Altitude readings?

[Meta4]

You are saying that it actually users a barometer to calcualte height?...

Yes ... just like real airplanes do.

 

[lookout]

Problematic if your quad thinks it is 400 agl from your start point but is about to run into the side of a mountain from changing terrain.

 

[AZDave]

Problematic if your quad thinks it is 400 agl from your start point but is about to run into the side of a mountain from changing terrain.

Certainly so. I live next to a fairly tall mountain and I've come close to running into the side of it because I topped out at the 500 meter barrier that wouldn't let me go higher. Of course, I also have to be careful coming back home to avoid exceeding 400 feet AGL as I drop down.

For certain paths that would take me near the mountain (or across the large ravine near it) I considered using pre-defined flight plans (Litchi, Maven, etc) that allow you to specify height-above-ground until I realized that databases like the Google Earth elevation.api used by those apps isn't very accurate at all for rapid changes in elevation. I found it to be off by almost 50 feet in places. If I want to fly under 100 feet above the terrain in those cases I either fly manually very cautiously, or to fly a predetermined path I first manually set waypoints by flying there with the sticks and setting each waypoint individually once I get there.

 

[Yorkshire_Pud]

Ok, I've been having a problem with this too ... I always assumed the the drones used the triangulated gps signal to calculate height above sealevel, then reset that height to zero at the home point and caluclated flight heights from that. . You are saying that it actually users a barometer to calcualte height? this does not seem to be a very accurate way to calculate height, as air density can vary considerably over short distances, especially if there are some large rain clouds around ...

Put your drone into a plastic bag and switch everything on. Using your mouth, (so as to create only small changes in pressure) suck the air out of the bag or blow air into the bag, watch the height indicated in the app. I have just done this with a mini 2 and saw temporary indicated heights in excess of +100m and a low of -6?m, (without the motors running it tends to reset to zero). The +100m might have been erroneous as I had a navigation error at the end of the test. DO NOT do this for long, there will be limited ventilation in the bag and the drone may move heat.

 

[Felix le Chat]

Put your drone into a plastic bag and switch everything on. Using your mouth, (so as to create only small changes in pressure) suck the air out of the bag or blow air into the bag, watch the height indicated in the app. I have just done this with a mini 2 and saw temporary indicated heights in excess of +100m and a low of -6?m, (without the motors running it tends to reset to zero). The +100m might have been erroneous as I had a navigation error at the end of the test. DO NOT do this for long, there will be limited ventilation in the bag and the drone may move heat.

Here's a howler. Was on a commercial gig a couple of years ago on the coast. Took off approximately 6 feet above true sea level. Flew to target and started shooting. When I flicked a look down at the telemetry, it informed me that I was operating 47 metres below sea level... According to DJI.... my drone had become a submarine....... Not a readout I would place any confidence in. What does that tell you about the accuracy of your 120m AGL ceiling?

 

[Meta4]

Took off approximately 6 feet above true sea level. Flew to target and started shooting. When I flicked a look down at the telemetry, it informed me that I was operating 47 metres below sea level... According to DJI.... my drone had become a submarine....... Not a readout I would place any confidence in.

If this was the height showing on your screen, you've encountered something I've never heard of with DJI drones.
It's common for the indicated height to vary by 10 metres or so due to variations in air pressure, but not such a large difference.

Or are you confusing it with the common height difference seen in the image metadata like this example?

What does that tell you about the accuracy of your 120m AGL ceiling?

Not much since the drone doesn't show an AGL height.
But experience shows the the height shown in the app is close enough unless there's a rare malfunction.

 

[Felix le Chat]

If this was the height showing on your screen, you've encountered something I've never heard of with DJI drones.
It's common for the indicated height to vary by 10 metres or so due to variations in air pressure, but not such a large difference.

Or are you confusing it with the common height difference seen in the image metadata like this example?

Not much since the drone doesn't show an AGL height.
But experience shows the the height shown in the app is close enough unless there's a rare malfunction.

Wasn't confusing it with anything. The sea level (the blue wobbly stuff boats float on) was 6 feet lower than the sea wall I was standing on. The vertical height telemetry read negative 47 (-47) less than 5 mins into the flight.

 

[Meta4]

Wasn't confusing it with anything. The sea level (the blue wobbly stuff boats float on) was 6 feet lower than the sea wall I was standing on. The vertical height telemetry read negative 47 (-47) less than 5 mins into the flight.

Was this was a one-off event or did you commonly observe large differences between the actual height and that showing on the screen?

The flight data from that incident might be interesting to see.

 

[mobilehomer]

What did the weather do during this time?

 

[Meta4]

What did the weather do during this time?

A change in atmospheric pressure of 6 hectopascals would be required to show a 47 metre drop in indicated height.
For that to occur over just 5 minutes, it would have been accompanied by a memorable weather event.

 

[Felix le Chat]

What did the weather do during this time?

Sunny September day. Clear as a bell. I haven't noticed this kind of aberration since, but to be honest, after seeing that, I refused to put any faith in the vertical height telemetry and stopped paying attention to it, preferring to fly VLOS vertically as well as horizontally. Scared the crap out of me for a moment though because I had no way of gauging how high I was over a flooded estuary until I re-established eyes-on visual sighting.

 

[Meta4]

after seeing that, I refused to put any faith in the vertical height telemetry and stopped paying attention to it

What you've described is not at all normal.
The barometric sensor is usually extremely reliable, which is why it's the primary height sensor on all real planes.

If yours had such a serious malfunction once, you'd expect it to do it consistently.
It would be interesting to see if the flight data gives any clues to the mystery.

 

[Felix le Chat]

Sunny September day. Clear as a bell. I haven't noticed this kind of aberration since, but to be honest, after seeing that, I refused to put any faith in the vertical height telemetry and stopped paying attention to it, preferring to fly VLOS vertically as well as horizontally. Scared the crap out of me for a moment though because I had no way of gauging how high I was over a flooded estuary until I re-established eyes-on visual sighting.

Curiously enough - this was taken today with the Mini 3 Pro (it was my Mavic 2 Zoom that insisted it was 47 metres below sea level). Like I said: I really don't look at the vertical height reading any more because of a lack of confidence in its accuracy. With the steeple in the shot: I know that level with the weather vane: I'm at a little less than half the maximum altitude of 400' AGL. While I was editing the shot: I noticed the altitude/elevation metadata.

Looks like my Mini 3 Pro is also a midget submarine on the quiet.

 

[Meta4]

Curiously enough - this was taken today with the Mini 3 Pro (it was my Mavic 2 Zoom that insisted it was 47 metres below sea level). Like I said: I really don't look at the vertical height reading any more because of a lack of confidence in its accuracy.
While I was editing the shot: I noticed the altitude/elevation metadata.

This has nothing to do with the height shown on your screen (which should be reasonably accurate despite your misgivings).
The height on your screen would have shown the drone's height relative to the launch point.

If you use a good metadata viewer that displays all the metadata, you'll find it shows two different altitudes.
Relative Altitude is what shows on your screen and Absolute Altitude is a wildly inaccurate estimate of the drone's height above sea level.
Because of the flawed algorithm DJI use to calculate it, this height is commonly out by +/- 200 ft or more.
This is common and normal.
I showed an example in post #46.


Here's an extract from the metadata of one of my image files showing both altitudes.