Pressure Altitude and GPS Altitude – How Do They Differ?

The Deep and Steep Insight and Stratos altimeters use two separate sensors for altitude readings: a pressure sensor and a GPS receiver. The pressure sensor is used to calculate the altitude shown on your display. This is for consistency with other altimeters on the market. The GPS receiver records GPS derived altitudes in your tracks that are viewed in the app. There are subtle differences between the two altitudes and it is important to know how the differ when comparing data on your skydive.

Altitude Difference Between Pressure and GPS #

The pressure altitude displayed on your altimeter will difference from the GPS track for your skydive. This is mostly due to how pressure changes with the temperature and weather systems in your area. The difference between pressure and GPS altitude will generally increase as your altitude above the ground increases. The table below gives a very rough idea of how the altitudes can differ.

Why the Two Never Match – Technical Details #

If you want to get into the technical details of how these altitudes work, read on…

Pressure Altitude – Standard Air and Real Air #

Pressure altitude is based on the air pressure, and the air pressure changes as you ascend in the airplane or descend on your skdyve. The altimeter converts that air pressure into a height using a fixed, standard atmosphere — the aviation model that assumes 15°C at sea level and a set rate of pressure drop as you climb. However in the real world air is almost never standard and therefor doesn’t fit the model perfectly.

Warm air is less dense, so on a hot day you are physically higher than the pressure suggests. Cold air is more dense, so on a cold day you are lower. The aviation rule of thumb is roughly 4% of your height for every 10°C (18°F) the air is away from standard — about 500 feet at a 13,000 foot exit. Pilots measuring both on warm days have logged GPS altitudes 5 to 8 percent above the pressure altitude.

This difference grows with height. Near the ground it is usually very small, which is why the two agree well under canopy and disagree most at altitude.

GPS Altitude #

GPS Altitude is calculated as a height above a fixed reference called Mean Sea Level (MSL). The altitude accuracy of the GPS receiver is based mainly on how many satellites it can see. With a clear view of the sky and many satellites in view, the GPS can determine altitude within a few meters. GPS altitude is not affected by changes in temperature, weather, burbles, etc… So it is a more consistent altitude throughout the entire skydive.

You do not have to guess how good a given sample was. Every row of the GPS track carries its own vAcc figure — the receiver’s own estimate, in meters, of how good that sample’s altitude is — along with the number of satellites it used. See Understanding GPS Track Data for the full field list, and GPS Signal Best Practices for how to give the GPS the best chance of a clean fix.

Track View in the App #

Knowing those two causes, here is what to expect when you review the GPS tracks:

• The app calculates and displays altitude AGL for the GPS track. This means your landing should end at roughly 0 feet AGL.
• Expect a difference between the GPS track view altitude and a video of your altimeter — up to a few hundred feet at exit. It closes as you come down, and it is smallest under canopy.

Which Altitude to Fly #