I've calculated Thiessen polygons based on points (restaurants) in CA. (I'm hoping to calculate restaurant catchment areas.) Using the US Base map (from esri) I'd like to clip those polygons so that the polygons do not extend outside of CA. This would produce an accurate catchment area. When I've tried this two problems arise: 1) The polygons at the coast are indeed clipped so that they do no extend into the water area (Yay!), but those polygons at the state boundaries (with other states) do not stop at those bounds. These polygons then extend into NV, AZ and OR. 2) The polygons close to inland bodies of water (e.g. SF Bay) are clipped completely, leaving me with missing polygons altogether.
It seems the polygon definitions for the state boundaries do not work. I've tried the same with the county boundaries only to find the same problems.