R Plot Filled Longitude-Latitude Grid Cells on Map

Question

I have a data frame containing a number of (x,y,z) data points, (x,y) is the lower-right coordinate of a longitude-latitude cell of size w (e.g. a 1-degree grid). The z value has been averaged over this cell.

I'd like to plot these points in R so that the entire grid cell is filled with some colour derived from z.

The result would look something like one of these images:

The projection itself (e.g. Lambert conformal conic, equirectangular) isn't important, just the grid cell plotting.

My data is sparse: not every longitude-latitude cell will have data associated with it.

My hope would be a solution similar to

library(maps)
map("state")
grid_points(my_data,c("x","y","z"),0.5)

where 0.5 is the grid resolution above, indicating a 0.5-degree cell.

Any thoughts?

Thanks!

Answer 1

An alternative to using either spplot or image is to use ggplot2. The relevant geometries are geom_raster and geom_tile. The first is supposed to perform better and yield smaller files, and the second is more standard. The following example call:

ggplot(aes(x = x, y = y, fill = value), data = dat_grid) + geom_tile() + 
  geom_path(data = ant_ggplot)

orginates from this blogpost of mine. In addition, ggplot2 supports a range of projections through the mapproj package, see coord_map for more details.

The following is a working example (provided you've defined YOUR_DATA to have x,y,z columns):

library(ggplot2)
library(maps)
us_states <- map_data("state")
(ggplot(aes(x=x,y=y,fill=z),data=YOUR_DATA) + geom_tile())+geom_polygon(data=us_states,aes(x=long, y=lat, group=group), colour="black", fill="white", alpha=0)