Final answer:
Altimeters must be checked for accuracy against a known elevation and each other in different modes. They are crucial for aviation safety, navigation, and maintaining separation from terrain and other aircraft. The specific standard of accuracy can vary but is important for flight operations at any altitude.
Step-by-step explanation:
Altimeters are precision instruments used in aviation to measure an aircraft's altitude above sea level. For a known elevation, altimeters must be accurate to ensure the safety and proper navigation of the aircraft. The question likely refers to a regulatory or operational standard that specifies the accuracy within which altimeters must check when compared to a known elevation or relative to each other in different modes such as RESET or STANDBY.
Typically, altimeters should be checked for accuracy both on the ground (at a known elevation) and against another altimeter to confirm their readings are consistent. The standard of accuracy required can vary based on the elevation of the airport or the altitude at which the aircraft is flying. Without the specific value provided in the question, we can refer to standard measurement elevations like 10,000 ft. (3,000 m), 5,000 ft. (1,500 m), 2,000 ft. (600 m), 1,000 ft. (300 m), sea level, and below sea level, which are commonly used references for altitude verification checks.
Furthermore, when discussing altimeters, pilots and mechanics might also reference navigational charts which include data points such as national boundaries and mountain peaks to evaluate the performance of these instruments in various geographic contexts. Ensuring that altimeters are accurately calibrated is vital for maintaining altitude separation from terrain and other aircraft, particularly when flying in mountainous areas.