Re: [Qgis-developer] Area calculation issues

[Nyall Dawson]

On 3 February 2017 at 16:54, Neumann, Andreas  wrote:
> Dear Nyall,
>
> Thank you so much for analyzing the issue and also thank you in advance for
> porting the fix to 2.18!

Done now. I also added one of those polygons to the reference geometry
test to ensure that all other stats (perimeter, centroid, etc) match
PostGIS' results.

Nyall


>
> I opened the sample project with 2.14 and I can confirm that the area values
> of both planimetric and ellipsoidic area calculations are very close for all
> features.
>
> It is very much appreciated!
>
> Thanks,
>
> Andreas
>
> On 2017-02-03 01:36, Nyall Dawson wrote:
>
> On 1 February 2017 at 19:55, Neumann, Andreas  wrote:
>
>
> Hi Nyall,
>
> Thanks for the explanations. I should have read the explanations of the help
> text more closely. It explains it all.
>
> However, there still seem to be issues. Consider the following results
> below. First column is the result of $area, second column is
> area($geometry).
>
> Sometimes the results are very close, in other cases the difference is
> massive. Like in the very first row. 32'087 vs 190'605'589 square meters.
> This can't be explained with ellipsoidical vs. planimetric. In this first
> row, the result of area($geometry) (planimetric) seems correct, while the
> ellipsoidical result ($area) is massively wrong.
>
> How can this huge difference be explained?
>
>
> Ok - I've looked into this. It's not an issue in 2.14 or 3.0, just 2.18.
>
> It was most likely fixed by:
> https://github.com/qgis/QGIS/commit/abc0919 "Fix area calculation of
> compoundcurve rings if they contain a 2-vertex linestring"
>
> Committed to 2.14 (with unit tests), I forward ported to 3.0, but it
> hasn't been applied to 2.18. I'll do this.
>
> Nyall
>
>
>
>
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Re: [Qgis-developer] Area calculation issues

[Neumann, Andreas]

Dear Nyall, 

Thank you so much for analyzing the issue and also thank you in advance
for porting the fix to 2.18! 

I opened the sample project with 2.14 and I can confirm that the area
values of both planimetric and ellipsoidic area calculations are very
close for all features. 

It is very much appreciated! 

Thanks, 

Andreas 

On 2017-02-03 01:36, Nyall Dawson wrote:

> On 1 February 2017 at 19:55, Neumann, Andreas  wrote: 
> 
>> Hi Nyall,
>> 
>> Thanks for the explanations. I should have read the explanations of the help 
>> text more closely. It explains it all.
>> 
>> However, there still seem to be issues. Consider the following results 
>> below. First column is the result of $area, second column is area($geometry).
>> 
>> Sometimes the results are very close, in other cases the difference is 
>> massive. Like in the very first row. 32'087 vs 190'605'589 square meters. 
>> This can't be explained with ellipsoidical vs. planimetric. In this first 
>> row, the result of area($geometry) (planimetric) seems correct, while the 
>> ellipsoidical result ($area) is massively wrong.
>> 
>> How can this huge difference be explained?
> 
> Ok - I've looked into this. It's not an issue in 2.14 or 3.0, just 2.18.
> 
> It was most likely fixed by:
> https://github.com/qgis/QGIS/commit/abc0919 "Fix area calculation of
> compoundcurve rings if they contain a 2-vertex linestring"
> 
> Committed to 2.14 (with unit tests), I forward ported to 3.0, but it
> hasn't been applied to 2.18. I'll do this.
> 
> Nyall

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Re: [Qgis-developer] Area calculation issues

[Nyall Dawson]

On 1 February 2017 at 19:55, Neumann, Andreas  wrote:
>
> Hi Nyall,
>
> Thanks for the explanations. I should have read the explanations of the help 
> text more closely. It explains it all.
>
> However, there still seem to be issues. Consider the following results below. 
> First column is the result of $area, second column is area($geometry).
>
> Sometimes the results are very close, in other cases the difference is 
> massive. Like in the very first row. 32'087 vs 190'605'589 square meters. 
> This can't be explained with ellipsoidical vs. planimetric. In this first 
> row, the result of area($geometry) (planimetric) seems correct, while the 
> ellipsoidical result ($area) is massively wrong.
>
> How can this huge difference be explained?

