[QGIS-Developer] Length measurement weirdness (again)

[Patrick Dunford]

It was always my impression that these measurements can never be that 
accurate as the projections are approximated? Or do you know otherwise?


On 04/05/18 11:18, Nyall Dawson wrote:
> On 3 May 2018 at 23:26, Paolo Cavallini <cavallini at faunalia.it> wrote:
>> Hi all,
>> the measurements of a EPSG:4326 line appear correct in any SRS, both
>> with the interactive info tool and writing on the table the value of
>> $length, *if* the OTF reprojection is active (including 4326).
>> If it is not, the result is grossly overestimated (298 m instead of 228,
>> etc.).
>> Am I missing something, or this is a serious bug?
> It's not a bug as much as very confusing 2.x behavior. The obscure
> link between a "rendering" setting (OTF projection) and ellipsoidal
> calculations (area/distance) is extremely confusing for users. We
> couldn't change this in 2.x because of the API freeze and how embedded
> this logic was in many areas of QGIS code and API.
>
> That's why in 3.0 the whole OTF logic was reworked, so that OTF is
> always on for rendering and doesn't the choice of ellipsoidal
> calculations or not is totally based on the project's ellipsoid
> setting.
>
> So unfortunately for 2.18 there's really no choice but to ensure that
> OTF projection setting is enabled.
>
> Nyall
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