Points to note:

The altimeter set to the correct, and relevant, barometric pressure should be used for legal separation and airspace compliance, i.e. the barometric altimeter is the only legal primary reference for altitude.

Altitude reporting from a transponder is a Flight Level (FL) based on 1013.25 hPa (29.92 inHg). This will be the source that triggers an infringement report.

GPS derived altitude should be regarded as advisory only.

Barometric settings:

Altitude: Elevation above mean sea level (AMSL) as set on an altimeter set to the relevant Aerodrome QNH, or, in the UK, the Regional Pressure Setting (RPS), where RPS is the “lowest forecast QNH within a designated altimeter setting region, and is used as an altitude pressure datum for Air Systems flying at or below the TA (Transition Altitude), away from aerodrome circuit and approach patterns.

Height: Height above a reference point based on the atmospheric pressure a that point, typically an Aerodrome reference point – QFE. When an altimeter is set to QFE it should read zero when the aircraft is at the reference point.

Flight Level (FL): The altimeter is set to the standard sea level pressure setting of 1013.25 hPa (29.92 in Hg) and is sued to report flight level when above the Transition Altitude (TA).

Altitude vs Height vs Elevation:

Altitude is vertical distance above mean sea level (AMSL) as indicated on a barometric altimeter set to QNH.

Height is the vertical distance above a specified reference point, usually a point on an aerodrome.

Elevation is the height above mean sea level (AMSL)of a fixed point on the ground.

GPS Altitude:

GPS altitude is based on satellites (GNSS). It can be Height above the ellipsoid (mathematical Earth shape), or AMSL if the receiving device can calculate a geoid correction.

GPS systems measure geometric height above the WGS‑84 ellipsoid. To get sea‑level altitude (MSL), it applies H = h − N (ellipsoid height minus geoid height). Around the world, the ellipsoid can sit about −100 m to +70 m from the geoid (your device or app may or may not correct this).

Error Sources or Common Mistakes:

Altimeter:

Setting Error – 1 hPa = 27 to 30 Ft.

Temperature (non‑ISA): Colder than ISA = you are lower than indicated over terrain; hotter = higher.

Instrument/position error: Static source and installation errors.

Wrong setting: QFE set instead of QNH, Regional Pressure set instead of QNH.

GPS:

Vertical geometry: Vertical GPS accuracy is worse than horizontal.

GPS Receiver\Aerial Position: Canopy/wing/fuselage/structure blocking the sky and signal reflections (multipath) add error. Potential error on receivers:

Good sky view: 30 to 100ft

Poor satellite position/blockage: intermittent 100 to 300+ ft

Altitude relative to other traffic (EC Caveat) – source https://support.easyvfr4.aero)

The reliability of “relative altitude” between you and another target depends entirely on the chain of sources and conversions used. Many Electronic Conspicuity (EC) systems pass on information from other sources, convert barometric to GPS and vice versa, and in the process the original altitude reference is in most cases lost. Two identical systems that share the same conventions tend to match well; mixed or unknown chains can accumulate errors into the hundreds of feet.

Best‑case scenarios

FLARM ↔ FLARM: Usually consistent because both sides use the same processing and altitude reference handling.

Partner‑app traffic (EasyVFR with SafeSky ↔ EasyVFR with SafeSky): Both systems treat each aircraft and traffic GPS altitudes as AMSL (geoid) for consistency, using the same conversion rules. This makes those targets more consistent with each other.

Mixed/unknown chains (caution)

ADS‑B/Mode S barometric altitude fused into an app that prefers GPS‑MSL.

Network rebroadcast converting between barometric and GPS altitudes.

Devices estimating barometric altitude from GPS or vice versa.

Result: The displayed relative altitude may include combined errors (your source + target source + conversions). The vertical margin needed can easily be hundreds of feet. Knowing “something is nearby” is still very helpful — but always scan above and below the horizon rather than staring exactly where the app points.

Operational guidance

Treat EC relative altitude as advisory unless you know both ends use the same altitude reference consistently.

Maintain see‑and‑avoid and legal altimetry; EC enhances awareness but is not a separation guarantee.