Ok - I've looked into this. It's not an issue in 2.14 or 3.0, just 2.18.

It was most likely fixed by:
https://github.com/qgis/QGIS/commit/abc0919 "Fix area calculation of
compoundcurve rings if they contain a 2-vertex linestring"

Committed to 2.14 (with unit tests), I forward ported to 3.0, but it
hasn't been applied to 2.18. I'll do this.

Nyall
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Re: [Qgis-developer] Area calculation issues

[Giovanni Manghi]

Hi all,

> Hi Andreas and Nyall,
>
> Please, share the conclusions here, after you find the problem.

I'm also very interested in know the results or help test if a sample
dataset is available.

cheers!

-- G --
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Re: [Qgis-developer] Area calculation issues

[Pedro Venâncio]

Hi Andreas and Nyall,

Please, share the conclusions here, after you find the problem.

Thanks!
Pedro



2017-02-01 10:02 GMT+00:00 Andreas Neumann :

> Sure - I'll send it to you as a geopackage file.
>
> Andreas
>
>
>
> On 01.02.2017 11:01, Nyall Dawson wrote:
>
>> On 1 February 2017 at 19:55, Neumann, Andreas 
>> wrote:
>>
>>> Hi Nyall,
>>>
>>> Thanks for the explanations. I should have read the explanations of the
>>> help text more closely. It explains it all.
>>>
>>> However, there still seem to be issues. Consider the following results
>>> below. First column is the result of $area, second column is
>>> area($geometry).
>>>
>>> Sometimes the results are very close, in other cases the difference is
>>> massive. Like in the very first row. 32'087 vs 190'605'589 square meters.
>>> This can't be explained with ellipsoidical vs. planimetric. In this first
>>> row, the result of area($geometry) (planimetric) seems correct, while the
>>> ellipsoidical result ($area) is massively wrong.
>>>
>>> How can this huge difference be explained?
>>>
>>> Can you share your dataset?
>>
>> Nyall
>>
>
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Re: [Qgis-developer] Area calculation issues

[Andreas Neumann]

Sure - I'll send it to you as a geopackage file.

Andreas


On 01.02.2017 11:01, Nyall Dawson wrote:

On 1 February 2017 at 19:55, Neumann, Andreas  wrote:

Hi Nyall,

Thanks for the explanations. I should have read the explanations of the help 
text more closely. It explains it all.

However, there still seem to be issues. Consider the following results below. 
First column is the result of $area, second column is area($geometry).

Sometimes the results are very close, in other cases the difference is massive. 
Like in the very first row. 32'087 vs 190'605'589 square meters. This can't be 
explained with ellipsoidical vs. planimetric. In this first row, the result of 
area($geometry) (planimetric) seems correct, while the ellipsoidical result 
($area) is massively wrong.

How can this huge difference be explained?


Can you share your dataset?

Nyall


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Re: [Qgis-developer] Area calculation issues

[Nyall Dawson]

On 1 February 2017 at 19:55, Neumann, Andreas  wrote:
>
> Hi Nyall,
>
> Thanks for the explanations. I should have read the explanations of the help 
> text more closely. It explains it all.
>
> However, there still seem to be issues. Consider the following results below. 
> First column is the result of $area, second column is area($geometry).
>
> Sometimes the results are very close, in other cases the difference is 
> massive. Like in the very first row. 32'087 vs 190'605'589 square meters. 
> This can't be explained with ellipsoidical vs. planimetric. In this first 
> row, the result of area($geometry) (planimetric) seems correct, while the 
> ellipsoidical result ($area) is massively wrong.
>
> How can this huge difference be explained?
>

Can you share your dataset?

Nyall
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Re: [Qgis-developer] Area calculation issues

[Neumann, Andreas]

Hi Nyall, 

Thanks for the explanations. I should have read the explanations of the
help text more closely. It explains it all. 

