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: 
There is a pretty comprehensive description of the issues at [this link](http://www.borgeltinstruments.com/GPSvsPressurealtitude.pdf).

Briefly - the conversion of pressure to altitude makes certain assumptions about the "standard atmosphere" - that is, it works best when the temperature and the lapse rate (change in temperature with altitude) exactly matches that from the model. In the real world, this is almost never the case. For example, on a hot day the air will be less dense, so the weight of a column of air will be smaller - and so when the pressure seems to have dropped by say 200 HPa, you will underestimate your altitude. The example calculation in the link shows that when the surface temperature on a dry day is 42°C, the error at 10,000 feet could be as much as 800 ft - which is considerably bigger than the typical error in a GPS (although GPS is less accurate in the vertical direction, you should routinely be able to get height within about 50 feet when you have an unobstructed view of the sky, and ionospheric activity is not unusually high). In principle it is possible to make corrections for these things - but a simple mechanical pressure-based altimeter probably doesn't.

On the other hand - if everybody else is flying by barometer, you might be "right" about your GPS based altitude, but "wrong" compared to other planes in the vicinity.

So it is advisable to use the same method as everyone else, so you don't crash into each other. On the other hand, when it comes to your final glide, the GPS is more likely to get you home safely on a hot day.
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.