(I'm not sure if this is an appropriate question for TCS.SE, but I'm not sure what is more appropriate).
When using Baker's Technique to find an approximation to the minimum dominating set of a planar graph, why must we overlap 2 BFS layers in every partition? I understand that for vertex cover each partition needs to overlap 1 layer to handle cases where an edge in the overlapped layer can only be covered by another vertex in the layer.
However, to my knowledge, vertex cover is just a reduction of dominating set, and any vertex cover is also a dominating set. Why do we need to overlap 2 layers for dominating set?
Of course, links to papers are great too--I just haven't found anything explaining the reason yet.
EDIT: David has a great answer below. There's also a paper describing the issue (with pictures) and a correction to Baker's original algorithm for Dominating Set here: http://www.mdpi.com/1999-4893/6/1/43 (PDF link)