[Qgis-user] Raster Calculator Error Questions

Mon Feb 15 13:51:01 PST 2021

Hello,

Yes, I did remove the spaces and fixed it as mentioned in the various
emails. It is working now.  Newbie mistake I guess.

Thank you all for the wonderful support.  I appreciate it!

Wanda

On Mon, Feb 15, 2021 at 3:53 PM Nicolas Cadieux <njacadieux.gitlab at gmail.com>
wrote:

> Thayer’s instructions look good.
>
> Start by building query one step at a time.  You can separate the parts
> using the pipe sign ||.  Then, change the || for the operators OR. That
> would help you trouble shoot the operation.
>
>  I think he is right.  The problem is likely the space between <=.
>
> Nicolas Cadieux
> https://gitlab.com/njacadieux
>
> Le 13 févr. 2021 à 10:57, Thayer Young <thayeray at yahoo.com> a écrit :
>
> 
> Hi Wanda,
>
> Please try removing the spaces between the operators, the tool should not
> run with those spaces:
>     they should be: <= and >= not < = and > =
>
> Also make sure that the name of the raster layer is EXACTLY the same as
> what you have in your Layers panel (what the ESRI work calls Table of
> Contents). The double quotes around the name are there just in case you
> have a space in the name of the raster Layer. You can safely remove them if
> there are no spaces. The parenthesis are also not necessary, because of the
> order of operations (the same as in basic algebra). The <= and >= will
> calculate before the OR, so there is no need for parenthesis. You would
> only need parenthesis if the expression were more complicated.
> The following should work equally well:
>
> Aspect at 1<=90 OR Aspect at 1>=270
> Aspect at 1 <= 90 OR Aspect at 1 >= 270
> "Aspect at 1" <= 90 OR "Aspect at 1" >= 270
> ("Aspect at 1" <= 90) OR ("Aspect at 1" >= 270)
>
> But if your layer is named "Aspect" but you enter "aspect", the raster
> calculator will say the expression is valid but then show a red banner
> error after it is run.
> This is because the expression looks fine to the raster calculator tool,
> but it does not check for the existence of the layer until you press OK. In
> this case it can not find a layer "aspect" because the layer is named
> "Aspect".
>
> I have tried reproducing your problem using the OSGeo4W Network Installer
> QGIS 10.10 without GRASS, but I am not able to. If the recommendations
> above are not helpful, you may need to give more specifics about the errors
> you are seeing and which installation you are using. I hope this helps :-)
>
> -Thayer
>
> P.S. I did have a problem with the extent not being set correctly when I
> had more than one DEM. Everything ran without errors, but the result was
> empty, except for a thin band where the two DEMs overlapped. You may need
> to be careful to set this to the correct extent, so click on your Aspect
> layer and then Selected Layer Extent.
>
>
>
> ======================================================
> Date: Fri, 12 Feb 2021 14:12:49 -0500
> From: Wanda Norman <wanda.mnorman2015 at gmail.com>
> To: Nicolas Cadieux <njacadieux.gitlab at gmail.com>
> Cc: qgis-user <qgis-user at lists.osgeo.org>
> Subject: Re: [Qgis-user] Raster Calculator Error Questions
> Message-ID:
>     <CAFN0wn0gq9wJY3=URkK0MHDuJCE_BvwEowH2_L7hbxeX6kM0vA at mail.gmail.com>
> Content-Type: text/plain; charset="utf-8"
>
> Thanks Nicolas for the quick response.  But I am still having issues.
>
> I did upgrade to the 3.16 version and I am still unable to write the
> formula.  I tried:  ( "aspect at 1" < = 90) OR ( "aspect at 1" > = 270) - I get
> Expression is not valid. As soon as I put anything other than "aspect at 1" I
> get a message that the expression is not valid.
>
> This is the first time using the raster calculator and it looks as if I
> can't move forward with the lesson without this step.
>
> Any other suggestions?
>
> Thanks for your help!
> Wanda
>
>
>
> On Fri, Feb 12, 2021 at 10:08 AM Nicolas Cadieux <
> njacadieux.gitlab at gmail.com> wrote:
>
> > Hi,
> > I have not used the raster calculator in a while but I am not sure about
> > you use of the \.
> > Examples in the user manual (
> >
> https://docs.qgis.org/3.16/en/docs/user_manual/working_with_raster/raster_analysis.html#raster-calculator
> )
> > use this as examples.
> >
> > ("elevation at 1" < 50) * 1 + ("elevation at 1" >= 50) * 2
> >
> >
> > I would try (expression 1) OR (expression 2).  Layers names must be
> placed
> > between quotes ??.
> >
> > You could also update your QGIS version to 3.16.
> >
> > Nicolas Cadieux
> > https://gitlab.com/njacadieux
> >
> > Le 12 f?vr. 2021 ? 09:09, Wanda Norman <wanda.mnorman2015 at gmail.com> a
> > ?crit :
> >
> > ?
> >
> > Greetings friends:
> >
> >
> >
> > I am working on lesson 7.3.5 in the User Guide / Manual (QGIS 3.10) ?
> > Following along: Using the Raster Calculator.
> >
> >
> >
> > I am having difficulty with the calculation formula as indicated below:
> >
> >
> >
> > The manual states this formula should be used:
> >
> >                Aspect at 1 <=90 OR Aspect at 1 >= 270
> >
> > Which I am assuming is incorrect because a) I get an error when I run it;
> > and b) when I set the parameters using the expression operators the
> formula
> > looks like this:
> >
> >                ?Aspect at 1? <=90 OR ?Aspect at 1? >=270
> >
> > However, I still get the error:
> >
> > Error parsing formula
> >
> > Execution failed after 0.02 seconds
> >
> >
> >
> > With some help ? I then tried this formula
> >
> > \?Aspect at 1?\ <=90 OR \?Aspect at 1?\ >=270
> >
> > Which worked for this assignment.
> >
> >
> >
> > However, moving on to lesson 7.3.6 ? More criteria: which takes me to
> > 21.12.2. Calculate slope (less than 2 and 5 degrees)
> >
> > I get the same error as above again.
> >
> >
> >
> > I am steadily working my way through the Manual and would appreciate any
> > help.  I am using QGIS version 3.10.9-A Coruna (64 bit) on a Windows PC.
> >
> >
> >
> > Thank you!
> >
> > Wanda Norman
> > _______________________________________________
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> > Qgis-user at lists.osgeo.org
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> >
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