However, there still seem to be issues. Consider the following results
below. First column is the result of $area, second column is
area($geometry). 

Sometimes the results are very close, in other cases the difference is
massive. Like in the very first row. 32'087 vs 190'605'589 square
meters. This can't be explained with ellipsoidical vs. planimetric. In
this first row, the result of area($geometry) (planimetric) seems
correct, while the ellipsoidical result ($area) is massively wrong. 

How can this huge difference be explained? 

 

Regarding the second issue - yes, it makes sense that OTF has no
influence and that always the project CRS is used. 

Thanks! 

Andreas 

On 2017-02-01 10:30, Nyall Dawson wrote:

> On 1 February 2017 at 18:55, Neumann, Andreas  wrote: 
> 
>> Hi,
>> 
>> Regarding field calculator:
>> 
>> I wonder what the difference between $area and area($geometry) is?
> 
> $area respects project settings like ellipsoid choice and area units.
> 
> area(geometry) is just a plain cartesian calculator.
> 
>> They deliver different results - sometimes quite close, but sometimes with a
>> hug difference.
> 
> The results will vary (sometimes by a huge amount) depending on the
> geometry's crs and ellipsoid choice. Eg area($geometry) for geometries
> in geographic coordinates in mostly meaningless, and the same for
> projections like web mercator.
> 
>> I have to add, that the data contains circular arcs.
> 
> Should not affect anything.
> 
>> Another user reported me that area calculation results are different
>> depending on OTF is on or off (I have to ask which QGIS version, he did not
>> tell).
> 
> Yes - switching off OTF will reset the ellipsoid to none. Switching it
> on resets the ellipsoid choice back to the ellipsoid matching the
> project CRS.
> 
> This was very confusing behaviour. The good news is that I changed
> this a week or so ago in qgis 3. Now OTF has no impact on area or
> length calculations, and that *only* depends on the project ellipsoid
> choice.
> 
> (There was talk at the time that we should totally remove the OTF off
> option. I'm in favour of this and will probably tackle it soon).
> 
> Nyall

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Re: [Qgis-developer] Area calculation issues

[Nyall Dawson]

On 1 February 2017 at 18:55, Neumann, Andreas  wrote:
> Hi,
>
> Regarding field calculator:
>
> I wonder what the difference between $area and area($geometry) is?

$area respects project settings like ellipsoid choice and area units.

area(geometry) is just a plain cartesian calculator.

> They deliver different results - sometimes quite close, but sometimes with a
> hug difference.
>

The results will vary (sometimes by a huge amount) depending on the
geometry's crs and ellipsoid choice. Eg area($geometry) for geometries
in geographic coordinates in mostly meaningless, and the same for
projections like web mercator.


> I have to add, that the data contains circular arcs.

Should not affect anything.

> Another user reported me that area calculation results are different
> depending on OTF is on or off (I have to ask which QGIS version, he did not
> tell).

Yes - switching off OTF will reset the ellipsoid to none. Switching it
on resets the ellipsoid choice back to the ellipsoid matching the
project CRS.

This was very confusing behaviour. The good news is that I changed
this a week or so ago in qgis 3. Now OTF has no impact on area or
length calculations, and that *only* depends on the project ellipsoid
choice.

(There was talk at the time that we should totally remove the OTF off
option. I'm in favour of this and will probably tackle it soon).

Nyall
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[Qgis-developer] Area calculation issues

[Neumann, Andreas]

Hi, 

Regarding field calculator: 

I wonder what the difference between $area and area($geometry) is? 

They deliver different results - sometimes quite close, but sometimes
with a hug difference. 

I have to add, that the data contains circular arcs. 

Tested with 2.18.x. OTF off. 

Nyall - I remember that you dived into this issue in one of the past bug
fixing efforts? Can you explain it? I can deliver a small dataset
demonstrating the issue. 

--- 

Another user reported me that area calculation results are different
depending on OTF is on or off (I have to ask which QGIS version, he did
not tell). All data sets in the projects use a meter based system. The
data set of the assumedly wrong area calculations are in the same
coordinate system as the project. 

Thanks, 

Andreas  

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