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I have a census tiger line file for counties available from the Census website.

I am trying to display this file on a ggmap, but fortify seems to jumble the regions. I can't find anyone else who has run into this error, but I'm certainly not doing anything out of the ordinary...

library(maptools)
library(ggmap)
library(rgeos)

# read shapefile
WGS84 <- CRS("+proj=longlat +ellps=WGS84 +datum=WGS84 +no_defs +towgs84=0,0,0")
Counties <- readShapeSpatial("tl_2013_us_county.shp", proj4string = WGS84)

# limit to Ohio
Counties <- Counties[ Counties$STATEFP == 39, ]

# fortify
Counties.fort <- fortify(Counties, region = "GEOID")

# get base map and plot
OhioMap <- get_map("Ohio", source = 'google', maptype = 'terrain', zoom=7)
ggmap(OhioMap) + geom_polygon(aes(x=long, y=lat), fill = 'grey', alpha=0.4, 
                              color = 'black', data = Counties.fort)

enter image description here

2

As it turns out, the solution is to group on the region id. I think I knew this once, but hopefully this can save someone else some Google time.

ggmap(OhioMap) + geom_polygon(aes(x=long, y=lat, group=group), fill = 'grey', alpha=0.4, 
                              color = 'black', data = Counties.fort)

enter image description here

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