When talking about altitude we primarily focus on the altimeter. Whether digital or analog, the altimeter displays a number that provides important information—our altitude above mean sea level (msl).
The accuracy of the analog altimeter depends on pilots setting the correct atmospheric pressure in the Kollsman window using the small knob protruding from the instrument. For example, if the ATIS broadcast or our weather briefing tells us the local pressure is 29.96 inches of mercury, we have to dial in that pressure reading. The altitude indicated on the instrument should now be within 75 feet of our actual height msl. It is also important to check weather en route and adjust the reading in the Kollsman window to reflect atmospheric pressure for our current location.
Remember, the indicated altitude read from the altimeter is not an indication of our height above the parking lot below us, or the radio tower, or the mountain that is angling upward to ever more impressive heights. The altimeter can only tell us our altitude above msl and that can lead to some mental math problems that are critical to our safety of flight.
For those who fly over flat areas of the country, like Florida, the msl and above ground level (agl) figures are often 200 feet or less apart. On the other hand, in mountainous areas like Colorado the difference between msl and agl can be thousands of feet.
This means two different aircraft, one flying over Miami and the other near Denver, both of which are entering a traffic pattern established at 1,000 feet agl, will see very different indications on their altimeters. For example, an aircraft that enters the traffic pattern at Miami Executive Airport at 1,000 feet agl will be flying at 1,010 feet msl because the airport elevation is 10 feet. An aircraft entering Colorado’s Centennial Airport traffic pattern at 1,000 feet agl will be flying at 6,885 feet msl because the airport elevation is 5,885 feet.
With practice and a commitment to remember that the altimeter does not know whether you’re over the ocean or the mountains, you will understand indicated altitude better and use that information to keep yourself safe.