That Vitruvius must have been well practised in surveying is shown by his descriptions of surveying instruments, especially the water level or chorobates, which he compared favourably with the groma, a device using plumb lines. They were essential in all building operations, but especially in aqueduct construction, where a uniform gradient was important to provision of a regular supply of water without damage to the walls of the channel. He described the hodometer, in essence a device for automatically measuring distances along roads, a machine essential for developing accurate itineraries, such as the Peutinger Table.