[Qgis-user] Wrong area count QGIS 3.20+?

Tue Jan 4 11:03:57 PST 2022

Dear Sebastian,

I am very grateful to you for taking on my case in such depth.
Now everything is clear to me, thank you kindly for your answer.
I will spread this knowledge further on local QGIS community and Polish internet QGIS forums.

with best regards
and greetings to the entire community of this amazing open-source project
Jarosław Sadowski

_____________________________

Jarosław Sadowski
Ekspert ds. Baz Danych i Analiz Przestrzennych | Biuro Strategii i Planowania, Projektowania i Inżynierii Podprogramu Kolejowego
Database and Spatial Analysis Expert |Railway Subprogramme Strategy & Planning, Design & Engineering Department
e: jaroslaw.sadowski at cpk.pl<mailto:jaroslaw.sadowski at cpk.pl>

From: Sebastian Gutwein <bas at rdgland.com>
Sent: Monday, January 3, 2022 3:47 PM
To: Sadowski Jarosław <jaroslaw.sadowski at cpk.pl>
Cc: qgis-user at lists.osgeo.org
Subject: Re: [Qgis-user] Wrong area count QGIS 3.20+?

Nie otrzymujesz często wiadomości e-mail z adresu bas at rdgland.com<mailto:bas at rdgland.com>. Dowiedz się, dlaczego jest to ważne<http://aka.ms/LearnAboutSenderIdentification>

Ok to add to the fun I tried the calculations in ArcPro and interestingly if I run it using calculate field with this expression !shape.geodesicArea at hectares<mailto:!shape.geodesicArea at hectares>! I get the QGIS 3.22 value of 31271112 but if I run Geoprossessing Add Geometry Attributes I get the QGIS 3.16 value of 31271108.
[cid:image001.png at 01D801A6.3464F4A0]
So if you want them to match I guess you should use the python expression in ArcGIS Pro!

I guess we are back to the old adage of All Measuring is Guessing.
-Bas

On Mon, Jan 3, 2022 at 8:43 AM Sebastian Gutwein <bas at rdgland.com<mailto:bas at rdgland.com>> wrote:
Yes that was my read as well except previously it was the Grass method and now it is Geographic Lib method because that is theoretically more accurate and is the same method that is used in Postgis and other places. The switched happened because someone noticed the difference between QGIS's calculations and Postgis's calculations. If I am reading the pull requests correctly even Grass has switched to the Geographic Lib method. I'm surprised that ArcGIS Pro uses the same method as the Grass method.

On Mon, Jan 3, 2022 at 2:57 AM Sadowski Jarosław <jaroslaw.sadowski at cpk.pl<mailto:jaroslaw.sadowski at cpk.pl>> wrote:
Sebastian,

thank you very much for your precise answer and materials sent - I have read them carefully.

So, as far as I can understand, QGIS has moved in version 3.20 to a new way of counting areas (other libraries? GRASS algorithm instead of Geographic lib? I have actually noticed a different nomenclature and definition of EPSG 2180 for some time in new versions...), that will be more accurate?

I must admit that, for me as a GIS operator, it is surprising and sounds like bad news - so far surfaces have always been the same in various GIS software, whether or not it was open-source or commercial (ArcGIS Pro). So from this moment it will be that I will get different surfaces depending on the software it was calculated? :(

with kind regards
JS

_____________________________

Jarosław Sadowski
Ekspert ds. Baz Danych i Analiz Przestrzennych | Biuro Strategii i Planowania, Projektowania i Inżynierii Podprogramu Kolejowego
Database and Spatial Analysis Expert |Railway Subprogramme Strategy & Planning, Design & Engineering Department
e: jaroslaw.sadowski at cpk.pl<mailto:jaroslaw.sadowski at cpk.pl>

From: Sebastian Gutwein <bas at rdgland.com<mailto:bas at rdgland.com>>
Sent: Saturday, January 1, 2022 7:13 AM
To: Sadowski Jarosław <jaroslaw.sadowski at cpk.pl<mailto:jaroslaw.sadowski at cpk.pl>>
Cc: qgis-user at lists.osgeo.org<mailto:qgis-user at lists.osgeo.org>
Subject: Re: [Qgis-user] Wrong area count QGIS 3.20+?

