How to attach a simple data.frame to a SpatialPolygonDataFrame in R?

Question

I have (again) a problem with combining data frames in R. But this time, one is a SpatialPolygonDataFrame (SPDF) and the other one is usual data.frame (DF). The SPDF has around 1000 rows the DF only 400. Both have a common column, QDGC

Now, I tried

oo <- merge(SPDF,DF, by="QDGC", all=T) 

but this only results in a normal data.frame, not a spatial polygon data frame any more. I read somewhere else, that this does not work, but I did not understand what to do in such a case (has to do something with the ID columns, merge uses)

oooh such a hard question, I quess...

Thanks! Jens

Answer 1

Let df = data frame, sp = spatial polygon object and by = name or column number of common column. You can then merge the data frame into the sp object using the following line of code

sp@data = data.frame(sp@data, df[match(sp@data[,by], df[,by]),]) 

Here is how the code works. The match function inside aligns the columns so that order is preserved. So when we merge it with sp@data, order is correctly preserved. A quick check to see if the code has worked is to inspect the two columns corresponding to the common column and see if they are identical (the common columns get duplicated and it is easy to remove the copy, but i keep it as it is a good check)