create hexagonal cells grid using lat/lon coordinates

Question

I would like to create a spatial grid with hexagonal cells using WGS84 coordinates (ie cells defined by 2 coordinates X=Latitude and Y=Longitude)

So, this is what I was thinkin about :

library(ggplot2);library(hexbin)
X<-seq(-10,20,by=0.1) # create coordinates vectors X and Y
Y<-seq(35,65,by=0.1)
z<-rnorm(301,0.5,1)
df<-as.data.frame(cbind(X,Y,z)) # create data frame with a z value for each cells (X,Y)
pl<-ggplot2(data=mat,aes(x=X,y=Y,z=z))+stat_summury_hex(fun=function(x) sum(x))
plot(pl)

But doing this does not provide what I wanted.

So, my question is : how to do a spatial grid with hexagonal cells using lat/lon coordinates ?

And second question : how to create a grid centered from one point (that would represent the centroid, and not the left bottom corner as usual?)

Answer 1

If I understand properly, you're looking for expand.grid() :

xy <- expand.grid(X=X,Y=Y)
z<-rnorm(nrow(xy),0.5,1)
df<-as.data.frame(cbind(xy,z)) # create data frame with a z value for each cells (X,Y)
head(df)
pl<-ggplot(data=df,aes(x=X,y=Y,z=z))+stat_summary_hex(fun=function(x) sum(x))
plot(pl)

As for the second question, I'm not sure, but since all hexagons are the same size and will require the same operation to center, you can shift them uniformly by changing X and Y appropriately. Perhaps this can also be done via arguments also, not sure.

[[Edit July 23]] second question was how to get a data.frame of hex coordinates. Took some digging, but here's an example:

library(hexbin)
coords <- hcell2xy( hexbin(x=X,y=Y))
head(coords)
      x        y
1 -10.0 35.00000
2  -9.5 35.86603
3  -8.5 35.86603
4  -9.0 36.73205
5  -8.0 36.73205
6  -7.5 37.59808

hcell2xy() is the key function called by ggplot2 , and you may need to be explicit about specifying the argument xbins , which is determined automatically inside ggplot2 , but appears to default to 30 in both cases.

[[Edit 3, to include z level]] This is an answer to the comment asking for z levels as well. Ripped from ggplot2:::hexBin

hb <- hexbin(x=X,y=Y)
# Convert to data frame
data.frame(
        hcell2xy(hb), 
        count = hb@count, 
        density = hb@count / sum(hb@count, na.rm=TRUE)
)

You can choose whether to use count or density for colors later, but warning: those are different from your z variable fed to ggplot2 . If you'd like to summarize based on some other statistic, then I suggest you also look into the guts of those functions to see how things are passed around. That's what I've been doing.