Altitude above sea level during flight

[Meta4]

I didn't see anything in the OP about accuracy. The question was if GPS altitude was available. It is in the logs.

I went to some effort to explain in post #10 that there's no such thing as "GPS Altitude" with DJI drones and explain what shows as "GPS Altitude in the Exif info.

The GPS altitude in DJI drones is as accurate as any other GPS reciever. 10-20m accuracy certainly isn't suitable for landing an aircraft. It may be plenty accurate for locating the point in space where a photo was taken, depending on the application.

What DJI label as GPS Altitude isn't accurate at all and can have an error of hundreds of feet.
That's not all ... it varies from day to day, with the weather.

Look at the Absolute Altitude shown in the metadata for these two shots of the same channel marker taken 7 months apart.

If the number actually came from GPS, do you think it would show this kind of variation?
The heights are shown on metres.
Where sea level is is obvious in the pictures and you shouldn't need numbers to tell the actual height of the drone (approx 25 ft).
-16.5 metres = 54 feet below sea level.
-120 metres = 394 feet below sea level !!!

 

[Yorkshire_Pud]

I am surprised no one suggested using Litchi to do repeat missions. Maybe I missed something?

post 13 ?

 

[WanderLost]

I understand the height shown on my display while I fly my DJI Mavic 2 drone is from launch point. What I would like to know is if there is a way, while I am flying (not when I download and view on my computer later), to see my actual GPS altitude above sea level. I am trying to replicate the same GPS altitude to retake photos from virtually the same location as before. I can view specific details of exact location and GPS altitude from the properties of the prior photos on my computer, but when I am in the field and flying I seem to only be able to see the elevation above launch point. Does anyone happen to know a way I can see GPS altitude during flight? Thanks for any help.

Maybe it's time to buy an Enterprise drone with RTK. With the proper corrections from something like a DJI D-RTK 2 Mobile station that would allow you to have centimeter level precision. You could fly all your waypoints on different occasions and not worry at all about barometric air pressure changes, etc. and have the accuracy you want.

 

[AMann]

Maybe it's time to buy an Enterprise drone with RTK. With the proper corrections from something like a DJI D-RTK 2 Mobile station that would allow you to have centimeter level precision. You could fly all your waypoints on different occasions and not worry at all about barometric air pressure changes, etc. and have the accuracy you want.

Centimeter accuracy horizontally, a prosummer grade RTK drone still does not have vertical accuracy - only of about 3cm or something like that. And it would have to hover stationary in the air for a while to get that good of a vertical fix. Only the much more expensive survey-grade drones can get 1.5-3 cm vertical accuracy.

The barometric measurement is still the most accurate a consumer drone can do. And you really don’t have to worry about barometric pressure changes if you use the same home point to start from each day. Your drone will start out with a fairly good barometric height AGL estimate any day you fly from the same home point because it resets the barometer when taking off from ground level. Then your controller shows how high it is above that point as it’s flying, Litchi uses that information as well.

 

[Droning on and on...]

It's a fact that DJI drones record GPS altitude in the log file. Please don't take my word for it, examine one yourself.

It appears it's as accurate as GPS altitude is typically known to be.cThis is neither "accurate" or "inaccurate", as some have stated here. It is a measurement with known error range, just like the barometric altimeter.

This error range may, or may not be accurate enough, it's application dependent. For example simply wanting to have an altitude within a hundred feet of how high you are on a mountain when a flight took place. "this picture was taken at 6500' looking out from the north slope of Mt. Reallytall..."

A firm belief that this information is too inaccurate to be useful might rather be a lack of imagination...

 

[AMann]

It's a fact that DJI drones record GPS altitude in the log file. Please don't take my word for it, examine one yourself.

It appears it's as accurate as GPS altitude is typically known to be.cThis is neither "accurate" or "inaccurate", as some have stated here. It is a measurement with known error range, just like the barometric altimeter.

This error range may, or may not be accurate enough, it's application dependent. For example simply wanting to have an altitude within a hundred feet of how high you are on a mountain when a flight took place. "this picture was taken at 6500' looking out from the north slope of Mt. Reallytall..."

A firm belief that this information is too inaccurate to be useful might rather be a lack of imagination...

But if you’re trying to re-create a photo, say of buildings close by, even minor differences in height AGL will easily show in the photos.

 

[MS Coast]

This error range may, or may not be accurate enough, it's application dependent. For example simply wanting to have an altitude within a hundred feet of how high you are on a mountain when a flight took place. "this picture was taken at 6500' looking out from the north slope of Mt. Reallytall..."

A firm belief that this information is too inaccurate to be useful might rather be a lack of imagination...

Or possibly, "This picture was taken at a depth of 394 feet at the number 4 channel marker at ..."

