sentences:  Generally this was at a lock or an artificially constricted part of the canal so that the boat had to pass within inches of the toll point unable to evade the toll.On canals where the fee was based on cargo weight it also put the boat in a convenient place to read the gauging mark height from the water line.On busy canals which were built with a towpath on either side such as the Birmingham Canal Navigations BCN New Main Line the toll house may have been built on an island between two constricted channels so that one toll point could collect from boats travelling in each direction.