Color a layer of the map based on the location of dots on another layer

Question

Thanks for helping me in GIS 101 Problem #2, now I have followup questions:

I created a map as below:

The map is created using the following script:

ct <- readShapeSpatial("housect_37800_0000_2010_s100_census_1_shp/wgs84/housect_37800_0000_2010_s100_census_1_shp_wgs84.shp")

ct_mod <- fortify(ct,region="SLDLST10")
chart <- ggplot(data=ct_mod,aes(long,lat))
chart <- chart + scale_x_continuous(limits=c(-73.8,-71.7),breaks=seq(-74,-71,0.1))
chart <- chart + scale_y_continuous(limits=c(40.9,42.1),breaks=seq(40,43,0.1))
chart <- chart + geom_polygon(fill="grey80",aes(group=group))
chart <- chart + geom_path(color="white",aes(group=group))  
# coord_equal make x-y same scale
chart <- chart + coord_equal()

chart1 <- chart + geom_point(data=hosp.list,aes(x=coord_x,y=coord_y))  

This is map of the Connecticut Census State Legislative District Map with several hospital located, you can see a few regions dot and a few does not. I want to fill the regions with dot using one colour, and regions without dot using another colour. Can anyone give me a clue? Thanks.

Answer 1

You could do a point in polygon operation to determine which regions contain a dot. You can then subset the regions that have dots and plot them with a different color.

Answer 2

Quick: use the over(pts,poly) function to return which polygon each point is in. [You might have to construct pts as a SpatialPoints object from your data frame]. Then the return value tells you which polygons have points in. Add a new column to your SpatialPolygonsDataFrame (ct) and then colour it in the usual ggplot way.

Sorry can't say more but there should be enough for you to work it out. See package:sp and the over function.