I’m reading Skyfaring by Mark Vanhoenacker, and thinking about altitude.

In my home office on the second floor, my phone app indicates we are 49 feet above sea level, with a possible error of 15 feet. If you are familiar with GPS, you know that my three-dimensional position is actually computed based on my phone’s distance from three or four satellites, each of which is moving. So I’m not shocked when I take my phone on a little walk about the house, return to the same spot, and find myself at 13 feet with a possible error of 45 feet.

Meanwhile, veloroutes.org says the elevation of my house is 55 feet, but freemaptools.com/elevation-finder says it is 46.4 feet, based on the house address. I wonder what methods those sites use.

Happily for me, this is just a matter of curiosity. Not so for a commercial airplane.

The most crucial time for the flight crew members to know their plane’s altitude is when landing, so then they use radar, bouncing a signal off the ground directly below the plane. The radar on the fuselage is programmed to display the distance from the ground to the bottom of the tires hanging from the deployed landing gear. Once the plane actually lands, its weight will compress the tires, so the radar display will indicate that part of the plane is underground.

When the plane is just leaving or approaching an airport, radar might bounce off another plane or a building and give a false reading, so the plane uses an altimeter, which calculates altitude based on air pressure. Air pressure is much more capricious than water pressure, because air is much more compressible. Although our atmosphere is fifteen miles thick, half of its air is crammed into the lower 3.5 miles. Air pressure also changes over the 24-hour light cycle, and due to characteristics of both prevailing and migrating weather systems.

The altimeter has to be calibrated to local conditions on the ground before takeoff, and adjusted during the flight as conditions change. When coming in for a landing, pilots may be given a local altimeter adjustment from the controller. Sometimes they get information from each other while in the air.

During the high-altitude cruising portion of a commercial flight, planes need to be separated by a minimum vertical distance, yet since their altimeters were calibrated at wildly different departure locations, readings might disagree. So pilots switch the altimeters to standard mode when they cruise. The algorithm results may vary from reality by hundreds of feet, but the relative readings among all planes will match. Planes cruise at tens of thousands of feet, so a few hundred won’t put any plane in danger of hitting anything on the ground.

What if a pilot forgets to switch over?

Mountains show up on pilots’ maps as Not Airspace. Navigating mountains combines altitude and location, and though planes use GPS now, it’s not the ultimate solution you might imagine. Beyond the scope of this blog.

When you see your altitude on the seatback screen, it may not be exactly right. In fact, even the pilot rarely knows exactly how high the plane is at any given moment. Earth is both bumpy and not-quite-spherical, making altitude is a moving target.

Should a simple question have a simple answer? Perhaps there are no simple questions. Certainly life seems devoid of absolutes.