Skydiving altimeters use pressure sensors to calculate your altitude above the ground. Every pressure altimeter approximates its height above the ground, and the approximation differs between brands, between models, and even between two units of the same model.
Accuracy and Differing Altitudes Between Brands #
At times we get asked the question: is my altimeter accurate? The answer: It is as accurate as a pressure sensor altimeter can be — the sensors we use have a precision of 1 meter or better. But the true accuracy of an altimeter depends on much more than how precise its sensor is, which is why your Deep and Steep altimeter will not always agree with another altimeter on the airplane. Let’s cover some of the reasons for altitude display differences.
Software Differences #
Every digital altimeter is a small computer taking continuous pressure readings to calculate an altitude above ground level (AGL). The most direct way to do that is to save the air pressure value at takeoff and compare it against air pressure readings during the skydive.
Manufacturers also apply their own corrections on top of that — adjusting the display altitude based on your vertivcal speed, canopy vs. freefall, burbles, and so on. Altimeters from different brands can show different altitudes on the same skydive purely because of the software they run.
Saved Pressure on Takeoff #
Air pressure changes throughout the day. While on the ground, every altimeter is updating it’s ground-reference pressure every few minutes. At takeoff that value is locked in as the ground reference for the skydive.
Different altimeters may lock-in the ground pressure at slightly different moments — just before takeoff, or partway down the runway — so each one can end up with a ground reference that differs from the next by a few tens of feet.
| Source of the difference | How much |
|---|---|
| Software corrections | Generally under 50 feet, but as much as 200 feet in extreme freefall cases |
| Ground pressure saved at takeoff | 20 to 40 feet, even between two altimeters from the same manufacturer |
Comparing Accuracy #
So which altimeter is correct? To be honest – None of them.
We know because we build altimeters that also carry GPS, and GPS data is highly accurate and unaffected by burbles and the other problems that disturb a pressure sensor. Testing our own GPS data against third-party data from the high-end GPS units found in aircraft makes it easy to see that pressure sensor based altitude is a rough indication of your height above the ground. We tested other major brand of skydiving altimeter to confirm this is true of anything on the market today.
None of the altimeters on the plane are telling you a true distance from the ground. They are all approximations that can change with altitude, speed, burbles, air temperature and the weather. A pressure altimeter can be within 20 or 30 feet of the truth, and it can also be incorrect by several hundred feet. At higher altitudes (for instance above 6,000 feet) they will tend to be farther away from the true altitude.
Safely Transitioning to a New Altimeter #
Warning
During canopy flight a new altimeter or audible may read 20 to 50 feet higher or lower than your previous altimeter. Until you have re-learned the sight picture, increase your canopy setup altitudes — especially for swoops or any aggressive turn close to the ground. Always use visual cues instead of relying soley on an altimeter.
When changing altimeter models or manufacturers it is important to fly conservatively. Raise your setup altitudes slightly and use the same process of evaluation and sight picture you used to learn your previous canopy altitudes.
After a few skydives you will know what altitude is correct for your canopy on the new equipment.
