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DJI - Waypoints - Minimum Altitude
3152 32 2016-1-20
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nigelw
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I've noticed recently I'm not able to set waypoints below 10m altitude.  When did this happen?  Last September I was able to set waypoints at negative altitudes with just a warning, now I am unable to set them below 10m.  This make it all a bit useless for me, since I tend to stand on mountains & fly both above & below the home point.

Can this be reverted back or disabled or something?
2016-1-20
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SHamers
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I do this with FPV Camera - and there you have only a (min) startup altitude - after the first waypoint you can go below zero.
2016-1-20
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nigelw
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SHamers Posted at 2016-1-20 12:58
I do this with FPV Camera - and there you have only a startup altitude - after the first waypoint yo ...

Thanks, I'll have a go later today.
2016-1-20
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nigelw
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SHamers Posted at 2016-1-20 12:58
I do this with FPV Camera - and there you have only a (min) startup altitude - after the first waypo ...

Yes, it works...I couldn't remember if I'd done it that way last time.  Thanks.
2016-1-20
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SHamers
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Perfect, have fun!
2016-1-20
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FPV Booster
lvl.4

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FPV Camera allows "Set waypoint height from -200m (-656ft) to 500m (1640ft), and the default height is 30m (98ft)."

http://fpvbooster.com/index.php? ... ew=article&id=7
2016-1-21
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jay6c1
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FPV Booster Posted at 2016-1-21 08:44
FPV Camera allows "Set waypoint height from -200m (-656ft) to 500m (1640ft), and the default height  ...

Wow... So with this app, can i set waypoints just 1 meter above the ground?
i want to fly parallel to me.
2016-1-25
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FPV Booster
lvl.4

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jay6c1 Posted at 2016-1-26 10:33
Wow... So with this app, can i set waypoints just 1 meter above the ground?
i want to fly parallel ...

Yes, you can even set negative value, say if you take off from a hill and flying down hill.

If you set 1 meter above ground, make sure to turn on VPS for more accurate altitude.
2016-1-25
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DJI-Ken
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Yes, you cannot set negative altitudes. Seems like the other program the other post works though.
2016-1-25
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nigelw
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DJI-Ken Posted at 2016-1-26 06:02
Yes, you cannot set negative altitudes. Seems like the other program the other post works though.

Sorry, I should have said, I tested it in the Go app & yes, you can.  You just need to set the first waypoint above 10 metres, same as FPV camera.
2016-1-26
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nigelw
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jay6c1 Posted at 2016-1-26 02:33
Wow... So with this app, can i set waypoints just 1 meter above the ground?
i want to fly parallel ...

If you do, you may get more than you bargained for.  Mine is never accurate enough to fly itself 1m off the ground.  Make sure you have you finger on the mode switch if you do it.
2016-1-26
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Geebax
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nigelw Posted at 2016-1-26 19:27
If you do, you may get more than you bargained for.  Mine is never accurate enough to fly itself 1 ...

I regularly do it at about 2 Metres and it flies pretty well. I use it to do tracking shots along the deck of bridges. But I do it at almost the slowest speed.
2016-1-26
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johnsr
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Great info, thanks Nigel, I thought that I was limited to 10 meters for all the waypoints in the Go App! Not sure what the barometer's altitude accuracy is, but I would guess not better than +/- 2 meters on separate runs (anyone have a more solid value). So yes I'll keep a finger ready to switch modes when doing skimming shots.
2016-1-26
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nigelw
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johnsr Posted at 2016-1-26 09:55
Great info, thanks Nigel, I thought that I was limited to 10 meters for all the waypoints in the Go  ...

Barometer accuracy seems to vary quite a bit.  Sometimes mine can be bang on, other times it seems to over estimate the height.  My worst case is a landing height of 24 metres!  Worth knowing when setting RTH values.

Check your own flight records because no doubt yours will be different (hopefully better).  Just go into each record & fast forward to the end when the aircraft's landed & check the height.  It should say zero if you landed at the home point, anything else is an error.
2016-1-26
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johnsr
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nigelw Posted at 2016-1-26 14:04
Barometer accuracy seems to vary quite a bit.  Sometimes mine can be bang on, other times it seems  ...

