How to convert a spatialpolygon to a SpatialPolygonsDataFrame and add a column to the attribute table

Question

coast<-readShapeSpatial("coastline.shp")
landc<-readShapeSpatial("landcover.shp")
ro<-readShapeSpatial("roads.shp")
bc<-gBuffer(ro,width=100)
landc$ratings=1
landc$ratings[landc$LANDUSE_ID==4]=0 

Above, I take any category that has 4 and in the new column put it as 0.

At this point, I want the column named ratings as well for the bc, where it will take 0 if it is inside the buffer and 1 if it is outside. The problem is that the bc is SpatialPolygons and it doesn't contain the attribute table.

Obviously to add a column to a SpatialPolygon object you have to convert it to a SpatialPolygonsDataFrame, but I don't know how.

I tried this:

buf_df<-as.data.frame(bc)
s_po<-SpatialPolygonsDataFrame(bc,buf_df)
s_po$ratings=0

but this error pops up:

row.names of data and Polygons IDs do not match 

Answer 1

What do the "coast", "ro" and "bc" objects have to do with your problem? The issue may lie in that you are using "readShapeSpatial". Have you tried readOGR in rgdal? If you are reading a polygon shapefile, readOGR will result in a SpatialPolygonsDataFrame object.

If you in fact, do have a SpatialPolygons object and want to coerce into SpatialPolygonsDataFrame the specified dataframe will need to have its rownames match the polygon ID's in the polygons slot. Here is a quick example.

library(sp)

# create some SpatialPolygons with ID's "2" and "3"
( p <- SpatialPolygons(list(Polygons(list(Polygon(cbind(c(2,4,4,1,2),c(2,3,5,4,2)))), "2"),
     Polygons(list(Polygon(cbind(c(5,4,2,5),c(2,3,2,2)))), "3"))) )
class(p)    

# Create a dataframe and display default rownames
( p.df <- data.frame( ID=1:length(p)) ) 
rownames(p.df)

# Try to coerce to SpatialPolygonsDataFrame (will throw error)
p <- SpatialPolygonsDataFrame(p, p.df) 

# Extract polygon ID's
( pid <- sapply(slot(p, "polygons"), function(x) slot(x, "ID")) )

# Create dataframe with correct rownames
( p.df <- data.frame( ID=1:length(p), row.names = pid) )    

# Try coersion again and check class
 p <- SpatialPolygonsDataFrame(p, p.df)
 class(p) 

# Now we can add a column
p@data$ratings <- 1:2 

# Or modify an existing one
p[p$ID < 2 ,] <- 5

Answer 2

Try:

#Code taken from the question:
s_po <- SpatialPolygonsDataFrame(bc, buf_df, match.ID = F) 

match.ID avoids the requirement of rownames for match polygons ID

Answer 3

It is quite simple:

library("rgdal")
polygons <- readOGR('path_to/file.shp',
                      layer = 'file')
class(polygons)
>[1] "SpatialPolygonsDataFrame"
>attr(,"package")
>[1] "sp"

poly_df <- as.data.frame(polygons)
# do some staff with "poly_df" that doesn't support SpatialPolygonsDataFrame
# then convert it to SPDF back again
s_poly <- SpatialPolygonsDataFrame(polygons, poly_df)
# add new column to SPDF:
s_poly$new_column <- "some data" 

When the Error: "row.names of data and Polygons IDs do not match" arise this solution seems to be helpful: rename IDs of the dataframe to match IDs of the polygons:

newdata <- data.frame(whatever you want in here)
row.names(newdata) <- (however the new polygons are labeled)
polygons <- SpatialPolygonsDataFrame(polygons, newdata)

Answer 4

I find the following solution generally works.

First, create an empty dataframe with ID as field:

df <- data.frame(ID=character(), stringsAsFactors=FALSE )

Then get the IDs of spatial polygon bc:

for (i in bc@polygons ) { df <- rbind(df, data.frame(ID=i@ID, stringsAsFactors=FALSE))  }
# and set rowname=ID
row.names(df) <- df$ID

Then use df as the second argument to the spatial dataframe conversion function:

spatial_df <- SpatialPolygonsDataFrame(bc, df)

As df and spatial_df are dataframe objects, columns can be easily added

Answer 5

I used this solution from Roger B. https://stat.ethz.ch/pipermail/r-help/2005-December/085175.html

yourDF <- as.data.frame(yourPolygon)

set rowname=ID from polygon

row.names(yourDF) <- sapply(slot(yourPolygon, "polygons"), function(x) slot(x, "ID"))

Then creates back your Spatial Data Frame with:

yourNewPolygon<- SpatialPolygonsDataFrame(yourPolygon, yourDF)

Test any operation on it like

yourNewPolygon<- elide(yourNewPolygon, rotate=-50)