Hello,

I flew my phantom 4 over a region to collect sample data where it captured a couple of buildings. I processed the data and found that the altitude of the building is 63 meters.

Now, I again went to the same location after 2 days. Flew the same drone with the same camera and same place. Collected sample data. This time, I found that the altitude of the building is shown as 25 meters. Why so?

Is it due to the changes in MEAN SEA LEVEL?

If YES, how can I get the ZERO set everytime before flight?

Please help me out with this.

Thank you.

If you are reading altitude as displayed by the screen - then the altitude is referenced NOT to sea Level - but to the point of take off. Sea Level has absolutely nothing to do with what a Phantom displays.

If you were to take off from point A and fly ascending up alongside a tree or structure ... lets say its 50m high..... the BAROMETRIC pressure CHANGE would provide the basis for giving the approximate altitude change and display ~50m give or take a bit.

Now land ... power down ... move to a different spot that is for example 20m higher than the first. Power up and do the same ascending up the same tree or building ... now the altitude shown will be less the ground elevation difference between the two locations ... ie display will now show ~30m. If you were to drop down to ground level at the tree / building - you would read -20m or so.

If you are flying from the same spot for both your flights you mention where you find a reduction of 38m ... that is very unusual ... its not uncommon to find maybe a slight change because of pressure variance ... but certainly not that amount...

Nigel

The only way to get a zero datum for a structure and to get a reasonable height reading for it - is to power up at the foot of the structure, so the AC registers ZERO.

Then without powering off ... pick up the AC and move to your take off spot ... ignoring the change in 'height' / altitude reading on the screen.  Then when you take off and fly up or over the structure - you will then have its height relative to the spot it stands on.

The only way you can have relative to Sea Level is to correct that displayed reading separate to the AC by using the Topgraphic information for that spot ... which is obtained from Survey maps etc.

Nigel

The solution is simple.  Make your home point on a  surface  near each building, level with the structure . Be sure check for any steel  in close proximity before powering up.. Fly to the top and your altitude should be fairly close to the actual height.