Barometric altimetry is the technique that determines the altitude of a location by measuring atmospheric pressure. Since pressure decreases with altitude in a predictable way, a pressure measurement can be converted to altitude. This is a practical application of the smartphone barometer.
Discover FizziQ
How to measure it in class
With the FizziQ app, the barometer allows measurement of altitude differences.
Steps:
- Open FizziQ and select the “Barometer” instrument
- Note the pressure P1 at the base of a building
- Go to the top of the building (roof, top floor)
- Note the pressure P2 at the top
- Calculate the altitude difference: Delta h approximately equals (P1 - P2) / 0.12 (in meters if P is in hPa)
- Compare with GPS altitude or known building height
Scientific activities on this topic
- Measuring the height of a building: https://www.fizziq.org/en/activities/altimeter/
- Altitude profile of a hike
- Barometer/GPS comparison
Learn more
Measurement principle:
Atmospheric pressure decreases by approximately 12 Pa (0.12 hPa) per meter of altitude near ground level. This relationship allows conversion of a pressure difference into an altitude difference.
Precision and limitations:
| Error source | Impact |
|---|---|
| Weather variations | plus or minus several hPa |
| Temperature | Modifies air density |
| Humidity | Minor effect |
| Sensor | plus or minus 1 hPa typical (plus or minus 8 m) |
Relative vs absolute altimetry:
- Relative: altitude difference between two points measured simultaneously - very accurate
- Absolute: altitude relative to sea level - requires a reference (QNH setting)
Applications:
- Altimeter watches for hiking
- Aircraft altimeters (complement to radio altimeter)
- Indoor navigation (floor detection)
- Barometer-assisted GPS
Formula
Simplified formula (low altitude): Delta h = (P1 - P2) / (rho x g) approximately equals (P1 - P2) x 8.5
where:
- Delta h: altitude difference (m)
- P1, P2: pressures (Pa)
- rho approximately equals 1.2 kg/m cubed: air density
- g = 9.81 m/s squared: acceleration due to gravity
More precise formula: h = H x ln(P0/P) with H approximately equals 8500 m
Application examples
- A standard floor (3 m) corresponds to approximately 0.35 hPa variation
- The Eiffel Tower (330 m): Delta P approximately equals 40 hPa between ground and top
- Modern smartphones use the barometer to improve indoor GPS
- Parachutists use a barometric altimeter to trigger opening
FAQ
Q: Why is GPS not sufficient? A: GPS has a vertical accuracy of plus or minus 10-20 m outdoors and does not work indoors. The barometer can detect a floor change (3 m).
Q: How to correct for weather variations? A: For quick relative measurements (climbing a building), the weather does not have time to change. For absolute altitude, you need to know the sea level pressure (QNH).
Q: What is the realistic accuracy with FizziQ? A: In quick relative measurement: plus or minus 1-2 m. In absolute altitude: plus or minus 10-30 m depending on weather conditions.
Q: Can a smartphone detect which floor I am on? A: Yes! Modern smartphones use the barometer for indoor navigation and emergency services (vertical location).
Related concepts
Atmospheric Pressure - Barometer - Altitude - GPS - Barometric Formula - Meteorology