The documentation of the st_union function of sf is not clear (for me!) on one point: what are the action and result of the function when applied to an sfc with a single feature, with the parameter by_feature=FALSE (default)? It seems that, according to the text of the documentation, nothing should happen and that the value returned should be the same as the one-feature sfc provided.
I ask this question because it is rather common to find proposals, advices for chaining st_union to st_combine, especially in the context of the use of st_difference (see the sf manual, at geos_binary_ops, or ?st_intersection):
the use of "st_difference(SF1,st_union(st_combine(SF2)))" is proposed.
Two questions in order to understand:
. Because st_combine returns a single-feature sfc, how could st_union have any effect on such an object?
. Is there any simple case, any example, where st_union, with by_feature=FALSE, applied to a single sfc has any effect?
Extract of the documentation of st_union: "If st_union is called with a single argument, x, and by_feature is FALSE all geometries are unioned together and an sfg or single-geometry sfc object is returned. If by_feature is TRUE each feature geometry is unioned. This can for instance be used to resolve internal boundaries after polygons were combined using st_combine."