Geometric vs. Barometric Altitude
There are two main ways of determining altitude: geometric measurement and barometric measurement.
Geometric altitude measurement is typically performed using GPS, although there are other methods which would give the same result. A geometric altitude measurement tells you exactly how far you are from the ground. This is the measurement you would expect to get if you had a very long ruler. In practice, making geometric altitude measurements requires specialized electronic equipment like a GPS.
As we increase our altitude, atmospheric pressure decreases. We can plot this change as follows:
Conventional barometric altimeters make use of this relationship. First, they measure the air pressure. Then, they convert this measurement to an altitude using a model similar to the one we see above. For example, if the air pressure is 69 kPa, the altitude would be about 10,000 ft above sea level.
Differences Between Geometric and Barometric Altitude
Barometric altitude measurement relies on a "standard" atmospheric model. However, in practice there are many factors which affect atmospheric pressure. As skydivers, we see this all the time when we zero our altimeter in the morning, but by the afternoon it is a few hundred feet off. Because of this, a barometric altitude measurement rarely agrees perfectly with a geometric measurement. The following article discusses these differences in detail, and includes some real-world measurements which tell us how big the difference can be:
The bottom line is that geometric and barometric measurements can commonly differ by 5-10%. For skydivers, this means that at a geometric altitude of 10,000 feet, your barometric altimeter might read 9,500 feet. That is, when your FlySight tells you you're at 10,000 feet, you may look at your altimeter and see an altitude of 9,500 feet. This is perfectly normal.