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Real altitude (ASL) as shown in Video Caption data

Nico El Greco

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In recent releases of DJI Go 4, Altitude is captured in the realtime video metadata recorded in the video's subtitle sidecar file (.srt file). These SRT subtitle files are created automatically when the Video Caption option is enabled.

The altitude recorded in this data is the real altitude above sea level (ASL) at that particular lat/lon. It is NOT the height relative to the take-off point, which is what is displayed in DJI Go 4. See attached screen shot.

When flying to remote objects such as mountain tops etc. it's very useful to be able to see the height ASL as you are flying, rather than the height relative to the take-off point, because it's easy to know the ASL height of these landmarks with great accuracy from geo/topographic map sources.

Eg, I want to fly up to the peak of a mountain top I know with certainty is 452m ASL. There is a ruined castle on top I want to film. It would be very useful to be able to see the height ASL in DJI Go 4 as I am flying. That way I know how close I am to the summit.

I know others have posted solutions whereby you first measure your height ASL at your takeoff point via some external means (eg GPS or topo map) and then keep adding that to the height displayed in DJI Go 4 to get your height ASL. However, doing this mental calculation is a big distraction from flying / filming and is error-prone.

What I am asking is if anything knows a way to get DJI Go 4 to display the height ASL - exactly the same way it's recorded in the Video Caption data. Obviously, the aircraft or the controller knows this information - else how does it get into the Video Caption data?

If it cannot, I think it would be a great option for DJI to add to a future release of Go 4.

Thanks in advance.
 

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I think you will find that the ASL is inaccurate, I have seen significantly negative ASL in the metadata of images taken when the drone was over my head and I was stood in a carpark by the seashore. I would be very careful if you intend to use it to 'maintain' ground clearance
 
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I think you will find that the ASL is inaccurate, I have seen significantly negative ASL in the metadata of images taken when the drone was over my head and I was stood in a carpark by the seashore.
Like this example?
i-hdNtRvZ-L.jpg
 
Nahhhh your looks warmer than mine was lol........although the value might be similar, I will have to see if I can find the photo
 
In recent releases of DJI Go 4, Altitude is captured in the realtime video metadata recorded in the video's subtitle sidecar file (.srt file). These SRT subtitle files are created automatically when the Video Caption option is enabled.

The altitude recorded in this data is the real altitude above sea level (ASL) at that particular lat/lon. It is NOT the height relative to the take-off point, which is what is displayed in DJI Go 4. See attached screen shot.

When flying to remote objects such as mountain tops etc. it's very useful to be able to see the height ASL as you are flying, rather than the height relative to the take-off point, because it's easy to know the ASL height of these landmarks with great accuracy from geo/topographic map sources.

Eg, I want to fly up to the peak of a mountain top I know with certainty is 452m ASL. There is a ruined castle on top I want to film. It would be very useful to be able to see the height ASL in DJI Go 4 as I am flying. That way I know how close I am to the summit.

I know others have posted solutions whereby you first measure your height ASL at your takeoff point via some external means (eg GPS or topo map) and then keep adding that to the height displayed in DJI Go 4 to get your height ASL. However, doing this mental calculation is a big distraction from flying / filming and is error-prone.

What I am asking is if anything knows a way to get DJI Go 4 to display the height ASL - exactly the same way it's recorded in the Video Caption data. Obviously, the aircraft or the controller knows this information - else how does it get into the Video Caption data?

If it cannot, I think it would be a great option for DJI to add to a future release of Go 4.

Thanks in advance.
Actually it isn't too hard to calculate. I live at 8200ft/2500m MSL (ASL). I have a mountain 1 mile from my house that is 9078ft/2766m MSL. For me to get to the top I would need to fly at 878ft/267m AGL (Above Ground Level), but would be more comfortable to fly at 950ft/290m to clear trees at the top. Just take the height of what you want to fly to, subtract your altitude of where you're launching from and you know how high to fly. In my case: 9078ft-8200ft=878ft.
 
I think you will find that the ASL is inaccurate, I have seen significantly negative ASL in the metadata of images taken when the drone was over my head and I was stood in a carpark by the seashore. I would be very careful if you intend to use it to 'maintain' ground clearance
I am guessing this "Altitude" you see in the video caption data is in fact "GPS elevation", as it's displayed to 6 decimal places like the GPS lat/lon. All these XYZ data appear to be coming from the GPS receiver. There has been much written about the inaccuracy of GPS elevation data (eg here), so I definitely see your point. Still, allowing for the inherent inaccuracy, it would still be nice as an option to enable the display of GPS altitude, along with perhaps a confidence or error estimate to help you understand what limitations might be caused by the positions of the GPS satellites in view at that point in time.
 
Actually it isn't too hard to calculate. I live at 8200ft/2500m MSL (ASL). I have a mountain 1 mile from my house that is 9078ft/2766m MSL. For me to get to the top I would need to fly at 878ft/267m AGL (Above Ground Level), but would be more comfortable to fly at 950ft/290m to clear trees at the top. Just take the height of what you want to fly to, subtract your altitude of where you're launching from and you know how high to fly. In my case: 9078ft-8200ft=878ft.
Thanks for the suggestion. As you say it isn't hard to calculate but does require a little planning in advance to find your take-off elevation. Also, keep in mind that if you use a GPS device to figure out your take-off elevation, you will suffer from the same inaccuracy in GPS elevation that PhiliusFogg mentioned the drone has, so you have the same dangers in getting the AGL number wrong... The most accurate way would be to use a topo map to determine both your take-off elevation as well as the elevation of the mountain top you wish to fly to. Subtract the two and there's your AGL number.
 
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I am guessing this "Altitude" you see in the video caption data is in fact "GPS elevation", as it's displayed to 6 decimal places like the GPS lat/lon.
It most likely comes from the same source as the Exif "GPS" Altitude, which isn't GPS at all for most DJI drones.
It's worse the GPS data and it would not be possible to give any error estimate.
Believe it or not, what's labelled in Exif as GPS Altitude comes from a rough conversion of atmospheric pressure assuming standard atmospheric pressure.
The error is all over the place, varying as local air pressure does.
The example I posted in #3 shows a "GPS" Altitude of -68 metres when the drone was actually 55 metres above sea level - an error of 123 metres (400 ft).
 
Thanks for the suggestion. As you say it isn't hard to calculate but does require a little planning in advance to find your take-off elevation. Also, keep in mind that if you use a GPS device to figure out your take-off elevation, you will suffer from the same inaccuracy in GPS elevation that PhiliusFogg mentioned the drone has, so you have the same dangers in getting the AGL number wrong... The most accurate way would be to use a topo map to determine both your take-off elevation as well as the elevation of the mountain top you wish to fly to. Subtract the two and there's your AGL number.
Oh I totally agree with you. There will always be some error in GPS accuracy and always need to account for it. I always try to err on the side of caution so if my calculations would work out to be 100ft for example, then I'd probably fly at 150ft just to be safe. I also agree that it would be helpful if the MSL would be displayed but I'm sure you'd have to build in an extra margin of safety with that also.
 
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