[R] How to average values from grid cells with coordinates

[Jim Lemon]

Hi lily,
It's not too hard to do it using dataframes. Getting the indexing
right is usually that hardest part:

# these values are the centers of the black cells
lat<-rep(28:38,11)
lon<-rep(98:108,each=11)
pop<-sample(80:200,121)
# just use the data.frame function
blackcells<-data.frame(lat=lat,lon=lon,pop=pop)
plot(0,type="n",xlim=c(97.5,108.5),ylim=c(27.5,38.5),
 xlab="Longitude",ylab="Latitude")
abline(h=27.5)
abline(h=lat+0.5)
abline(v=97.5)
abline(v=lon+0.5)
text(blackcells$lon,blackcells$lat,pop)
# the red cells will be centered on the corners of 4 black cells
lat2<-rep(seq(28.5,34.5,by=2),4)
lon2<-rep(seq(99.5,105.5,by=2),each=4)
redcells<-data.frame(lat=lat2,lon=lon2,value=NA)
display the red cells
rect(lon2-1,lat2-1,lon2+1,lat2+1,border="red",lwd=2)
nblackcells<-dim(blackcells)[1]
nredcells<-dim(redcells)[1]
for(redcell in 1:nredcells) {
 close4<-rep(NA,4)
 closen<-1
 for(blackcell in 1:nblackcells) {
  if(abs(blackcells[blackcell,"lat"]-redcells[redcell,"lat"]) < 1 &&
   abs(blackcells[blackcell,"lon"]-redcells[redcell,"lon"]) < 1) {
   close4[closen]<-blackcells[blackcell,"pop"]
   closen<-closen + 1
  }
 }
 cat(close4,"\n")
 redcells[redcell,"value"]<-sum(close4)/4
}
library(plotrix)
boxed.labels(redcells$lon,redcells$lat,redcells$value,col="red")

Jim