Thanks Bryco. I have thought of a ray method. First, find the boundingbox for all geometry on the layer, next, scan this boundingbox area with a horizontal/vertical line or ray and use intersectwith to find out where the geometry borders are. It is surely possible to use that method, but it's SLOW. And confusing, when the scan ray hits an edge of a entity and thus returns only one hit (or an odd number of hits).