[GRASS-dev] geodesic distances for measuring and buffers, even when working in planar coordinate system ? [was: Re: [GRASS-user] What distance is being measured and used for buffers ?]

[Markus Metz]

On Tue, Sep 4, 2012 at 6:57 PM, Moritz Lennert
<mlennert at club.worldonline.be> wrote:
> On 01/09/12 18:02, Moritz Lennert wrote:
>>
>> Leaving below mail as record of my original issue, I would to raise the
>> fundamental question of whether it would be feasible
>>
>> 1) to (optionally) provide geodesic instead of planar distances when
>> measuring, even if the location is in a projected coordinate system.
>> E.g. QGIS provides the possibility in distance measuring to check a box
>> to activate geodesic distance
>>
>> 2) to (optionally) allow the creation of buffers based on geodesic
>> distances, again in a projected location, which would imply non-circular
>> buffers.
>>
>
> Exploring my exploration of this in the hope that someone might share an
> interest:
>
> r.buffer actually provides the possibility of geodesic buffering when used
> in a lat-long location. Would it be difficult to implement the same in
> v.buffer ?

The short answer is yes, it will be difficult. The GRASS-internal
vector buffering algorithm has a number of bugs, the only vector
buffering method that is AFAICT bug-free is v.buffer in trunk with
GEOS support which uses the GEOS buffering algorithm which in turn
does not (yet?) support geodesic distances in latlong.

Markus M