Thanks, I'll do some tests. I based my guess on an estimated barometric pressure measurement at around +/- 0.1 hPa level (like pocket altimeters) and mild weather (less than 1 hPa per hour change). On a windy day with the barometer changing rapidly, well... 1hPa represents 8 meters. Your value seems a lot worse, so my guess must be too simplest.
2016-1-26
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johnsr
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Back from flight tests. WOW I was really too optimistic! On a series of repeats of the same low level waypoint mission the height AGL of the Phantom varied by +/- 3 meters (full range) with respect to three different physical reference points. Admittedly repeats close in time were better. The bird acts as if the homepoint altitude has drifted from one repeat to the next (circuit board temperature compensation, or?). Using this for low level filming, it seems that it would be a good idea to land and re-establish the homepoint altitude before each repeat of the programmed mission. Even in that case, as Nigel remarked, take care...
2016-1-26
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johnsr
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nigelw Posted at 2016-1-26 14:04
Barometer accuracy seems to vary quite a bit.  Sometimes mine can be bang on, other times it seems  ...

Thanks for the suggestion Nigel. I went back through 26 recent flight logs (too lazy to do more) and made the following plot of the homepoint "0.0 AGL" altitude drift over the flight (typically 16-17 minutes). As one can see the shift of the homepoint altitude measured at landing compared to take-off varies from -3.5 meters below it to +3.5 meters above. It seems fairly random so I couldn't think of a trick to improve things when flying repeat missions. A point set at 5 meters above a surface, skimming a trail, road or bridge (for example) could easily be re-flown at headheight (or lower!) on a repeat flight. Seems quite obvious why the Waypoints should not be selected too low...
Furthermore, I don't see how this would change with a third-party software.

Altitude drift over flight duration

Altitude drift over flight duration

2016-1-28
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nigelw
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johnsr Posted at 2016-1-28 14:29
Thanks for the suggestion Nigel. I went back through 26 recent flight logs (too lazy to do more) a ...

Thanks for testing!  it's good to see I'm not the only one with this issue.  It's not likely anything can be done to fix it, it's just the nature of the beast.  It is important to know about it though for obvious reasons.

I was flying mine up a river in Scotland a couple of weeks ago on waypoints & could see it losing altitude as it went.  It was a good job I wasn't just looking at the monitor because I wouldn't have seen it happening.  Admittedly I was attempting to fly as close to the water as possible, so I had good reason to keep an eye on it.
2016-1-28
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FPV Booster
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The Important Notes section of FPV Camera states:

• Aircraft height (altimeter) may easily vary 15-20m (50-66ft) off the actual height after aircraft flying and heating up several minutes. You are recommended to let IMU warn up properly on the ground especially in cold weather, and set waypoint height minimum 25m (82ft). Cold IMU calibration trick is NOT recommended which might incur more discrepancy in altimeter reading.

For example, if there is a tree 30m height and you set the first waypoint to 40m above that tree to maintain a 10m gap and the last waypoint is also set to 40m above the same tree after 10 minutes mission flight. The aircraft might reach the first waypoint well above the tree, but when it reaches the last waypoint 10 minutes later, it might hit the tree due to discrepancy of barometer though the altimeter is still reporting 40m height.


2016-1-28
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johnsr
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nigelw Posted at 2016-1-28 16:07
Thanks for testing!  it's good to see I'm not the only one with this issue.  It's not likely anyth ...

Yes, nothing beats keeping an eye on it especially when you're near some surface. I after this exercise I think that I will stay with manual piloting for those close shots.
Good flying!
2016-1-28
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johnsr
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FPV Booster Posted at 2016-1-28 17:04
The Important Notes section of FPV Camera states:

• Aircraft height (altimeter) may easily vary 15 ...

Thanks for the comment FPV Booster. After my test and previous experiences I would suspect that they are being somewhat conservative on a 15-20 meter drift safety margin, especially if the craft has be correctly warmed up and the homepoint was reset before preparing the waypoint mission. If you can see what you are doing, I would assume that a 10 meter margin is relatively safe (keeping a finger on the P-A-F mode switch as Nigel said to cut out if needed). Unless of course the weather is changing rapidly or the risk includes possible third-party dammage. Nevertheless the message is clear: take care if a waypoint mission passes over or under anything.
2016-1-28
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nigelw
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FPV Booster Posted at 2016-1-28 16:04
The Important Notes section of FPV Camera states:

• Aircraft height (altimeter) may easily vary 15 ...

