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

Sat Oct 15 12:20:45 PDT 2016

Hi Nyall (and list)

A small projecto with UTM and GEO layers can be downloaded from dropbox:

https://www.dropbox.com/s/c4cfb48r5326vrk/Cajamarca.zip?dl=0

Attribute table has the correct area in AreaKm2 field (and in these
unities). "AreaOTFoff" is the area given by field calculator when "On The
Fly transformation" is deactivated. "AreaOTF_on" is the area with it
activated.

So: when I have a GEO layer, I can't calculate correct area directly in
that layer: I've to reproject to UTM and deactivate automatic
transformation of SRCs, to use field calculator expecting correct values.

OS : Ubuntu 16.04
QGIS : 2.16.3
SRC settings: by default (ellipsoide WGS84 for measures)

Thank you for your interest

Carlos Cerdán

2016-10-14 15:14 GMT-05:00 Nyall Dawson <nyall.dawson at gmail.com>:

> On 15 Oct 2016 12:51 AM, "Carlos Cerdán" <sig.upagu at gmail.com> wrote:
> >
> > 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?
>
> Hi Carlos,
>
> Can you please share your file? Cut it down to just a few polygons and let
> me know what area you expect to see. Email direct to myself.
>
> Thanks!
>
> Nyall
>
> >
> > 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/
> general_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-
> calculating-area-depending-on-file-type-or-projection-tp5290228.html
> >> >>>> To start a new topic under Quantum GIS - User, email [hidden
> email]
> >> >>>> To unsubscribe from Quantum GIS - User, click here.
> >> >>>> NAML
> >> >>>
> >> >>>
> >> >>> ________________________________
> >> >>> View this message in context: Re: inconsistenty when calculating
> area depending on file type or projection?
> >> >>> Sent from the Quantum GIS - User mailing list archive at Nabble.com.
> >> >>>
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