Plotting elements from shapefiles in R

Question

stackoverflow community, I have two shapefiles of the municipalities of Japan. I'm using R to create separate plots for each of the municipalities. I can make this work with one shapefile, but the identical syntax fails with the other. Here are the code and the URLs for the data: ``` library(sp) library(maptools) # Map A - this one works # Please download: http://www.filefactory.com/file/z26nirxoz53/n/JPN_adm_zip # Enter your path for readShapePoly japanMapA = readShapePoly("JPN_adm/JPN_adm2") names(japanMapA) plot(japanMapA[japanMapA$ID_2 == 1199,]) # Map B - this one doesn't work # Please download: http://geocommons.com/overlays/173340.zip # Again, enter your path for readShapePoly japanMapB = readShapePoly("japan_ver71") names(japanMapB) plot(japanMapB[japanMapB$JCODE == 45382,]) ``` The error it throws is: ``` Error in plot(japanMapB[japanMapB$JCODE == 45382, ]) : error in evaluating the argument 'x' in selecting a method for function 'plot': Error in japanMapB[japanMapB$JCODE ==…

Accepted Answer

I think this comes down to an issue with appropriate sub-setting.

Your `japanMapB` object consists of some metadata and a series of polygons for each shape stored in `japanMapB@polygons`. So, you have:

```
> length(japanMapB$JCODE)
#[1] 1902
> length(japanMapB@polygons)
#[1] 1902
```

As @PaulHiemstra notes though, you have some `NA` values in your `JCODE` variable

```
> table(is.na(japanMapB$JCODE))

#FALSE  TRUE 
# 1894     8 
```

Which means you get NA results when trying to index the municipalities you want to plot.

```
> table(japanMapB$JCODE==45382,useNA="always")

#FALSE  TRUE  <NA> 
# 1893     1     8 
```

Wrapping in `which` solves this:

```
which(japanMapB$JCODE == 45382)
#[1] 1802
#You will now select to plot only the 1802th polygon stored in the data object
plot(japanMapB[which(japanMapB$JCODE == 45382),])
```

Plotting elements from shapefiles in R

Solution

Problem