Question: I am not a pilot and have come to this site to ask a question about the measurement of height. This is as a result of the comments made as a result of a question in the Physics Stack Exchange forum ["Does barometer improve gps accuracy?"](https://physics.stackexchange.com/questions/261185/does-barometer-improve-gps-position-accuracy/261240#261240) 

I feel that there will be more of you on this site who have real time experience with flying and so can answer my question which is:

*Which is more accurate for height measurement: GPS or barometer?*

I have noted that those engaged in geocaching often have GPS units with built-in barometers to improve height accuracy.

One of my friends who is a pilot responded as follows:

> 
> A light aircraft relies on a barometer that is set to the standard
>  1013 millibars when in cruise at height (or the regional QNH when
>  lower) but they set to the local reading for the airfield (QFE) when
>  planning their decent. Commercial aircraft also have ground proximity
>  radar but they also have a standard set of old fashioned flight
>  instruments in case the electronics fail. My old aircraft GPS did
>  indeed read out height as well but you were told never to rely on it!
> 
> 
>

Answer: 
This depends on the accuracy of the barometer and the weather, since the weather influences the air pressure. So even if you're standing still at the same altitude the barometer will show you different readings. This is why a pilot constantly has to adjust his altimeter. An altimeter is designed to measure the altitude above a fixed level. This is why pilots have to calibrate their altimeters before take-off with ATC.

The GPS system, on the other hand, works with an accurate clock and the known positions of the satellites. The satellite sends out packages which are time stamped. The receiver receives these time-stamped packages and uses its own calibrated clock to determine the sequence. Comparing this to the know positions of the satellite at certain positions allow the use of triangulation formulas to solve for the position of the receiver.

The GPS system is owned by the US military and they have this graph about the accuracy improvements over the years.
[![source: http://www.gps.gov/systems/gps/performance/accuracy/URE.pdf](https://i.stack.imgur.com/EwrZD.png)](https://i.stack.imgur.com/EwrZD.png)

So to answer your question, it is determined by the equipment you use, but cost wise it is cheaper to get an accurate reading from a GPS since it is also less influenced by the weather.
To amend already existing answers:

Very close to the ground, GPS altitude is unusable for aviation purposes. Vertical accuracy of GPS position is roughly 1.5 times that of the lateral position. Unaugmented GPS signal might have position of over 10 meters, giving even larger vertical error. For that reason, vertical GPS position is not used for approaches if some augmentation method is not available. Even then, GPS altitude is not used to determine approach minima, which is defined as barometric altitude.

There is also problem of momentary degradation of GPS accuracy due to satellite constellation or interference from building, terrain, weather, jamming etc. which would be unacceptable if the signal would be used for terrain clearance.

During takeoff and landing, barometric altitude is referenced to local air pressure, making it very accurate locally at low altitude. In cruise as mentioned, the accuracy does not matter as much as aircraft maintain suitable clearance from the terrain in the area, so even in case of lateral navigation error there will not be a collision.

The benefits of barometric altitude are separation from other aircraft, as any aircraft arriving from distant corners of world are using same barometric reference of 1013hPa (29.92 inHg) during cruise, and the matter of aircraft performance. Aircraft performance depends on local air pressure and temperature, so fixing the altitude to the barometric reference reduces aircraft performance to a single variable problem, ie. temperature.