So Many Altitudes

I recently overheard a couple of pilots talking about various kinds of altitude. “How many altitudes are there?” one asked. I don’t recall the details of any answer he got but, alas, the answer is that there are many. If you use the wrong one, you might get into an uncomfortable situation.

Aviation Safety has discussed altimetry errors in numerous articles over the years. See November 2015’s “Your Altimeter Is Lying” for an example. Meanwhile, let’s stick to the more conceptual errors.

Why We Need Altitude…

It should be obvious why we need altitude: We don’t want to hit anything, whether terrain or other aircraft. The real issue is how to measure altitude. And we’re not very good at it. That’s not because we aren’t clever enough to do it, it’s because altitude is, at best, a fuzzy concept.

In the bad old days—there are no good old days in aviation, no matter what old guys like me might claim—there really wasn’t any need to measure altitude. Flying VFR (or, in the old days, CFR, for contact flying rules) looking out the window was the only data you needed. That’s still true today, with a big if.

For example, if you fly at night or on instruments, you can’t rely on looking out the window, so you need a vertical reference. Suppose I want to fly from my home field to the beautiful Oregon coast. There are mountains in the way. In the bad old days, we would look at a sectional chart and see a mountain peak with a charted elevation of 3424 feet near the route. At night or in IMC, that means that I need to measure my altitude to make sure I am higher. How much higher is another topic.

…And How We Measure It

The first step in measuring something is deciding where zero is; that’s part of the datum. This is not an easy choice. The elevations of one spot might differ by hundreds of feet depending on the choice of datum, and the difference can vary from day to day. The elevation of the sea itself varies from place to place and even from year to year, although these variations are usually too small to affect aviation. (One exception with which many Floridians are acutely and recently aware is the storm surge associated with tropical storms and hurricanes. — Ed.)

The ocean has tides; at the Bay of Fundy, the difference between high tide and low tide is 53 feet. So measuring by comparing height to sea level has to use an average sea level. The current standard in the U.S. is mean sea level or msl. In Canada, heights are measured above sea level, asl. The 3424-foot-high peak on my route to the coast is that many feet above msl. But where is “sea level” measured? It’s more complex than it appears at first, but the current standard is Galveston, Texas. (Galveston is at zero feet, but the local airport, KGLS, is at six feet msl.) Nailing all this down was an incredibly complex effort involving dozens of tide gauges observed for many years.

This may seem old-fashioned, but it’s the raw data that flight planning apps use when showing you a profile view of the terrain and major obstacles along your route. 

Collision Avoidance

Another reason we need to measure altitude is to avoid hitting each other. To do that, we have to agree where zero feet is. But things get fuzzy again, because altimeters measure air pressure. And they do it backward: Altimeters measure how much air is above you, exerting less and less pressure as you climb, while what you really care about is how much air is below you. Air pressure varies as you move across the surface of the Earth, which is why you have to adjust your altimeter setting on a long cross-country. (Unless you are above the transition altitude, where all altimeters are set to 29.92 in. Hg. In the U.S. and depending on ambient pressure, that’s at FL180, also known as the base of Class A airspace.)

We start with an altimeter set to field elevation, which was determined by the kind of detailed survey conducted at Galveston. The conversion from pressure to altitude is based on the International Standard Atmosphere, or ISA. The training literature often features a table showing the ambient pressure at various pressure altitudes. You don’t have to use it in flight: the altimeter or avionics manufacturer “knows” that at, say, 19 inches of pressure, the device should indicate just below 12,000 feet msl.

But here’s a catch: ISA also models a temperature for each altitude. Standard sea level temperature is 15 degrees C, and the temperature decreases about two degrees C for each 1000 feet of climb. That’s the Standard Lapse Rate. If you’re flying along in seven-degree C air and your friend is in five-degree air, you won’t collide, unless there is a weather front between you. That won’t work in the higher atmosphere of the troposphere, where temperature does not vary with altitude. No one recommends using temperature for terrain avoidance, because temperature and lapse rate are far more variable than pressure.

The variability of lapse rates is also the basis of Cold Temperature Altimeter errors. This was discussed in a March 2022 Aviation Safety article’s sidebar. When temperature is colder than standard, the barometric altimeter reads higher than it should. “High to low, look out below” applies to temperature as well as altimeter setting. The correction table, which should be applied to segments of instrument approaches, is in section 7-3 of the Aeronautical Information Manual.

For example, if the temperature at the altimeter source is -20 C, an altimeter at 5000 feet agl reads 710 feet higher than it should. That can put you uncomfortably close to terrain, because intermediate segments are only 1000 feet agl, even in mountainous terrain.

Radar Altimetry

Another source of altitude data is a radar (or radio) altimeter. It emits radar energy straight down, giving the pilot (and autopilot) very accurate information about how high the aircraft is above the ground. If you have one, you can generally program an alarm to go off when you reach MDA or DA on an approach. They’re less useful at higher altitudes; most only read up to 2000 or 2500 feet. This altitude is independent of temperature or barometric pressure.

Many autopilots use radar altitude as a cue to calm down the control inputs near the surface. When the radar altitude is below a certain value (e.g., 500 feet), the autopilot’s control manipulations are reduced substantially, as if listening to every CFII who has ever reminded an instrument student to use small corrections near the bottom of a precision approach. Again, this is independent of temperature or barometric pressure.

GPS

The global positioning system gives pilots another source of altitude information. This has led to a lot of confusion, because GPS altitude is based on an absolute position in space: x-, y-, and z-coordinates. A complicated mathematical transformation based on a model of the Earth’s surface converts these to latitude, longitude and altitude. This altitude can be significantly different from barometric altitude, depending on temperature and local barometric pressure; GPS altitude may or may not guarantee clearing that 3424-foot peak on my route to the coast.

On a recent flight, I compared GPS and barometric altitude during the climbout. The surface temperature at 200 feet msl was nine degrees C, a little colder than standard. The altimeter setting was 30.13, a little higher than standard. The altimeter error at the surface was 20 feet. (Remember, if the altimeter error at the surface is more than 75 feet, the altimeter is considered unreliable.) I had a friend with me who has an engineering degree and was thrilled to help record the data.

The first thing to notice is that the GPS altitude was consistently lower than the barometric altitude; on a warm day the GPS altitude would be higher. The observed lapse rate, the change in temperature with altitude, was pretty close to minus two degrees per 1000 feet. The outside air temperature gauge was hard to read, so there may have been some error in the temperature data.

The worst error was 178 feet at 6000 feet agl. There were some scattered cumulus clouds whose tops were around 4000 feet msl, so once above them we were in a different airmass.

The Bottom Line

The various ways of measuring altitude each have advantages and disadvantages. It’s good to know how they differ, especially if you find yourself depending on one of the non-standard measures.