Makes perfect sense.  I'll check my records again to see if accuracy improved or not after using several batteries consecutively.  It's a shame DJI haven't issued such a warning.
2016-1-28
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nigelw
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nigelw Posted at 2016-1-28 18:16
Makes perfect sense.  I'll check my records again to see if accuracy improved or not after using s ...

Most of the time accuracy improves as the aircraft warms up, although not always by much.  It's a bit unpredictable.
2016-1-28
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Geebax
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nigelw Posted at 2016-1-29 09:11
Most of the time accuracy improves as the aircraft warms up, although not always by much.  It's a  ...

My aircraft seems to maintain the height quite well at about 2M, I figured it was the VPS working. But you said you were flying along a river, in which case it might not maintain height well in that situation. My subjects are not over water and have nice contrasty objects to reference.
2016-1-28
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nigelw
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Geebax Posted at 2016-1-28 22:38
My aircraft seems to maintain the height quite well at about 2M, I figured it was the VPS working. ...

I turned VPS off as it doesn't make much difference in altitude & I tend to fly a lot over surfaces that might confuse it, i.e. long grass, snow or water.
2016-1-28
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johnsr
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Geebax Posted at 2016-1-28 23:38
My aircraft seems to maintain the height quite well at about 2M, I figured it was the VPS working. ...

Agree with Nigel, I rarely use VPS when flying outdoors. Last Summer I had a minor crash when filming very low in the woods. I suspected at the time that the VPS had gotten confused by the poorly defined surface. Since then I generally leave it off.
2016-1-29
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FPV Booster
lvl.4

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It's fairly easy to test it out with barometer only (VPS off), and check the height upon landing after a flight. When VPS is on, it would correct altimeter reading within 3 meters before landing.
2016-1-29
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nigelw
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FPV Booster Posted at 2016-1-29 14:43
It's fairly easy to test it out with barometer only (VPS off), and check the height upon landing aft ...

That's what I thought.  Unfortunately you can't tell from the flight records whether it was on or off.  I suspect the few times it landed at the same height it took off from, it was due to VPS.  Last time I flew it had defaulted to on, after an app update I think.
2016-1-29
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nigelw Posted at 2016-1-30 00:33
That's what I thought.  Unfortunately you can't tell from the flight records whether it was on or  ...

Turn off VPS before takeoff. Check altimeter reading right before landing (say 1 meter above ground) and take a screenshot. Then you know the discrepancy of barometer after 10-20 mins flight. This is mainly due to temperature changed inside aircraft after flight.
2016-1-29
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nigelw
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FPV Booster Posted at 2016-1-30 04:31
Turn off VPS before takeoff. Check altimeter reading right before landing (say 1 meter above groun ...

No need to the screenshot, it shows in all the flight records.
2016-1-30
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djiuser_5303WiHqCJh4
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The min altitude setting would have saved my mavic from drowning. I took off, GPS locked went over the lake and visually set my height so I could see the wind ripples so about 6ft or 2m up flew across the lake turned it around and made the return but it plowed right into the lake. The recording showed it at -5m at majority of flight up to 20m when I swooped up and then came back down.
Flight mode was in regular not sport.
Not impressed.
2018-8-18
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Labroides
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djiuser_5303WiHqCJh4 Posted at 2018-8-18 15:56
The min altitude setting would have saved my mavic from drowning. I took off, GPS locked went over the lake and visually set my height so I could see the wind ripples so about 6ft or 2m up flew across the lake turned it around and made the return but it plowed right into the lake. The recording showed it at -5m at majority of flight up to 20m when I swooped up and then came back down.
Flight mode was in regular not sport.
Not impressed.

The min altitude setting would have saved my mavic from drowning. I took off, GPS locked went over the lake and visually set my height so I could see the wind ripples so about 6ft or 2m up flew across the lake turned it around and made the return but it plowed right into the lake.
it plowed right into the lake??
Have a chat with the person who was doing the driving.
Flying close to obstacles is always risky, whether it's tree branches or a lake.

If you fly your own drone into a lake, you only have yourself to be unimpressed with.
2018-8-18
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Mark The Droner
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Altitude fluctuations happen sometimes.  If you had more experience, you'd have known that.  They can happen in windy conditions due to varying air pressure.  They can also happen due to minor IMU issues.  Either way, you should be looking for them and expecting them and be prepared to adjust for them.  Sorry - but this is clearly pilot error.
2018-8-18
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