For typical drone operations like the one the OP described, the so-called GPS altitude doesn't seem to have much value. With the red marker and Mt. Reallytall examples here, it seems that a similar or better estimate of the altitude a drone photo was taken at could be obtained by consulting a map rather than relying on the GPS altitude figure.

 

[Meta4]

It's a fact that DJI drones record GPS altitude in the log file. Please don't take my word for it, examine one yourself.

I won't take your word for it because I've examined the data closely.
Did you see post #21?

It appears it's as accurate as GPS altitude is typically known to be.
...
A firm belief that this information is too inaccurate to be useful might rather be a lack of imagination...

Stop using that imagination and read posts #10 & #21.
There is no "GPS Altitude" and the number you are thinking is "GPS Altitude" is junk and doesn't come from GPS.

 

[WanderLost]

Centimeter accuracy horizontally, a prosummer grade RTK drone still does not have vertical accuracy - only of about 3cm or something like that. And it would have to hover stationary in the air for a while to get that good of a vertical fix. Only the much more expensive survey-grade drones can get 1.5-3 cm vertical accuracy.

The barometric measurement is still the most accurate a consumer drone can do. And you really don’t have to worry about barometric pressure changes if you use the same home point to start from each day. Your drone will start out with a fairly good barometric height AGL estimate any day you fly from the same home point because it resets the barometer when taking off from ground level. Then your controller shows how high it is above that point as it’s flying, Litchi uses that information as well.

I understand which is why I suggested an Enterprise drone like the Mavic 3E.

 

[pbr19]

Is there a handheld device that can give you the altitude of the take off point and then can calculate the driver's altitude from your height on the screen? Just asking.

My Elevation app

 

[jaja6009]

With DJI drones, any Enterprise drone that has the ability to receive corrections via an RTK module, the elevations in the image metadata are in ellipsoid elevations (GNSS), but it will also have the Relative to takeoff value in the Relative Elevations part of the metadata. If the Enterprise drone is not receiving corrections it will still give elevations in ellipsoid elevation, but it will be as inaccurate as a consumer type GPS elevation (cell phone, GNSS receiver not receiving corrections). Think 1-15 meters with the 1 and 15 being extremes.

This is easily proven.

Here is a map derived from a Mavic 3E that was receving corrections in NAD83(2011) with ellipsoid elevations, processed in Agisoft Metashape. Low centimeter accuracy as compared to Check Points and having the Point Cloud overlaid onto state produced, authoritative LiDAR data.
That's me on the left and in the tents are people sleeping on the beach somewhat illegaly.


Control Points were measured by an Emlid RS2 receiving corrections in NAD83(2011) with ellipsoid elevations.
Control Point 1 on the left has an ellipsod elevation of -31.937 meters. This number is negative because in my area the ground is below the ellipsoid.

The flight height for the mission was 200ft AGL relative to the takeoff point, which was the aforementioned Control Point.

All of my images are around 28 to 29 meters above the ellipsoid as shown here in Agisoft.



This 200ft for the flight mission was obtained by the barometer and is listed as the Relative Altitude.
The ellipsoid elevation was obtained from the GPS Altitude

200 feet = ~ 61 meters

Take my takeoff spot's ellipsoid elevation of ~ -32 meters (It took off from the Control Point) and add the flight height of ~ 61 meters and we have ~29 meters which matches the elevations in my images imported into Agisoft automatically. The accuracy readings are the GNSS Standard Deviation as reported by the GNSS module receiving corrections.

I have over 100 maps that will all show this to be true.

If further proof is needed, tomorrow I will take the Emlid Reach RS2 and either a Mavic 3E or P4RTK and compare the current position of the Multi-Constellation, Multi Band GNSS rover with images taken by either of the aforementioned drones sitting next to it and show that indeed, Enterprise drones do use GNSS for their image elevation and this is in ellipsoid elevation.

Further, I previously used a P4PV2 to map with an installed PPK kit from BAAM. With this kit I could use the aircraft to mark Control Points by placing it on the point, having the X grid on screen, take an image of the point nadir and then factor in an offset. I would obtain the Control Point's coordinates and ellipsoid elevation from the image metadata and apply an offset to account for the location of the GNSS module on the aircraft. Using this method was compared to an Emlid RS2 on the same Control Point and they were always within sub cm.


OP, this will not work for you since, your non-Enterprise drone uses some straight up incorrect vodoo in its reported GPS elevation/altitude.

 

[123taff]

I am in the UK and use Memory Map software on my PC and "Memory Map for all" app on my iphone . I have Ordinance survey 1:25k mapping of the whole of the UK. The cursor position on the map not only gives the British national positioning location, but the elevation above Newlyn mean sea level. I can plan a route on my PC software and then transfer it to memory map so that using the iphone I can go to the exact spot where the SL altitude is known.
If I take off from there then I then have the drone data of the height of the flight and it would be pretty simple to replicate that on a subsequent flight should I want a comparison.
Don't know if that suggestion might be helpfull?

