Hi. The Radar Altimeter will give you the height of the lowest wheels (Mainwheels) from the ground +/- 2 feet at 50 feet. If you have rising ground just before the runway threshold, then it might be possible that the flight deck would be lower than 50 feet as the Wheels pass 50RA, but unlikely this close to touchdown, especially as the RA is an important part of an autoland system. Have a good read of your FCOM.