[R] Midpoint between coordinates

[Mafalda Viana]

The arithmetic mean was my first approach and to nearby points it
doesn't make much difference. However, when the distance between the 2
points gets bigger this is no longer accurate enough. So yes, I was
thinking on spherical geometry, midpoint considering the great circle
distance or similar.

Thank you
Mafalda

On 23 July 2010 13:30, David Winsemius <dwinsemius at comcast.net> wrote:
>
> On Jul 23, 2010, at 7:13 AM, Mafalda Viana wrote:
>
>> Dear R users,
>>
>> I need to find the coordinates for the point (midpoint) located half
>> way between two pairs of coordinates (lon1,lat1 and lon2,lat2)
>> assuming a straight line between them. What would be the best way? I
>> tried to find an answer in the help archives but without success. I
>> would greatly appreciate any help.
>>
>> df<- data.frame(lon1=c(-4.568,-4.3980), lat1=c(59.235,56.369),
>> lon2=c(-5.123,-4.698), lat2=c(60.258,59.197) )
>
> Wouldn't that just be the arithmetic average of the values? Or to you have
> some need for a more accurate calculation based on spherical geometry?  Or
> some thing that will handle some other coordinate weirdness?
>
>> df$midlong <- apply(df[,c(1,3)], 1, mean)
>> df$midlat <- apply(df[,c(2,4)], 1, mean)
>> df
>    lon1   lat1   lon2   lat2 midlong  midlat
> 1 -4.568 59.235 -5.123 60.258 -4.8455 59.7465
> 2 -4.398 56.369 -4.698 59.197 -4.5480 57.7830
>>
> --
>
> David Winsemius, MD
> West Hartford, CT
>
>
>



-- 
Mafalda Viana
Department of Zoology
School of Natural Sciences
Trinity College Dublin
Dublin 2
Ireland

(+353) (0) 872829850

vianam at tcd.ie

http://www.tcd.ie/Zoology/research/research/theoretical