[Qgis-user] inconsistenty when calculating area depending on file type or projection?

[Carlos Cerdán]

Hi Nyall

I'm afraid that 2.16 has still this issue. I've loaded an UTM-17 south
layer (my zone) and a Lat-long layer and:

1. SRC was seted in UTM

2. In UTM layer, if OTF SRC transformation is active, I get different area
than if it's deactivated. Correct value is the last one.

3. In Lat-long layer, if OTF SRC transformation is active, calculated area
is same as the wrong value of first layer. I can't get the correct value in
this layer, so I have to reproject into a new one and do step 2 (with OTF
deactivated).

What about a general option to set the prefered SRC to calculate areas and
lengths with OTF active?

Regards from Peru

Carlos



2016-10-12 18:06 GMT-05:00 Nyall Dawson <nyall.dawson at gmail.com>:

> On 12 Oct 2016 11:56 PM, "Carlos Cerdán" <sig.upagu at gmail.com> wrote:
> >
> > AFAIK, It also is needed to turn off "on the fly SRC transformation" to
> get correct area values.... Or QGIS has fixed this point?
>
> Everything should be fixed in recent versions, and I very (VERY) much want
> to know if any issues are still encountered.
>
> Calculating area/length is a core task for a GIS and we need to make sure
> it's rock solid. (Which it should be since 2.16!)
>
> Nyall
>
>
> >
> > If you can't get correct area values, check out about it....
> >
> >
> > 2016-10-12 7:40 GMT-05:00 DelazJ <delazj at gmail.com>:
> >>
> >> Hi,
> >> To complete Nicolas answer, you should check what are the measurements
> options set in Project --> Project Properties --> General tab.
> >> See also http://docs.qgis.org/2.14/en/docs/user_manual/introduction/g
> eneral_tools.html#measuring
> >>
> >> 2016-10-12 14:07 GMT+02:00 Nicolas Cadieux <
> nicolas.cadieux at archeotec.ca>:
> >>>
> >>> Hi,
> >>> You may be calculating square degrees and not metres.  It can depend
> on the crs depending on the tools you are using.
> >>> Nicolas
> >>>
> >>> Le 11 oct. 2016 à 08:54, Martina Schäfer [via OSGeo.org] <[hidden
> email]> a écrit :
> >>>
> >>>> I experienced some confusion with calculation of area using the field
> calculator in QGIS version 2.16.3. Since I'm using MapInfo Professional as
> well, I mainly use tab-files that I can open in both programmes, but
> occasionally I save as shapefile since this used to be the default in QGIS.
> >>>>
> >>>> When comparing files, I coincidently realized that there was a
> mismatch in calculated area for the shapefile and the tab-file for exactly
> the same polygons! I used the field calculator in the attribute table in
> both cases, but for the shapefile the resulting areas were almost doubled
> in area compared to the tab-file. Any idea why this is happening?
> >>>>
> >>>> I also realized similar differences when calculating area in a file
> where projection has been converted from SWEREF99TM (a Swedish national
> projection) to WGS84. There differences occurred in both the tab and
> shapefile compared to the area calculated for the same tab-file in MapInfo.
> Again I find this very confusing!
> >>>>
> >>>> I need to rely on the area-calculations thus I really hope someone
> here can explain to me what is happening!
> >>>>
> >>>> Thanks in advance,
> >>>> Martina
> >>>>
> >>>>
> >>>>
> >>>>
> >>>> ________________________________
> >>>> If you reply to this email, your message will be added to the
> discussion below:
> >>>> http://osgeo-org.1560.x6.nabble.com/inconsistenty-when-calcu
> lating-area-depending-on-file-type-or-projection-tp5290228.html
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> >>>> NAML
> >>>
> >>>
> >>> ________________________________
> >>> View this message in context: Re: inconsistenty when calculating area
> depending on file type or projection?
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> >>>
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