Question

I am trying to use the function fortify in R to plot the map. But I am always getting an error while processing the data. I read the shape file using the following code:

shape <- readShapeSpatial("Lond_City/lond_city.shp")

The shapefile can be found on the following dropbox location:
https://www.dropbox.com/sh/d4w6saailydtu1r/5oIa56xV6S

Then I tried to use the following command :

shape1 <- fortify(shape,region="data")

In the above code, what should I put in Place of “data”. This is the place which seems confusing. I can open the data or shapefile in R. However, when I run the fortify line I get the following error:

Error in ‘[.data.frame'(attr, , region) : undefined columns selected

Can anybody tell me how to properly fortify a shape file with example ? The reason I want to use fortify is that I want to combine the shape file with the data points.

Thank you so much.

Jdbaba

Answer 1

Your problem is that you need to give fortify a column that actually exists. If you type str(lon.shape) from my example code below, you will see that there is no region column:

> str(lon.shape)
Formal class 'SpatialPolygonsDataFrame' [package "sp"] with 5 slots
  ..@ data       :'data.frame': 1 obs. of  7 variables:
  .. ..$ ons_label: chr "00AA"
  .. ..$ name     : chr "City of London"
  .. ..$ label    : chr "02AA"
  .. ..$ X_max    : num 533843
  .. ..$ y_max    : num 182198
  .. ..$ X_min    : num 0
  .. ..$ y_min    : num 0
  ..@ polygons   :List of 1
  .. ..$ :Formal class 'Polygons' [package "sp"] with 5 slots
  .. .. .. ..@ Polygons :List of 1
  .. .. .. .. ..$ :Formal class 'Polygon' [package "sp"] with 5 slots
  .. .. .. .. .. .. ..@ labpt  : num [1:2] 532464 181220
  .. .. .. .. .. .. ..@ area   : num 3151465
  .. .. .. .. .. .. ..@ hole   : logi FALSE
  .. .. .. .. .. .. ..@ ringDir: int 1
  .. .. .. .. .. .. ..@ coords : num [1:771, 1:2] 531027 531029 531036 531074 531107 ...
  .. .. .. ..@ plotOrder: int 1
  .. .. .. ..@ labpt    : num [1:2] 532464 181220
  .. .. .. ..@ ID       : chr "0"
  .. .. .. ..@ area     : num 3151465
  ..@ plotOrder  : int 1
  ..@ bbox       : num [1:2, 1:2] 530967 180404 533843 182198
  .. ..- attr(*, "dimnames")=List of 2
  .. .. ..$ : chr [1:2] "x" "y"
  .. .. ..$ : chr [1:2] "min" "max"
  ..@ proj4string:Formal class 'CRS' [package "sp"] with 1 slots
  .. .. ..@ projargs: chr "+proj=tmerc +lat_0=49 +lon_0=-2 +k=0.9996012717 +x_0=400000 +y_0=-100000 +ellps=airy +units=m +no_defs" 

Instead, try using name, like this:

lon.df <- fortify(lon.shape, region = "name")

Which produces this data frame:

> head(lon.df)
      long      lat order  hole piece            group             id
1 531026.9 181611.1     1 FALSE     1 City of London.1 City of London
2 531028.5 181611.2     2 FALSE     1 City of London.1 City of London
3 531036.1 181611.5     3 FALSE     1 City of London.1 City of London
4 531074.0 181610.3     4 FALSE     1 City of London.1 City of London
5 531107.0 181609.3     5 FALSE     1 City of London.1 City of London
6 531117.1 181608.9     6 FALSE     1 City of London.1 City of London

Here’s one way to go about it, start to finish:

library(rgdal)
library(ggplot2)
library(rgeos)

shape.dir <- "c:/test/london" # use your directory name here

lon.shape <- readOGR(shape.dir, layer = "lond_city")
lon.df <- fortify(lon.shape, region = "name")

ggplot(lon.df, aes(x = long, y = lat, group = group)) +
    geom_polygon(colour = "black", fill = "grey80", size = 1) +
    theme()

…resulting in the following: