “m@2" > 94) * 1) *
((“m@3" < 130 AND “m@3" > 98) * 1)
The floating point values is a why I believe we have a rounding bug in Raster
Calculator. I suspect that during the calculation a number is converted to
floating point, between 0 and 1 (eg 0.9995), and that causes the equation to
result in all 0s. Its unusual that the calculator is performing floating point
math because these are all integer values being operated on by value comparison
operators and other integer numbers without any division which would cause
floating point conversion or multiplication by a floating point. The numbers
are all whole integers. I was hoping the original author could chime in on the
tool and perhaps this is a bug.
I’m on Mac OSX using 2.10.1 release and see same results on 2.8 release.
I’m suspecting this may be a bug related to:
Raster calculator always returns float32 tiffs
https://hub.qgis.org/issues/10965 <https://hub.qgis.org/issues/10965>
Raster calculator produces only 0 values with conditional expression
https://hub.qgis.org/issues/11682 <https://hub.qgis.org/issues/11682>
There is this suggestion on the mailing list of how to resolve, however am a
bit confused on the methods. The workaround may be to reproject the raster
layer and I’m unsure why that’s required:
http://lists.osgeo.org/pipermail/qgis-user/2014-November/029873.html
<http://lists.osgeo.org/pipermail/qgis-user/2014-November/029873.html>
http://lists.osgeo.org/pipermail/qgis-user/2014-November/029878.html
<http://lists.osgeo.org/pipermail/qgis-user/2014-November/029878.html>
Thanks again,
Nick
> (1/("pm2-5-europe-2001-2010@1">240 AND
> "pm2-5-europe-2001-2010@1"<250))*(1/("pm2-5-europe-2001-2010@2">139 AND
> "pm2-5-europe-2001-2010@2"<145)) * (1/("pm2-5-europe-2001-2010@3">80 AND
> "pm2-5-europe-2001-2010@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 <[email protected]> <mailto:[email protected]> 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 <[email protected]>
>> > > <mailto:[email protected]> 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@1" < 238 AND “m@1" > 213 AND “m@2" < 123 AND “m@2" > 98 AND “m@3" <
>> > > 125 AND “m@3” > 99) * ((“m@1" < 238 AND “m@1" > 210) * "m@1") + ((“m@2"
>> > > < 123 AND “m@2" > 94) * “m@2") + ((“m@3" < 130 AND “m@3" > 98) *“m@3"))
>> > >
>> > > cheers
>> > >
>> > > Stefan
>> > >
>> > > > Nick Papadonis <[email protected]> <mailto:[email protected]>
>> > > > 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@1" < 238 AND “m@1" > 210) * 1) + ((“m@2" < 123 AND “m@2" > 94)
>> > > > * 1) + ((“m@3" < 130 AND “m@3" > 98) * 1)
>> > > >
>> > > > I then change the expression to only use values 2 and greater and this
>> > > > shows properly:
>> > > > b) ((“m@1" < 238 AND “m@1" > 210) * 1) + ((“m@2" < 123 AND “m@2" > 94)
>> > > > * 1) + ((“m@3" < 130 AND “m@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@1" < 238 AND “m@1" > 210) * 1) + ((“m@2" < 123 AND “m@2" > 94) *
>> > > > 1) + ((“m@3" < 130 AND “m@3" > 98) * 1) > 2.1
>> > > > ((“m@1" < 238 AND “m@1" > 210) * 1) + ((“m@2" < 123 AND “m@2" > 94) *
>> > > > 1) + ((“m@3" < 130 AND “m@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 <[email protected]>
>> > > > > <mailto:[email protected]> 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@1" < 238 AND “m@1" > 213 AND “m@2" < 123 AND “m@2" > 98 AND
>> > > > > “m@3" < 125 AND “m@3” > 99) * 1
>> > > > > b) ((“m@1" < 238 AND “m@1" > 210) * 1) * ((“m@2" < 123 AND “m@2" >
>> > > > > 94) * 1) * ((“m@3" < 130 AND “m@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@1" < 238 AND “m@1" > 213) * 1
>> > > > > d) (“m@2" < 123 AND “m@2" > 98) * 1
>> > > > > e) (“m@3" < 125 AND “m@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@1" < 238 AND “m@1" > 210) * 1) + ((“m@2" < 123 AND “m@2" >
>> > > > > 94) * 1) + ((“m@3" < 130 AND “m@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
>> > > >
>> > > > _______________________________________________
>> > > > Qgis-user mailing list
>> > > > [email protected] <mailto:[email protected]>
>> > > > http://lists.osgeo.org/mailman/listinfo/qgis-user
>> > > > <http://lists.osgeo.org/mailman/listinfo/qgis-user>
>> >
>>
>>
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