Add elevation data to a previously-extracted OSM data set, using a
pre-downloaded global elevation file from
http://srtm.csi.cgiar.org/srtmdata. Currently only works for SC-class
objects returned from osmdata_sc.
osm_elevation(dat, elev_file)An SC object produced by osmdata_sc.
A vector of one or more character strings specifying paths to
.tif files containing global elevation data.
A modified version of the input dat with an additional z_ column
appended to the vertices.