On some of the gps and 2d systems I've used they have an optional laser sensor on the machine. This allows the system to utilize the laser as a baseline or bench... as the sensor crosses the laser the system uses it to stay accurate if a satellite isnt available or there is tree cover. Its not used to grade by and you dont have to maintain the level with the laser, It just maintains accuracy of the system by continuously benching it in as you cross the laser then your machine based sensors do their thing. Alot of us have lasers already, if a delta with a laser sensor were added to the machine it could help keep the sensors benched in with perfect accuracy.