Extract borders between SpatialPolygonsDataFrame units.
a SpatialPolygonsDataFrame. This SpatialPolygonsDataFrame has to be projected (planar coordinates).
identifier field in spdf, default to the first column of the spdf data frame. (optional)
A SpatialLinesDataFrame of borders is returned. This object has three id fields: id, id1 and id2. id1 and id2 are ids of units that neighbour a border; id is the concatenation of id1 and id2 (with "_" as separator).
getBorders and getOuterBorders can be combined with rbind.
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