So basically for the side bcs I need to make the map values less than zero for all the values in the extruded column. This means substracting a big enough value so that when we move up the column and add the offsets the value stays negative. This clashes with the operation that substracts -maxint from the bottom/top bcs. This is why I've chose two different small enough numbers so that the substract operations will not lead to an underflow error. I realise this is not as robust as it could be.