[R-sig-Geo] sp::spDists*

[Edzer Pebesma]

The problem described below has been corrected ins sp 1.2-4 which is now
on CRAN; new releases of gstat, spgwr and spdep will follow to correct
for the same bug.

On 08/12/16 15:48, Edzer Pebesma wrote:
> While building support for geosphere distance functions in sf and
> comparing results from geosphere::distMeeuws with those of sp::spDists,
> I found a typo in the C code underlying the great circle distance
> computation functions in sp (sp::spDists, sp::spDistsN1) [*]:
> 
> https://github.com/edzer/sp/commit/d8374ff7efc6735cba9a054748c602bed0672f23
> 
> Before:
>> library(sf)
> Linking to GEOS 3.5.0, GDAL 2.1.0, proj.4 4.9.2
>> library(sp)
>> library(units)
>>
>> x = st_sfc(
> + st_point(c(0,0)),
> + st_point(c(1,0)),
> + st_point(c(2,0)),
> + st_point(c(3,0)),
> + crs = 4326
> + )
>>
>> y = st_sfc(
> + st_point(c(0,10)),
> + st_point(c(1,0)),
> + st_point(c(2,0)),
> + st_point(c(3,0)),
> + st_point(c(4,0)),
> + crs = 4326
> + )
>>
>> st_distance(x, y)
> Units: m
>         [,1]     [,2]     [,3]     [,4]     [,5]
> [1,] 1105855 111319.5 222639.0 333958.5 445278.0
> [2,] 1111387      0.0 111319.5 222639.0 333958.5
> [3,] 1127822 111319.5      0.0 111319.5 222639.0
> [4,] 1154693 222639.0 111319.5      0.0 111319.5
>>
>> d.sf = st_distance(x, y, geosphere::distMeeus)
>> d.sp = spDists(as(x, "Spatial"), as(y, "Spatial"))
>> units(d.sp) = make_unit("km")
>> d.sf - d.sp
> Units: m
>          [,1]         [,2]         [,3]         [,4] [,5]
> [1,] 1851.990 0.000000e+00 0.000000e+00 0.000000e+00    0
> [2,] 1842.938 0.000000e+00 0.000000e+00 2.910383e-11    0
> [3,] 1816.564 0.000000e+00 0.000000e+00 1.455192e-11    0
> [4,] 1775.032 2.910383e-11 1.455192e-11 0.000000e+00    0
>>
> 
> 
> 
> After:
> 
>> library(sf)
> Linking to GEOS 3.5.0, GDAL 2.1.0, proj.4 4.9.2
>> library(sp)
>> library(units)
>>
>> x = st_sfc(
> + st_point(c(0,0)),
> + st_point(c(1,0)),
> + st_point(c(2,0)),
> + st_point(c(3,0)),
> + crs = 4326
> + )
>>
>> y = st_sfc(
> + st_point(c(0,10)),
> + st_point(c(1,0)),
> + st_point(c(2,0)),
> + st_point(c(3,0)),
> + st_point(c(4,0)),
> + crs = 4326
> + )
>>
>> st_distance(x, y)
> Units: m
>         [,1]     [,2]     [,3]     [,4]     [,5]
> [1,] 1105855 111319.5 222639.0 333958.5 445278.0
> [2,] 1111387      0.0 111319.5 222639.0 333958.5
> [3,] 1127822 111319.5      0.0 111319.5 222639.0
> [4,] 1154693 222639.0 111319.5      0.0 111319.5
>>
>> d.sf = st_distance(x, y, geosphere::distMeeus)
>> d.sp = spDists(as(x, "Spatial"), as(y, "Spatial"))
>> units(d.sp) = make_unit("km")
>> d.sf - d.sp
> Units: m
>               [,1]         [,2]         [,3]         [,4] [,5]
> [1,] -2.328306e-10 0.000000e+00 0.000000e+00 0.000000e+00    0
> [2,]  2.328306e-10 0.000000e+00 0.000000e+00 2.910383e-11    0
> [3,]  0.000000e+00 0.000000e+00 0.000000e+00 1.455192e-11    0
> [4,]  0.000000e+00 2.910383e-11 1.455192e-11 0.000000e+00    0
>>
> 
> [*] assuming the source code of
> http://www.abecedarical.com/javascript/script_greatcircle.html is
> correct; I don't have access to the Meeuws book mentioned there under (1).
> 
> The same C function is used by gstat; I corrected that as well.
> 
> 
> 
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> 

-- 
Edzer Pebesma
Institute for Geoinformatics  (ifgi),  University of Münster
Heisenbergstraße 2, 48149 Münster, Germany; +49 251 83 33081
Journal of Statistical Software:   http://www.jstatsoft.org/
Computers & Geosciences:   http://elsevier.com/locate/cageo/

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