[R] Midpoint between coordinates

[David Winsemius]

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