find locations within certain lat/lon distance in r

Question

I have a gridded dataset, with data available at the following locations:

lon <- seq(-179.75,179.75, by = 0.5)
lat <- seq(-89.75,89.75, by = 0.5)
         


        

Answer 1

The dist* functions of the geosphere package are vectorized, so you only need to prepare better your inputs. Try this:

#prepare a matrix with coordinates of every position
allCoords<-cbind(lon,rep(lat,each=length(lon)))
#call the dist function and put the result in a matrix
res<-matrix(distm(cbind(mylon,mylat),allCoords,fun=distHaversine)<=500000,nrow=length(lon))
#check the result
identical(res,dd2)
#[1] TRUE

As the @Floo0 answer showed, there is a lot of unnecessary calculations. We can follow another strategy: we first determine the lon and lat range that can be closer than the threshold and then we use only them to calculate the distance:

#initialize the return
res<-matrix(FALSE,nrow=length(lon),ncol=length(lat))
#we find the possible values of longitude that can be closer than 500000
#How? We calculate the distances between us and points with our same lon 
longood<-which(distm(c(mylon,mylat),cbind(lon,mylat))<=500000)
#Same for latitude
latgood<-which(distm(c(mylon,mylat),cbind(mylon,lat))<=500000)
#we build the matrix with only those values to exploit the vectorized
#nature of distm
allCoords<-cbind(lon[longood],rep(lat[latgood],each=length(longood)))
res[longood,latgood]<-distm(c(mylon,mylat),allCoords)<=500000

In this way, you calculate just lg+ln+lg*ln (lg and ln are the length of latgood and longood), i.e. 531 distances, opposed to the 259200 with my previous method.