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DJI FPV Altitude Display gets off quickly

dougcjohn

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New to FPV, enjoying learning & playing with the DJI FPV in Normal Mode.
I‘m enjoying flying low, the speed element is fun close to ground.

I take off and On-Screen Altitude indicator states 3 Feet. I fly around pretty much remaining at the 2-5 Ft altitude at 8-30 mph speed, but notice the Indicated Altitude changes often to a higher altitude… Meaning indicates On-Screen a altitude of 5 Ft when really 2 Ft. So I land to recalibrate… I land and it shows - 3 To - 4 Feet at same LZ location. I take off showing Zero, fly around and it’s soon off again.
When Flying cinematic (Mavic, Inspire, Matrice, etc) I’m normally above 80-300 Ft, but haven’t noticed when landing the altitude goes Negative.

Flying FPV 2-4 ft, having altitude displayed accurately would be helpful. I‘m learning to judge distance to ground visually, but would like altitude to be accurate.
My practice area is my Yard… basically flat with a few mild rolls. I fly around and through multiple trees.
My Fence is about 4 Ft, and when I route over the fence I‘m not able to always tell I’m above 4 feet.

Is this noticed by anyone else, is this considered normal or a unique problem I’m experiencing?
This little craft is an outright blast to fly… I’m getting faster in turns and looking forward to improving FPV skills.
 

Kilrah

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Expecting anything more precise than +/- 5-10ft would be unreasonable especially given the flight speed, if you do fast climbs and descents it can actually become off by >100ft.
Get used to the view, it's much more reliable and doesn't take your eyes and concentration away to look at and process a number.
 

dougcjohn

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Expecting anything more precise than +/- 5-10ft would be unreasonable especially given the flight speed, if you do fast climbs and descents it can actually become off by >100ft.
Get used to the view, it's much more reliable and doesn't take your eyes and concentration away to look at and process a number.
I can accept that and get used to the view. You're correct, taking eyes off visual to focus on screen numbers is challenging! I am starting to feel 3' vs 5' too... which is pretty cool.

But that all said, I can take a older M600Pro with LB and A3Pro electronics or Inspire 2 around, up to 300-400 feet... change Altitude several times and maintain much better accuracy; Fly across a Ag field in pattern and it'll maintain steady Altitude the whole time.

Land them and they don't read out -4 to -6 ft at take-off LZ. Not an overly big issue, but was curious if these low flyers had more precise accuracy since they were intended to fly low. Thought the sensors on bottom would maybe be assisting.
 

thetechnobear

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for low level flying the 'visual' height meter, I found it pretty reliable.

GPS is never super accurate for altitude (mavics as well)... I don't think triangulation of height is particularly accurate, and is why many GPS systems are supplemented with barometers (which have their own issues) - and why DJI has visual sensors for landing.

also, even if it was accurate, its still not very useful for judging height above ground since it is reporting height above take off point NOT height above ground.

when I got my first Mavic (air), I didnt realise this and nearly flew it into the ground - basically my land rises gradually, so if you stick to the same altitude (above home point), you are slowly decreasing altitude.

the other thing Ive learnt the hard way is looking at these numbers (or any other numbers) whilst flying low is a sure way to crash - when flying low/fast, you need to concentrate on flying based on what you can see - and learning to judge height.
so them being 'inaccurate' is in the mid-term not really important at all.


what I did when I first got the DJI FPV was to practice flying low in N mode, since you have the obstacle avoidance to keep you out of trouble - so you get your eyes used to how low is 1m... once you are happy with that use S mode, now you can go faster , and you also still give visual warnings (yellow/red flashes)

then finally you can move to M mode ... but take that slowly, this is how I ended up crashing/damaging my dji fpv...
even at slow speed, if you travel very low, its easy to get caught out by the terrain and catch something e.g. a small rock, and it'll send the drone flying.
my main advice here, is make sure you use the Arm Bracers - I'm pretty sure, my drone would have survived with these... without an arm break is pretty easy.



finally, if you want to do a lot of really low level flying or close proximity - consider getting another drone to supplement the DJI drone.
I bought a 5", and cant tell you how much more confident I feel with it, since it'll survive a crash, and if it doesn't its easy/cheap to fix.

thats not to say I dont use the DJI FPV, to the contrary, as stated on other threads, even compared to my 5" its fantastic for things like longer distance flights, or more casual crusiing, cinematic flights... it feels better quality, and safer, and more battery.

