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: 
For surveying glacier movement, or a mountain range's growth/decay, GPS is more accurate.

For airplanes: refresh rate, reliability, and simplicity matter more. When it comes down to accuracy, ground augmented GPS provides ILS CAT I accuracy using [LPV](https://en.wikipedia.org/wiki/Localizer_performance_with_vertical_guidance). So alongside refresh rate, and accuracy, comes the question of application. For airplanes in cruise, where separation matters, reliable devices are best.

GPS signals can be jammed. As can be seen in the news lately in [California](https://www.aopa.org/news-and-media/all-news/2016/june/08/navy-cancels-planned-gps-outage-in-southern-california) and [Egypt](http://www.mirror.co.uk/news/world-news/fresh-terror-fears-hackers-trying-8074849). And GPS signal can be lost due to ionospheric interference. So, any equipment that doesn't rely on external sources is safer.
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.