a method for monitoring a vehicle tool including inputting tool data indicative of the location of the vehicle tool to a data processor , inputting data to the data processor on a topography model in which the vehicle tool is to operate , operating the data processing to compare the vehicle tool location with the model , outputting from the data processor to a visual display , data indicative of the difference in location between the vehicle tool and an adjacent surface of the model , displaying on the visual display an image of the topography model , a representation of the location of the vehicle tool and a representation of the distance of the vehicle tool to the adjacent surface of the model .