Given that if one where to fly in a straght 'trimmed' line at a 'fixed' height, does the Mavic adjust the height relative to the launch point using barometric pressure or GPS triangulation? Not that we can really fly far enough for it to be an issue (legal and battery power limited), but I'm interested from a technical point of view when doing ground surveys. Should one take into account the curvature of the earth - which for example would render a fall of 1.8 metres over a stright line distance of 4.8 kilometres - or in imperial, a 6 ft drop over a 3 mile straight line range? Maybe the margin of error is greater than the projected differance in elivation relative to the underlaying ground point. But it would interesting to know - does the Mavic fly in a geometrically straight line, or does it follow the curvature of the earth?

Interesting question.  Not sure of answer except to say GPS is not used in height measurement, only barometric pressure.

interesting question and interesting answer from Edm - i did not know that. i cannot provide an answer to your question but a possible practicle work around could be to split your project distance in half and fly from the midpoint of each half.
Therefore you would only be flying 1.2km either side of home point means your curvature fall is about 40cm (is that right?.more or less according to your figures).
in addition, going on what EdM says about barometric for height, then the risk of barometric variances over shorter distances should be more stable than may be over longer distances. Not to forget that if you fly out 4.8km in absolute ideal conditions you will/should (aged battery could reduce absolute potential) have enough juice to get home safely.
if you stop to take pics or hover about too long in a place or the wind picks up your risk of not getting home increases.

I guess you could use VPS altitude readings provided you stay within 12 m height

Altitude is assessed by the drone against the home point using the internal barometer and not the ground it is flying over using GPS. The only exception to this is when the downward vision system can detect the ground and you have both readings show in the telemetry within the app - this is how the 'terrain follow' flight mode is effective and equally why if you take off from the top of a hill at say 500' AGL and take off to 10 feet your altitude will still read 10' as you fly away from the hill despite being 510' AGL.