I answered this also in the Questions section.
The Radar Altitude value is removed from the display at 1500 feet when ascending and is again displayed at 1400 feet when descending. If the Flight Path sysmbol is not displayed, the Radar Altitude is displayed relative to the Aircraft Reference sysmbol.
The digital value is displayed in flight in ten foot increments between 50 and 1500 feet, five foot increments between 10 and 50 feet and one foot increments between -20 and 10 feet. Radar Altitude is not displayed on the ground.