

the same effect as iteratively unioning all individual polygons together). Unioning a set of overlapping polygons has the effect of merging the areas (i.e. The former corresponds to rgeos::gUnaryUnion, the latter to rgeos::gUnion.

If y is provided, all elements of x and y are unioned, pairwise (and by_feature is ignored). This can for instance be used to resolve internal boundaries after polygons were combined using st_combine. If by_feature is TRUE each feature geometry is unioned. If st_union is called with a single argument, x, (with y missing) and by_feature is FALSE all geometries are unioned together and an sfg or single-geometry sfc object is returned. St_combine combines geometries without resolving borders, using c.sfg (analogous to c for ordinary vectors).
