Extracts the the vector of unique polygon identifiers from
SFDF.
This function is useful when writing code to aggregate data by polygon and
calculate per-polygon statistics. Each unique simple features data frame will
have a different set of data columns but each is guaranteed to have a column
named polygonID that uniquely identifies each polygon.
This allows us to write code that aggregates by polygon without having to know whether the polygons represent, countries, timezones or HUCs, etc.
getPolygonID(SFDF)Spatial polygons dataset of interest.
Vector of polygon identifiers.