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

[lily li]

Hi Jim,

Thanks. It works. I now have more complex problems. If at each blackcell,
there are two variables such as pop and mood. For each variable, there are
daily records in one year, so 365 records for pop and 365 records for mood.
The averaged values for the redcells should be daily records too. What kind
of format do you recommend for this problem? Right now, I just get the
latitudes and longitudes into a dataframe. Thanks.

On Sun, May 20, 2018 at 3:52 AM, Jim Lemon <drjimlemon using gmail.com> wrote:

> 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
>

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