I'm working on a project where I'm looking at overlap in users circling an area in an image. Esentially, I make a SpatialPolygonsDataFrame where each user gets one row. Some users don't select anything. So, I'm trying to figure out how to include that information. When I attempt to create a "NULL" polygon, and then scale it up, either with something like
Polygon(matrix(rep(NA, 4), nrow=2)
or anysuch, none of these work.
1) Can I create an empty polygon? 2) or add a row to a SpatialPolygonsDataFrame with no matching SpatialPolygons object?