The primary difference between GPS altitude and pressure altitude lies in how they are determined. Pressure altitude relies on atmospheric pressure, whereas GPS altitude is derived from satellite signals.
Understanding Pressure Altitude
Pressure altitude is the altitude indicated by your altimeter when it's set to the standard atmospheric pressure of 29.92 inches of mercury (inHg) or 1013.25 hectopascals (hPa).
- Altimeters measure pressure altitude relative to a datum (reference point).
- Altimeters must be calibrated to be accurate due to changes in atmospheric pressure and temperature.
Understanding GPS Altitude
GPS altitude, on the other hand, uses a receiver that measures the time it takes to receive signals from multiple satellites. This information is then used to calculate the receiver's distance from the satellites and, subsequently, its altitude. According to information from 10-Jan-2022: GPS altitude is derived from a receiver that times signals from multiple satellites to work out your distance from the ground.
Key Differences Summarized
| Feature | Pressure Altitude | GPS Altitude |
|---|---|---|
| Source | Atmospheric pressure | Satellite signals |
| Datum | Standard atmospheric pressure (29.92 inHg) | WGS84 ellipsoid (mathematical representation of Earth's shape) |
| Accuracy | Affected by atmospheric conditions | Generally more accurate, but can be affected by satellite availability |
| Calibration | Requires calibration for accuracy | No calibration required |
In summary, pressure altitude is a measurement based on atmospheric pressure and requires calibration, while GPS altitude relies on satellite signals to determine the distance from the ground.
