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 <dwinsem...@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 via...@tcd.ie http://www.tcd.ie/Zoology/research/research/theoretical ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
