On the Litchi forum I recently asked a knowledgable member why Drones used barometric pressure to determine altitude rather than GPS which I would have thought more accurate. He qualified his answer by stating he was not an expert on GPS but he had read that GPS height is only accurate to only 10 to 20 metres. Can anyone throw more light on this matter?
Hi. I don’t know the actual numbers but deffo a barometer is much more accurate than the GPS derived reading. I think it’s something to do with the geometry/maths errors 
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This explains it quite nicely
As Steve says 
A barometer is very accurate down to 10cm
The set point is from the takeoff point and can be affected by changes in the atmospheric pressure
But because the flight is generally short (less than 30 minutes) it’s not really an issue
Also it’s better to have it separate from the GPS as it’s easy to lose satellites so even in atti mode you would still maintain allitude position
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Thanks for all your pertinent contributions. Some of the terminology contained therein allowed me to do some more informed searches online. In addition, it seems that the best satellite arrangement is to have satellites overhead and at least 3 on the horizon separated by 120 degrees. These 3 potentially contribute most to the accuracy in terms of elevation but unfortunately the signal from the horizon is poor and this affects the accuracy of their contribution. This is where I obtained this info, it’s short, sweet and to the point: Altitude Accuracy