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.
Most likely GPS is more accurate, after all, it can calculate your location in three-dimensional space within a few feet. However, it's not really that usable, since GPS measures altitude from the [geoid](https://en.wikipedia.org/wiki/Geoid). That said, new GPS-receivers should be able to correct the difference between the geoid and actual earth surface.

Baro-altimeters are also quite accurate, but only when the atmosphere is at [ISA](https://en.wikipedia.org/wiki/International_Standard_Atmosphere)-conditions. Baro-altimeters are calibrated to ISA-atmosphere, and when any condition varies from this, you get an error. Some of those, for example temperature error, can be compensated for, but removing all potential errors is a tedious task.

If I was geocaching and wanted the most accurate elevation information for my location, I would use an old-fashioned geographic map with elevation information.