[Qgis-user] Q calculates Area incorrectly

[Richard Duivenvoorde]

Hi John,

Please make sure you also sent replies to the list, then other people can also learn/respond on these issues.

(so others see below)

You have to try to make yourself familiar with the use of crs's in QGIS.
EPSG:4326 is data in degrees (LatLon coordinates), not very well suited for measuring area's
For calculations/analysis it is better to use a non geographical crs (like EPSG:27700, the british national crs I presume?).

Also between 3.4.5 and 3.16.2 A LOT has changed in the projection part of QGIS (and the library we use for that PROJ).

I created a polygon, both in 4326 and in 27700 here locally and (after downloading a so called correction grid for the UK), I get more or less the same values for both polygons (in both crs's) both in expressions, the measurement tool AND the (Derived) attributes when using the Identify Results.

Also note that QGIS can calculate area on the earth elipsoid (so taking the roundness in account),and Cartesian (when you use eg a national crs).
In the attribute calculator there is "$area" and "area" (which behave differently, see the function help)

Can you maybe try to create a new polygon file (in 27700) and copy paste the geometries to that file? See what the new area is then?

So short answer: I do not understand the huge difference you see, I cannot reproduce it.
Also please use the newest QGIS possible (3.16) if you want to compare values.

I hope others have better input for you.

Regards,

Richard Duivenvoorde

On 1/8/21 10:47 AM, johnparrottca at gmail.com wrote:
> Thanks, Richard
> 
> Both laptop and desktop are running Windows10Pro.
> The laptop is running Q 3.4.5, the desktop 3.16.2
> The original polygons EPSG is 4326, but the EPSG on the desktop shows as
> 27700.
> The data comprises several polygons.
> The areas generated by the Field Calculator differ from those measured with
> the measuring tool (which I checked against known features, and is
> accurate).
> For a polygon od 1.24 hectares, for instance, the calculator gives 2.27 ha
> I hope that may help?
> I would like to measure the polygons correctly!
> 
> John
> -----Original Message-----
> From: Qgis-user <qgis-user-bounces at lists.osgeo.org> On Behalf Of Richard
> Duivenvoorde
> Sent: 08 January 2021 08:38
> To: qgis-user at lists.osgeo.org
> Subject: Re: [Qgis-user] Q calculates Area incorrectly
> 
> On 1/8/21 9:24 AM, johnparrottca at gmail.com wrote:
>> A colleague digitised a number of polygons on my laptop 3 years ago.
>>
>> Ijustloaded these on my desktop and used Q(3.16.2)to calculate areas, but
> theyreport incorrectly.
> 
> Hi John,
> 
> Your message does not contain any information on which somebody can do
> something on it, or try to answer a question (which is also NOT there :-)
> ...
> 
> If you can add some info about:
> - crs/projection/epsg you used both for data and project (are you
> reprojecting? Is it a non-geographical crs?)
> - data you use (maybe make it avaialable, in a github issue?)
> - OS and QGIS version you use
> - why you think/know it is reporting incorrect (and 'how incorrect'
> exactly...)
> 
> Then maybe somebody can look into this and find a solution...
> 
> Regards,
> 
> Richard Duivenvoordd
> 
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