# Does baro altitude from ADSB represent height above ground level or height above mean sea level?

I am retrieving ADSB data from Opensky-network.org. I would like to know what the baro altitude represents, is it the height above ground level or above mean sea level ?

From the FAA's Frequently Asked Questions (FAQ) regarding ADS-B (Under the "Operations" section, 6th bullet point):

Can you please explain the altitude and velocity reports that ADS-B provides?

ADS-B reports two kinds of altitudes: barometric and geometric. Barometric or pressure altitude is the one pilots know best - this is the altitude that is displayed on the altimeter in the aircraft. Geometric altitude is calculated by GPS as the height of the aircraft above the earth ellipsoid. These two altitudes are not the same, but having both allows for applications that require one or the other as an altitude source and provides a means of verifying correct pressure altitude reporting from aircraft.

(emphasis is mine)

As noted above, the Barometric altitude is "Pressure Altitude" (defined as the height above a standard datum plane (SDP), which is a theoretical level where the weight of the atmosphere is 29.92 "Hg (1,013.2 mb) as measured by a barometer.) Source: FAA's Pilot Handbook of Aeronautical Knowledge, page 4-4.

Unless the already corrected local pressure set in your altimeter is 29.92 in. Hg, in order to know what the actual (True) altitude above mean sea level is a correction factor would have to be applied to the ADS-B reported altitude based on the actual atmospheric pressure outside.

• Your answer is correct but I’m not sure it answers the question the OP is asking. Yes, ADS-B is generally uncorrected pressure altitude rather than either GPS-derived geometric altitude or “Kolsman-corrected” barometric altitude. If the OP is a non-pilot they may not be able to infer what that means in terms of settling their intellectual curiosity about whether the altitude, no matter how derived, is meant to approximate AGL (above ground level) or MSL (above mean sea level). For the OP: most altitudes related to air traffic, including ADS-B are approximations of height above sea level. Nov 7, 2022 at 2:56
• @MaxR The "approximation" is critical for accuracy and the specific application. Geometric (GPS) altitude is used to prevent planes from crashing into objects on the ground, since somebody measured the geometric altitude of things on the ground. Barometric altitude is used to prevent planes from crashing into other planes in the sky, since planes are told to fly essentially at a certain atmospheric pressure (which above transition altitude, has no fixed relation to geometric altitude, due to weather). This is why both of them are important. Nov 9, 2022 at 3:29
• Obstacle clearance is not done using geometric altimetry. No matter how a tower is surveyed, including GPS, the FAA does not collect or distribute geometric data. You provide the height of a tower to the FAA on 7460-1 in both measured AGL and derived MSL, not height above the GPS-reference earth spheroid. This MSL reference datum is the only approved datum of record for obstacle related heights, and what is used for barometric correction by NOAA that you dial into the Kolsman. Nov 9, 2022 at 5:11
• @MaxR Nope, you're saying a bunch of things on administrative paper forms, but that's not how it actually works in the real world. GPWS units are designed to use GPS/GNSS data exclusively, because it avoids altimeter setting errors, which is one major cause of CFIT. Thus, the terrain/obstacle DB is all referenced to GPS height. What is "of record" is irrelevant. Nov 11, 2022 at 21:50
• Dear @Max R, dear user71659, you are both discussing the topic of height measurement from a different perspective, and you both bring valid points to the table. However, you don't have the compete picture and mix concepts of vertical datum (ellipsoid, MSL, AGL) with measurement type (geometric, barometric, geopotential) in one discussion. This is not the place to have that discussion, consider taking it to the hangar. Please be respectful to each other and try to understand where the other is coming from. Nov 12, 2022 at 8:28

I don't know what post-processing is done by the opensky-network, but the altimetry transmitted by ADS-B from the aircraft is the pressure altitude. This is the pressure measured by a static port on the aircraft, converted to altitude according to the formulas or tables of the ICAO standard atmosphere.

The ICAO Standard Atmosphere is hydrostatic atmosphere model which assumes a sea level pressure of 1013.25 hPa, a sea level temperature of 15 degrees Celsius and a lapse rate of -6.5 degrees / 1000 meters (up to 11 km, then the temperature is constant up to 20 km) .

The pressure altitude is equivalent to the Flight Levels used in air traffic control. A flight level is equal to 100 ft of pressure altitude.

The pressure altitude is not equivalent to the height above ground. The aircraft's altimetry system has generaly no way of knowing where the ground is (radar altimetry is not standard on every aircraft and is not connected to ADS-B).

The pressure altitude is also not equivalent to the height above MSL, because the assumptions of the ICAO standard atmosphere are typically different from the actual atmospheric pressure distribution.

ADS-B does also transmit geometric height, which is GPS derived and is expressed with respect to the WGS84 ellipsoid.