Nie otrzymujesz często wiadomości e-mail z adresu bas at rdgland.com<mailto:bas at rdgland.com>. Dowiedz się, dlaczego jest to ważne<http://aka.ms/LearnAboutSenderIdentification>

There was some discussion on this in the list serve back in January<https://eur06.safelinks.protection.outlook.com/?url=https%3A%2F%2Flists.osgeo.org%2Fpipermail%2Fqgis-user%2F2021-January%2F047638.html&data=04%7C01%7Cjaroslaw.sadowski%40cpk.pl%7C72f10e545ec748e873ca08d9cec7e6d0%7Cfa798250ca0b4a1bb47381cff4a1752b%7C1%7C0%7C637768180240837962%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000&sdata=n9NfD1jSBsKsrD9ujGf50us0H5McoNOnjZTet%2BvXFlo%3D&reserved=0> that resulted in this feature request<https://eur06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2Fqgis%2FQGIS%2Fissues%2F40888&data=04%7C01%7Cjaroslaw.sadowski%40cpk.pl%7C72f10e545ec748e873ca08d9cec7e6d0%7Cfa798250ca0b4a1bb47381cff4a1752b%7C1%7C0%7C637768180240837962%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000&sdata=7md2NSCR87fR2Uhv2tMwrisF9%2FbiulNQH%2BuTPwqCqok%3D&reserved=0> and this pull request<https://eur06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2Fqgis%2FQGIS%2Fpull%2F41726&data=04%7C01%7Cjaroslaw.sadowski%40cpk.pl%7C72f10e545ec748e873ca08d9cec7e6d0%7Cfa798250ca0b4a1bb47381cff4a1752b%7C1%7C0%7C637768180240837962%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000&sdata=I7pSrQ9eHBnY%2BOq6xN5BTbUV2ZkcJOP0k30teMLdJUk%3D&reserved=0>. I tested your polygon, to the best of my limited ability, and the areas were indeed different in QGIS 3.16 and 3.22 as would be expected from that change . However it would be hoped that the area measurement would be more accurate in 3.22.

On Fri, Dec 31, 2021 at 6:49 AM Sadowski Jarosław <jaroslaw.sadowski at cpk.pl<mailto:jaroslaw.sadowski at cpk.pl>> wrote:
Dear all,

I am asking for help with QGIS error/bug or verification of proper usage of basic QGIS functionality "$area".
>From version 3.20 upwards, it counts areas differently than before (3.18 calculated area is different than 3.20). Up to version 3.18 everything was fine, equal to calculations from ArcGIS Pro (geodesic mode).
Anyone have an idea where the differences come from?
This is example vector [cid:image002.png at 01D801A6.3464F4A0]  PL_2180.zip<https://eur06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fcpkonline-my.sharepoint.com%2F%3Au%3A%2Fg%2Fpersonal%2Fjaroslaw_sadowski_cpk_pl%2FEVs0-TiEvPdBjMaqXrEvGfoBI0VussbJaYHdDVFfQnEYug%3Fe%3DVDY3rf-&data=04%7C01%7Cjaroslaw.sadowski%40cpk.pl%7C72f10e545ec748e873ca08d9cec7e6d0%7Cfa798250ca0b4a1bb47381cff4a1752b%7C1%7C0%7C637768180240837962%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000&sdata=AcHBFAywXe35DZ3kZWTRk%2FuaUEp145TjOwmJKp7sf64%3D&reserved=0>: Polish borders in EPSG: 2180, in the table of attributes you can see the areas (hectares) calculated for different versions of the software.

with wishes for more success in the new year
Jarosław Sadowski

_________________________________

Jarosław Sadowski
Ekspert ds. Baz Danych i Analiz Przestrzennych | Biuro Strategii i Planowania, Projektowania i Inżynierii Podprogramu Kolejowego
Database and Spatial Analysis Expert | Railway Subprogramme Strategy & Planning, Design & Engineering Department
e: jaroslaw.sadowski at cpk.pl<mailto:jaroslaw.sadowski at cpk.pl>
m: +48 532 720 230

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Sent from a phone. Amazing! But possibly garbled. Sebastian Gutwein Regenerative Design Group 1 Chevalier Ave Greenfield MA Bas at rdgland.com<mailto:Bas at rdgland.com> (631) 241-1018
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