I visted Monsal Head in Derbeyshire a couple of weeks ago, for those who don’t know it, it’s a fabulous vantage point overlooking Monsal Dale and the viaduct carring the track bed of the former Mildland line. In addition to myself there were two others with their drones and a thought crossed my mind. If I went further out and did a RTH then I would have to factor in the additional depth below me in the RTH setting. I wondered if the altimeter was barometric does it use the optical sensors additionally for the near to the ground readings. Anyhow here’s a photo I took on the day.
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DJI Drones use the TOAL point as their reference, so what appears on the screen is the height difference from where you took off - meaning you need to manually factor in the changes in terrain height…
This does mean that if you are flying up the side of a mountain you can exceed 400ft above the TOAL point, however, you must stay within 400ft of the ground…
And if flying over a valley then you will need to descend as the terrain descends to stay within 400ft of the ground below you…
All covered in the CAA DRONE code -
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Yes it’s a barometer
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Generally yes, they use Barometric pressure. But also some drones may also use GPS data depending manufacturer and software.
Thanks Suzanne, I get all that, my real question is it barometric coupled to optical for near the ground. When I used to fly I would set in the barometric pressure on the altimeter to either read 0 (QNH) or the actual height (QFE) So DJI set to zero at the take off point but barometric alone cannot be accurate enough on it’s own when you’re relatively close to the ground. So my question is does it use a combination of optical? or whatever for the near to ground.
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Optical isn’t generally used for height. It does get used for secondry positioning. Ultrasonic sensnors are sometimes used for height measurement at low heights
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It must use a secondary sensor I’ll check it out with DJI thanks
The drone code actually states the nearest point on the earths surface, this may not necessarily be the ground below you. 
The downward vision sensor is not used above 10m only the barometer is used for height calculations
The accuracy is variable and is just for reference
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Thanks I didn’t think the barometric alone would be good for near to ground readings