During the landing flare, the cockpit can sometimes be twenty/thirty feet or more above the rear landing gear, especially with larger aircraft.
(source: valkaviation.com)
In some aircraft, you can hear the system call out altitude when on final, ie "500... 200... 100... 50... etc".
Where does the altimeter measure altitude from? Does it measure from the bottom of the rear landing gear, or from the cockpit itself? How do pilots know when the rear landing gear will touch down?
There are a few types of altimeters but they all generally do the same thing and can be calibrated to anything if need be. Where they measure from and what they consider 0 are really 2 different questions.
A pressure altimeter reads from the pito-static port (many planes have more than one of these for redundancy) and they are generally located forward on the air frame and often on the side somewhere.
((737 static port) source)
Radar Altimeters will most likely mount to the bottom of the aircraft (since that is the way the radio beam needs to go).
(source)
According to this discussion radar altimeters are on the belly and 0ft is calibrated to mains on the ground, after that altitudes will be read out in negative values.
The callouts (50, 40,30,20,10 feet, etc) is the function of two radio altimeters which provide the height of the main landing gear above the runway.
The altitude callout is usually given using the radio altimeter. The radio altimeter (Tx/Rx) is usually located in the forward fuselage and calibrated to tell the altitude of the main landing gear above the ground.
Location of Boeing 757 radio altimeter; image from flightdatacommunity.com
This patent gives details of a system used for calibration of the radio altimeter. From the document:
To ensure that a radio altimeter indicates zero altitude when the aircraft is on the ground, an on-ground calibration system utilizes a calibration switch to initiate a calibration radio altimeter signal from the radio altimeter's transmitter section which is reflected off of the ground and received by the altimeter's receiver section. The time delay between transmission and reception of the signal is measured and a value representing the delay is stored in the altimeter's microprocessor. This value corresponds to zero altitude and can be used as the relative zero point for calculating altitudes of the aircraft when it is in flight.
This gives the basics of operation of the altimeter. You can set the MLG altitude as the zero point for landing (of course once the aircraft is in hangar and the landing gear is compressed, it will show negative values).