... but for real close up stuff, I found I was just always thinking about not crashing ... not just due to cost, but the inconvienience of sending it to DJI.
(bare in mind, a trip to DJI for repair will be $100-200+ , money you could use on getting another drone to crash ;) )

of course, once you've got better with the other drone, you'll also be better/more confident at flying the DJI FPV lower/faster - so its a win/win :)
 

dougcjohn

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@thetechnobear , great points!
I am currently using N mode, but turned the OA off after a few flights… didn’t like the slow down when trying to fly around and between trees. The visual height meter is getting better with each flight!

I did pickup and install the arm braces last week. It may help, but I’ve snagged a few bush twigs with the braces… but all good. Was disappointed that my local BestBuys haven’t started carrying accessories or even the kit yet… only online. Needed a few props and had to wait 5 days for B&H to provide. Broke a antenna stub (battery in pocket, cord pulled off counter) and was down waiting on part. No one could provide quickly, so purchased a 2nd Goggle from Amazon - next day. Ended up keeping 2nd Goggle for “Ride Alongs”.

I’ve tried S mode a few times… the speed and acceleration is great but my reaction isn’t quite there yet around obstacles at 3 ft altitude.

I’ve thought about a smaller FPV build, but was surprised I basically needed separate gear: goggles & transmitter. I’m pondering that route… this was originally a small venture off the cinematic platforms, more to see what everyone was chatting about and maybe for a bit of entertainment…. With intentions to return the unit. That grew into keeping kit, adding batteries, goggle foam, 2nd goggles for Wife /Friend to “Ride Along”, props, case, battery strap mount, straps, antenna spares, etc!

This little craft is a Blast and consuming!
 

thetechnobear

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I’ve tried S mode a few times… the speed and acceleration is great but my reaction isn’t quite there yet around obstacles at 3 ft altitude.

yeah, the closer you are to the ground the faster everything gets, still catches me out occasionally :)

I'm still occasionally getting caught out when I come swooping in from a height, and think im slow enough - but get to the ground, and am surprised how fast im actually going!

one thing I started doing, that helped a bit, was changing my approach to landing

I used to land like a helicopter, so coming it at a height (say 10-15m) , then hovering, landing - so all very vertical...

now, what I do is come in with an approach more like an aircraft so say 10m down to 1m (horizontally), then just hover/auto land the last 1m (or less).
this seems to be the most common way to land a traditonal fpv which you have to do manually, so give you good practice for that.
also, frankly, its a more fun way to land :) (and also works a bit better with the Motion controller I think )


note: the last bit of auto-land, I still do in N mode, as dji fpv drone has a nasty habit of 'toppling over' in M mode...


I’ve thought about a smaller FPV build, but was surprised I basically needed separate gear: goggles & transmitter. I’m pondering that route… this was originally a small venture off the cinematic platforms, more to see what everyone was chatting about and maybe for a bit of entertainment…. With intentions to return the unit. That grew into keeping kit, adding batteries, goggle foam, 2nd goggles for Wife /Friend to “Ride Along”, props, case, battery strap mount, straps, antenna spares, etc!

yeah, theres alot of investment in the DJI Kit, and as you say a real pity, you cant use more of it on other quads ... particularly the controller.
that said, I will say, now that Ive got used to the larger controller (a radiomaster TX16) - I am starting to prefer it to the DJI Controller v2 , albeit its not as good for carrying around.

but yeah, just becareful when you start taking more risks, just think... should I do this with something else?
(perhaps even a cheap cinewhoop... though, unfortunately, once you go to HD, I dont think you're going to want anything else ;) )

This little craft is a Blast and consuming!
indeed, don't let the 'traditional' fpv-ers fool/discourage you... the DJI FPV is a great flyer, and many features are just fantastic.

I'll give you an example...
Ive just been doing some range tests, comparing the air unit (5") vs the dji fpv drone (going to be an interesting video!)
the 5" was absolutely nerve racking, when getting to limits of range - even though I've got GPS rescue on it, and its fully tested, known to be working - but I still regard it as 'my last hope'
the dji fpv, I just send it... no quarms about it losing signal, I know it will return safely... and also the degradation is also just so much smoother.
... and thats not the best thing... but I wont spoil the highlight of the range test video to reveal the best bit ;)

thats the thing about the DJI FPV drone, its relaxing to fly
 

droneguy

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New to FPV, enjoying learning & playing with the DJI FPV in Normal Mode.
I‘m enjoying flying low, the speed element is fun close to ground.