 

[jaja6009]

To use a different example instead of a drone derived map I conducted another example.

I placed a P4RTK and an Emlid RS2 next to each other outside. I was up against my house so the P4RTK was in and out of FIX. Both were receiving corrections via NTRIP from a paid account from a local provider that sends corrections in NAD83(2011) ellipsoid elevations.



Emlid RS2 in FIX showed an ellipsoid elevation of -31.184 m below the ellipsoid.





In the RTK menu of the P4RTK controller with a FLOAT I had -31.181 m below the ellipsoid. The elevation jumped around due to the FLOAT status.

Sorry it was getting dark out and my hand was shaky hence the blur.



I took an image and the image metadata showed in GPS Altitude -31.3 meters below ellipsoid. Again this number jumped around due to being FLOAT, but had previously been FIX for a few seconds.




Upon import into Agisoft Metashape:

 

[Droning on and on...]

@jaja6009 not entirely sure what you're showing... Is the Agisoft Metascape software showing the GPS altitude from the drone's on-board GPS, stored with the picture? So this is compared to the extremely accurate RTK position, and its nearly as accurate?

 

[jaja6009]

I am showing that the camera'a metadata on Enterprise DJI drones only, gives an elevation obtained from the RTK (GNSS module and this is in the Absolute Altitude and GPS Altitude fields in the metadata.

GNSS natively reports first as ECEF (Earth Centered Earth Fixed) (XYZ) coordinates. Next software will take the XYZ and give coordinates as a geographic (3D) coordinate system with horizontal measured in degrees, minutes and seconds and elevation measured as above or below a theoretical ellipsoid, in this case NAD83(2011) and the GRS80 ellipsoid since my RTK module was receiving corrections in that datum and coordinate system. A further step can be added to take this ellipsoid elevation and transform it to an elevation that makes sense, an orthometric elevation or one over a geoid that represents sea level as compared to where sea level should be due to the gravity of the earth. This further step is completed by my photogrammetry software. This elevation would be NAVD88 using the newest geoid, Geoid 18.

This will not help the OP as just like Meta has stated and proven, non-Enterprise drones report in the GPS Altitude an elevation based on "who knows" and is wildly inaccurate.

 

[Thelastosirus]

Get a cheap rangefinder from Walmart. It might take a few tries but it should be able to pick up a M2 sized object in the air. Or go to Home Depot and get a Bosch laser measure. But those typically only go up to 165' but accurate to the inch.

 

[Filmarik]

I understand the height shown on my display while I fly my DJI Mavic 2 drone is from launch point. What I would like to know is if there is a way, while I am flying (not when I download and view on my computer later), to see my actual GPS altitude above sea level. I am trying to replicate the same GPS altitude to retake photos from virtually the same location as before. I can view specific details of exact location and GPS altitude from the properties of the prior photos on my computer, but when I am in the field and flying I seem to only be able to see the elevation above launch point. Does anyone happen to know a way I can see GPS altitude during flight? Thanks for any help.

The simplest way to replicate taking photos from the same point is using waypoints and taking off from exactly the same Home Point when replicating the flights. As for knowing the altitude of your drone during flight, the reading on your display is the only info you have available. It is derived from barometric altimeter built in your drone. It is accurate to about +/- 1m when more than 10m AGL but the longer you fly the greater the error might get, especially at long distance from Home Point or when the drone gets into vastly different environment i.e. very hot or cold temperature which can affect local barometric air pressure.
Caveat:
My 2c worth. If this is a paid job and the accuracy of the drone's altitude down to a few inches is a paramount requirement, you need to disclose to your client that you would NOT be able to guarantee that accuracy. Do not ask me how I know

 

[Droning on and on...]

My 2c worth. If this is a paid job and the accuracy of the drone's altitude down to a few inches is a paramount requirement, you need to disclose to your client that you would NOT be able to guarantee that accuracy. Do not ask me how I know

Or use the right tool for such a precision job: RTK.

 

[Meta4]

If this is a paid job and the accuracy of the drone's altitude down to a few inches is a paramount requirement, you need to disclose to your client that you would NOT be able to guarantee that accuracy.

Or use the right tool for such a precision job: RTK.

With the wideangle lens of the drone, for the job the OP described a height difference of +/- 5 metres won't make any discernable difference.

 

[Filmarik]

With the wideangle lens of the drone, for the job the OP described a height difference of +/- 5 metres won't make any discernable difference.

Based on the info provided by OP it might or might not be the case. If those photos are to be used as evidence in a litigation process, accuracy and hence altitude from were the photos were taken can be of a crucial importance.
I'd suggest to let the OP to be the judge of that..