[josm-dev] Geometric calculations and projections

[Matthias Julius]

Tobias Wendorff <tobias.wendorff at uni-dortmund.de> writes:

> Matthias Julius schrieb:
>> This is actually a good example why we might want to use Mercator for
>> calculating rectangles.
>
> Which Mercator are you talking about?
> - Spehrical Mercator / Web Mercator (it's using a sphere)
> - "real" Mercator (Earth is flattened)
> - Transverse Mercator (on sphere or ellipsoid?)
> - Universal Transverse Mercator (on sphere or ellipsoid?)

Spherical Mercator.

>
> If you want to use the "normal" Web Mercator, OSM and Google use,
> it would be better to use spherical geometry ... it's more accurate
> than calculation into a projection and back to LatLon.

The point is that things like the state of Wyoming only look
rectangular in spherical Mercator projection or similar.

>
> Spherical geometry allows you to calculate _directly_ on the sphere
> without using a projection ... you simple use LatLon in radian
> degrees.

True, but it's not really trivial.

>
> I can do some calculations about accurancy the next week, perhaps
> sphere is enough ... but as soon as you work on big objects, you
> might run into problems.

The question is: What is big?

When does the error become more than, say, 0.1 m?

Matthias