I take off and On-Screen Altitude indicator states 3 Feet. I fly around pretty much remaining at the 2-5 Ft altitude at 8-30 mph speed, but notice the Indicated Altitude changes often to a higher altitude… Meaning indicates On-Screen a altitude of 5 Ft when really 2 Ft. So I land to recalibrate… I land and it shows - 3 To - 4 Feet at same LZ location. I take off showing Zero, fly around and it’s soon off again.
When Flying cinematic (Mavic, Inspire, Matrice, etc) I’m normally above 80-300 Ft, but haven’t noticed when landing the altitude goes Negative.

Flying FPV 2-4 ft, having altitude displayed accurately would be helpful. I‘m learning to judge distance to ground visually, but would like altitude to be accurate.
My practice area is my Yard… basically flat with a few mild rolls. I fly around and through multiple trees.
My Fence is about 4 Ft, and when I route over the fence I‘m not able to always tell I’m above 4 feet.

Is this noticed by anyone else, is this considered normal or a unique problem I’m experiencing?
This little craft is an outright blast to fly… I’m getting faster in turns and looking forward to improving FPV skills.


There are a few factors that will affect the altitude reading or height above ground (both are calculated differently). DJI uses a number of sensors to determine the height. The downward facing Time of flight (ToF) sensor has an effective range of less than 10m while the vision sensor is good from 0.5m to approx 30m. The GPS is also used (it is displayed once you are above 10+m from what I recall) It would seem to be that the height in the goggles is a calculated value using the various sensors. When close to the ground the ToF and/or visions sensor are used. The problem is that when you pitch the quad forward/backwards for flight etc. the sensors are no longer pointing down and the reading get skewed. I noticed if you stop and hover in place for a min or two after a high speed run, it tends to get a good reading then displays a more accurate height but it's not immediate. I suspect that DJI uses the various sensors and calculate the height using data from the sensor arrays differently in different conditions.
 

dougcjohn

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There are a few factors that will affect the altitude reading or height above ground (both are calculated differently). DJI uses a number of sensors to determine the height. The downward facing Time of flight (ToF) sensor has an effective range of less than 10m while the vision sensor is good from 0.5m to approx 30m. The GPS is also used (it is displayed once you are above 10+m from what I recall) It would seem to be that the height in the goggles is a calculated value using the various sensors. When close to the ground the ToF and/or visions sensor are used. The problem is that when you pitch the quad forward/backwards for flight etc. the sensors are no longer pointing down and the reading get skewed. I noticed if you stop and hover in place for a min or two after a high speed run, it tends to get a good reading then displays a more accurate height but it's not immediate. I suspect that DJI uses the various sensors and calculate the height using data from the sensor arrays differently in different conditions.
Great points, hadn't explored all the sensors. Out of curiosity, where do you find all the info on the sensors? I haven't found much in-depth info on the craft or it's engineering. I wasn't aware the GPS didn't come into play until above 10M (30 Ft), although isn't the GPS being used on take-off for hover & stability?

I'll have to try the hover for 1-2 min to see if that changes altitude back to accuracy. I've found if I touch down & power back up it'll recalibrate. The altitude would be helpful, but I am learning to look at the ground distance a bit better; Still it would be nice to see it more close to accuracy. Tonight I flew around yard, in & around trees & bushes 2-4 Ft above ground and occasionally launched upward above house to practice coming back into a small open spot in yard. While angling a downward decent, I looked at altitude and it was indicating -8 Ft and craft was still 20+ feet above ground.
 

rktman

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Hmmm... I find the visual sensors to be pretty accurate close to the ground. Tenth of a foot accurate.

While you're flying 2–5 AGL which height are you watching? At that height the VPS measurement will be in red, and displaying in 0.1ft resolution. It's pretty accurate.

The GPS/baro altitude is subject to constant pressure variations and GPS altitude errors, which are measured in tens of feet.
 
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droneguy

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Hmmm... I find the visual sensors to be pretty accurate close to the ground. Tenth of a foot accurate.

While you're flying 2–5 AGL which height are you watching? At that height the VPS measurement will be in red, and displaying in 0.1ft resolution. It's pretty accurate.

