Yes, lasers. Even with the higher accuracy and more expensive real time dGPS systems the vertical accuracy isn't as good as horizontal. When you're looking for a very shallow gradient in a field for irrigation GPS just isn't as good as a laser level.
Much of the problem is geometry, while you're likely to have a good spread of satellites around your receiver horizontally they're all on one side of the vertical solution your receiver is calculating.
3 sats are needed for a two point fix, 4 for a one point fix, and 5+ help with accuracy if they're not clumped near each other (the constellation's always moving). At higher altitudes such as in a plane at FL 350 with sats below the receiver, vertical accuracy increases some compared to ground, which makes these obvious: http://www.gps.gov/applications/space/
Much of the problem is geometry, while you're likely to have a good spread of satellites around your receiver horizontally they're all on one side of the vertical solution your receiver is calculating.