On Thu, 14 Jan 2010, ONKELINX, Thierry wrote:
Dear Karl,
I think that the relative errors are more relevant than absolute errors.
The relative error on the equatorial circumference for "geo.dist" is
0.33%, which is not that accurate. The relative error on the meridional
distance is only about 0.085%, which should accurant enough for most
applications.
From the code of rdist.earth(), it is using a sphere with a 6378388m
parameter (which is like the International 1909 ellipsoid, but a sphere).
geod.dist() is using WGS84 a=6378137.0 rf=298.257223563:
http://code.google.com/p/r-oce/source/browse/trunk/R/misc.R
near line 280. The code under spDistsN1() uses WGS84 values, but with the
radius in km:
http://r-spatial.cvs.sourceforge.net/viewvc/r-spatial/sp/src/gcdist.c?revision=1.6&view=markup
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,
Roger
Just my 2 eurocents.
Thierry
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ir. Thierry Onkelinx
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tel. + 32 54/436 185
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-----Oorspronkelijk bericht-----
Van: r-sig-geo-boun...@stat.math.ethz.ch
[mailto:r-sig-geo-boun...@stat.math.ethz.ch] Namens Karl Ove Hufthammer
Verzonden: donderdag 14 januari 2010 10:57
Aan: r-sig-geo@stat.math.ethz.ch
Onderwerp: Re: [R-sig-Geo] Best way to calculate distance between
geographicalpoints
On Thu, 14 Jan 2010 10:27:17 +0100 Karl Ove Hufthammer <k...@huftis.org>
wrote:
I have found three functions for calculating the distance between
geographical points, 'geo.dist' in the 'oce' package, 'rdist.earth' in
the 'fields' package, and 'spDistsN1' in the 'sp' package. They all
give slightly difference answers, so my question is: which of them is
best, i.e., gives the most accurate answer?
One simple test would be to calculate the circumference of the earth at
the equator and the meridian. But this test gives conflicting answers.
According to Wikipedia, "http://en.wikipedia.org/wiki/Earth"; the
equatorial circumference is 40075.02 km and the meridional circumference
is 40007.86 km.
When calculating half the equatorial circumference, 'spDistsN1' is best,
and gives the *exact* answer, while 'rdist.earth' is close (less than
one km difference), and 'geo.dist' is far from the correct answer
(133.92 km difference). So it would initially seem that 'spDistsN1' is
the one to use, and one should avoid 'geo.dist'.
But when calculating the meridional distance, 'geo.dist' gives the exact
answer, while both 'rdist.earth' and 'spDistsN1' are both off by about
34 km (but in difference directions).
So which to use is still an open question ...
Here is the code I used:
library(oce)
library(fields)
library(sp)
# Equatorial
lt1=0
lt2=0
lg1=0
lg2=180
a=matrix(c(lg1,lt1),nrow=1)
b=matrix(c(lg2,lt2),nrow=1)
# Calculated distance
geod.dist(lt1,lg1,lt2,lg2)
rdist.earth(a,b, miles=FALSE)
spDistsN1(a,b, longlat=TRUE)
# Correct distance (according to Wikipedia)
40075.02/2
# Meridional
lt1=-90
lt2=90
lg1=0
lg2=0
a=matrix(c(lg1,lt1),nrow=1)
b=matrix(c(lg2,lt2),nrow=1)
# Calculated distance
geod.dist(lt1,lg1,lt2,lg2)
rdist.earth(a,b, miles=FALSE)
spDistsN1(a,b, longlat=TRUE)
# Correct distance (according to Wikipedia)
40007.86/2
--
Karl Ove Hufthammer
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