I have also found them to be pretty accurate with the exception of some specific scenarios like flying low over water then transitioning to tall flying over tall grass. In that situation, there seems to be a delay in calculating the proper height. I expect the gyro data is leveraged with the sensors to recalculate the height when flying at speed and I have noticed some situations where the calculations are clearly incorrect. I have however noticed that after a few moments of flying over solid terrain, the recalculation typically works well and the height becomes accurate once again.
 

rktman

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Well water, yeah, that's a toughie 😁😁

Parallax and size changes relative to vertical speed are the main methods estimating height with the visual and IR ToF sensors.

None of this works if the surface is not "solid" and unmoving. I've had issues over tall grass waving around in the wind. Where is the solid "ground" you're trying to measure distance from?

It's a tough problem.
 
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Kilrah

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Don't forget that the ToF sensors only look straight down when hovering. When flying forwards they are oblique, for which the software likely compensates since it knows the angle, but that won't be able to take into account that they're essentially looking at some place behind you now, not where you are, and even less where it would matter to be able to rely on the indication for you to know what to do i.e. in front of you.
 

droneguy

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Don't forget that the ToF sensors only look straight down when hovering. When flying forwards they are oblique, for which the software likely compensates since it knows the angle, but that won't be able to take into account that they're essentially looking at some place behind you now, not where you are, and even less where it would matter to be able to rely on the indication for you to know what to do i.e. in front of you.
Yeah exactly. That was my point as well.
 

dougcjohn

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Hmmm... I find the visual sensors to be pretty accurate close to the ground. Tenth of a foot accurate.

While you're flying 2–5 AGL which height are you watching? At that height the VPS measurement will be in red, and displaying in 0.1ft resolution. It's pretty accurate.

Well, bit embarrassed to state… I was checking the GPS Altitude from Home and not the Distance to Ground (DTG) indicator. Viewed various YouTubes & skimmed manual when received in hast to fly the craft.

Reread manual today and hadn’t given the DVS or IR sensors much attention. As you referenced, they’re 5-30 m distance with ideal .5-15 m. I noticed the screen DTG readout indicator without much thought. I’m used to larger GPS platforms… mindset… opps!

In reviewing manual, the DTG indicator is accurate up to 10m. Pg. 33 - #11. Distance to Ground indicator.
“Displays the current altitude information of the aircraft from the ground when the aircraft is less than 10 m above the ground.“

Appears the DTG indicator uses both DVS & IR Sensors. When you indicate ToF sensor, are you referring to the IR sensor? I didn’t find reference to ToF.

Appreciate all the Info from everyone!
I’ll go out later today and take note of the DTG reading.
 

droneguy

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Well, bit embarrassed to state… I was checking the GPS Altitude from Home and not the Distance to Ground (DTG) indicator. Viewed various YouTubes & skimmed manual when received in hast to fly the craft.

Reread manual today and hadn’t given the DVS or IR sensors much attention. As you referenced, they’re 5-30 m distance with ideal .5-15 m. I noticed the screen DTG readout indicator without much thought. I’m used to larger GPS platforms… mindset… opps!

In reviewing manual, the DTG indicator is accurate up to 10m. Pg. 33 - #11. Distance to Ground indicator.
“Displays the current altitude information of the aircraft from the ground when the aircraft is less than 10 m above the ground.“

Appears the DTG indicator uses both DVS & IR Sensors. When you indicate ToF sensor, are you referring to the IR sensor? I didn’t find reference to ToF.

Appreciate all the Info from everyone!
I’ll go out later today and take note of the DTG reading.

Yes the ToF is the IR sensor.
 

dougcjohn

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Thanks All… Resolved.
Flew it this evening, 3 Batteries.
Monitored the DTG indicator and it appeared spot on!
A few times, I’d jump from 3ft to 120 ft, then decend back to 3-4ft, the DTG would go from white to red once close to ground and accurate.

Nice to see, it helps verify my visual estimates were close. I thought I was 1.5 ft and it’d indicate 2 ft… I’d lift goggles to verify… looked more like 2 ft.
 

droneguy

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Just be careful over water. I had my spotter freak out a few times while I flew 2-3 ft over the water at 40-50mph only to find out it was more like 10-14 inches in reality. The DTG indicator seems to be consistently off over water.
 
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