[R-sig-Geo] Best way to calculate distance between geographicalpoints

[Roger Bivand]

On Thu, 14 Jan 2010, Karl Ove Hufthammer wrote:

> On Thu, 14 Jan 2010 12:27:04 +0100 (CET) Roger Bivand
> <Roger.Bivand at nhh.no> wrote:
>> So in principle geod.dist() and spDistsN1(..., longlat=TRUE) should yield
>> equivalent results, because their assumptions are the same.
>>
>> Several of the references suggest that the geographic pole should not be
>> input. It looks as though the oce implementation handles this more
>> gracefully than sp.
>>
>> Hope this helps,
>
> It does. Thanks.
>
> BTW, is there a reason 'spDistsN1' handle longitude 360 different from
> longitude 0? For example, the distance between 0,0 and 0,0 is 0, but the
> distance between 0,0 and 360,0 is about 1e-12. This is of course not a
> problem; I was just wondering on the reason (since 360,0 is for all
> practical *correctly* handled -- it's just not handled equal to 0,0).

The C function now tests for equality in the longitude and latitude pairs, 
but arguably should catch this. Equality is taken here as an absolute 
difference of less than .Machine$double.eps, about 2.220446e-16. I've 
committed a change to sourceforge, setting distance to 0.0 for

(|lat0 - lat1| < fuzz) && (|lon0 - lon1| < fuzz)

as before, but also:

(|lat0 - lat1| < fuzz) && (((|lon0| + |lon1|) - 360.0) < fuzz)

which seems to handle your case, and others - like -170, 190. Comments 
welcome.

Roger

>
>

-- 
Roger Bivand
Economic Geography Section, Department of Economics, Norwegian School of
Economics and Business Administration, Helleveien 30, N-5045 Bergen,
Norway. voice: +47 55 95 93 55; fax +47 55 95 95 43
e-mail: Roger.Bivand at nhh.no