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

Wed Oct 19 00:16:54 PDT 2016

Hi Nyall,

Splitting up ellipsoidal measurement from OTF reprojection would be nice.
I would love to be able to define the default measurement type (e.g. in general settings).
Possible to get it in QGIS 3.0?

Cheers
Stefan

-----Original Message-----
From: Qgis-user [mailto:qgis-user-bounces at lists.osgeo.org] On Behalf Of Nyall Dawson
Sent: 19. oktober 2016 02:59
To: Carlos Cerdán <sig.upagu at gmail.com>
Cc: qgis-user <qgis-user at lists.osgeo.org>
Subject: Re: [Qgis-user] inconsistenty when calculating area depending on file type or projection?

On 19 October 2016 at 02:22, Carlos Cerdán <sig.upagu at gmail.com> wrote:
> It was my fault. I was used to consider plane areas as "correct", 
> forgetting ellipsoidal areas. So, QGIS give us freedom to calculate 
> planar or ellipsoidal (geodetical) areas, but we must be careful about 
> settings; Is not it?. Sometimes, too much freedom is annoying!
>
> But, How can I calculate UTM (planar) area from a GEO layer in one step?

Under project properties, general tab - change "ellipsoid" to "None / Planimetric".

I keep meaning to split up the concept of ellipsoidal measurement from OTF reprojection. In my view these are two different concepts - one is just about display of data, the other relates to measurement. Ie, you should be able to perform ellipsoidal calculations even if OTF rendering is disabled.

I just keep forgetting to do it early in a release cycle for extensive testing...

Nyall

>
> Thank you very much for your time and so sorry if I'm doing basic questions.
>
> Carlos
>
>
>
> 2016-10-16 17:59 GMT-05:00 Nyall Dawson <nyall.dawson at gmail.com>:
>>
>> On 16 October 2016 at 05:20, Carlos Cerdán <sig.upagu at gmail.com> wrote:
>> > 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)
>>
>> Thanks for sending this through, but again - I can't find any issue here.
>>
>> The different area calculations come from the difference between 
>> measuring the area using a flat plane (OTF off, no ellipsoid set) vs 
>> measuring the area using an ellipsoid (OTF on, ellipsoid set under 
>> the project "general" settings). When OTF is on QGIS calculates the 
>> same area regardless of whether the geographic or projected source 
>> layer is used.
>>
>> Hope that clarifies!
>>
>> Nyall
>>
>>
>>
>> >
>> > 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/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-cal
>> >> >> >>>> culating-area-depending-on-file-type-or-projection-tp52902
>> >> >> >>>> 28.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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