[Qgis-developer] Distance calculation accuracy ?

[kimaidou]

Hi all

Thanks Richard for your answer.

I made some tests. Here are my conclusions :

* By default, QGIS will use the current project ellipsoid to compute
distances.
* You can change this behaviour in the Project Properties dialog ( Menu
Project > Project properties ), tab "General"
* The safest distance calculation is done when you use the same projection
between the project and the layers. I have seen some weird lengths while
using the map tool with OTF on, with a project in Pseudo Mercator (EPSG:
3857) and a projected layer ( EPSG:2154 ). Like 30% differences (which is
huge)
* You can select "Planimetric / None" instead of an ellipsoid in the
project properties option to avoid discrepencies between distance
measurements (for example inside projects with different OTR parameters).
But you won't then benefit from the precise calculation done by using the
projection ellipsoid.
* It seems many other softwares like ArcGis or grass , they use
"Planimetric / None" to compute the distance (and I read some deactivate
distance tool when OTF is on)

So it seems to me QGIS is advanced here in the field of calculating
distances (and areas, etc.), but the end user must know what he (she) is
doing to fully understand what is going on. Should we not use "Planimetric
/ None" as a default parameter, even if OTF is on ? Advanced users could
benefit of more precision by using the projection ellipsoid, but everyone
else will have consistent measurements ?

Cheers
Michaël

2015-10-26 9:09 GMT+01:00 Richard Duivenvoorde <rdmailings at duif.net>:

> On 26-10-15 08:48, kimaidou wrote:
> > Hi all
> >
> > Someone reported to me an issue with $length distance calculation and
> > the "Distance" map tool. It seems to him QGIS was not precise enough
> > compared to field data collection (about 10% or worst discrepency)
> >
> > I remember reading some discussions here about wrong distance
> > calculations in some contexts.
> > Has anyone a "cookbook" or good practices about distance calculations in
> > QGIS, regarding different data sources, contexts, SCR, etc. ( on the fly
> > reprojection or not, data provider, etc.)
>
> I know Nyall has been busy with this (see hist posts in both dev and
> user list).
>
> Regards,
>
> Richard
>
>
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