Altitude - what’s the point, and how do you ever know your actual altitude?

Hi all

I’ve searched all through the FAQs but I can’t seem to find an answer for this.

In piloting there are 5 altitudes (indicated, pressure, density, true and absolute). These are never zero, but my Mini 3 Pro frequently shows me negative altitudes. Consumer drones appear to use none of these, but instead a form of QFE - but without actually measuring air pressure. So ‘altitude’ is simply metres/feet above or below take-off point.

Sooo … if your TO point is higher than your current waypoint, how does everyone here manage to avoid a ground crash when flying at a reasonable distance away where it’s difficult to judge your exact height? Or a splashdown? The only way would be if the ground is exactly level to your takeoff-point, or it is exactly the same level as any waterways you’re flying over.

To sum up my question:

  1. What’s the point of DJI’s display of altitude in any meaningful context?
  2. How do you guys avoid ground or water crashes if you can’t detect true altitude?

Thanks in advance!

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In this scenario I’d usually be closer to the drone. or rely on FPV

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Personally speaking, what Chris said :point_up:

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So, the altitude display really isn’t useful for anything?

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Not really, it’s still a reference height either legislation wise or if you want to replicate shots multiple times at the same relative altitude for example. In terms of ground avoidance you’ll still get the radar (which is even in your screenshot) which is based off the sensors and so will be real-time relative to whats below you.

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It’s very accurate for AGL height at point of take off.

At the end of the day it’s a £500 plastic toy.

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If you pre-plan your missions with an app like Litchi you can set the altitude at each waypoint relative to your take-off point or relative to the ground below each point. So you can set up a flight where the aircraft is more or less at the same height above ground all the way along the route.

But, as has been previously pointed out, the Mk 1 eyeball is the best collision avoidance device :slight_smile:

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DJI drones at least do basically use QFE for the display - they do have barometric pressure sensors and base the altitude off those.
Most also have a low level “radar” altimeter to measure actual height above the ground. Typically these are ultrasonic or stereoscopic cameras and start working below about 30ft.

Then of course there’s the GPS altitude in the metadata which is absolute but woefully inaccurate due to ellipsoid errors etc…

As a light aircraft pilot and new drone pilot we use Qfe airfield elevation and qnh normal height above sea level .

When I take off in the aircraft I set my altimeter to the qfe setting then on route I’ll change it to the qnh ,when I get near to the landing airfield I’ll set it to the Qfe for that airfield .
Unfortunately a drone doesn’t have a method of resetting the calibration for the height .

There is also the quadrantle rule which separates aircraft by the heading they are flying
Hope this helps

Cheers
Dave

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If you’ve ever flown with an instrument that gives live AGL measurements you’d know the resolution is so low as to be useless for most drone operations.

Given there’s no sensible dataset to use and no sensible way to reliable measure true live AGL it’s unclear what alternative you’re suggesting.