The sp package is the package when it comes to handling spatial data in R. Getting to know how this package work can take some time, but it's well worth it. It can be a pain to import spatial data from for instance www.opentreetmap.org.
The other day for instance, I required an outline of Lake IJssel. How hard could it be to get that from openstreetmap? Well, getting the data is not all that hard, but converting it into something workable really got me pulling my hair out. In order to save future frustrations, I decided to share my solution with you.
The first step is to get the relation id number from openstreetmap. This can be easily obtained by searching the website for ‘Lake IJssel’. After clicking the first hit that represents a water body, you will find the relation id.
The second step is to download the data in R using this relation id. This can be done with the get_osm function of the osmar package. This data can be converted into a SpatialLines object using the as_sp function from the osmar package. However, I don't want a SpatialLines object; I want it to be a SpatialPolygons object. And that is the tricky bit, because there is no package available (that I know of) that can do this directly.
So this is the trick that I applied. First you need to join the lines in the SpatialLines object, using the gLineMerge function from the rgeos package (it took me a while to figure this one out!). Then convert it into a PolySet using the SpatialLines2PolySet function from the maptools package. Only then can it be converted into a SpatialPolygons object using the PolySet2SpatialPolygons function from the maptools package. If anyone knows of an easier way, pleas let me know in the comments.
So, there you have it. A SpatialPolygons object of Lake IJssel, that is actually useful for my purpose. The code below will do what I've described in this post; feel free to try it yourself! Note that it may not work on all openstreetmap data...
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