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Exercise

Converting from one CRS/projection to another

The process of converting from one CRS or projection to another is handled by the spTransform() methods in the rgdal package. spTransform() has methods for all sp objects including SpatialPolygonsDataFrame, but doesn't work on raster objects. This is because transforming a raster is a little more complicated; the transformed rectangular grid will no longer be rectangular. You can look at ?raster::projectRaster if you are curious about transforming rasters.

Transformation is simple. The first argument to spTransform(), x, is the spatial object to be transformed and the second, CRS, is a specification of the desired CRS. The CRS can be specified by a PROJ4 string, which you could construct by hand, but it's much easier to take it from an existing object (e.g. with the proj4string() function).

Time to get your two polygons datasets into the same CRS.

Instructions

100 XP
  • Transform neighborhoods to have the same CRS as nyc_tracts by using spTransform() with the CRS argument set to proj4string(nyc_tracts).
  • Verify the transformation by looking at the head() of coordinates(neighborhoods).
  • Check the datasets now line up by plotting neighborhoods, then plotting nyc_tracts with add = TRUE and col = "red", and finally plotting water with add = TRUE and col = "blue".