Incorrect EXIF GPS and Elevation

[Paulo Lobo]

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[TheAllrounder]

Well, I am referring to your image in post #16. My problem is, that I do not see where you get your data from. From the Exif file, I am only able to get relative and absolute altitude values. So could you please explain that a bit closer? I am really trying to understand what you are saying but I think I am missing a big point somewhere...

 

[sar104]

Well, I am referring to your image in post #16. My problem is, that I do not see where you get your data from. From the Exif file, I am only able to get relative and absolute altitude values. So could you please explain that a bit closer? I am really trying to understand what you are saying but I think I am missing a big point somewhere...

The data in the graph are from the flight DAT file, which does record GPS altitude AMSL. The EXIF file only has absolute and relative barometric altitudes, even though it incorrectly labels one of the absolute altitude fields as "GPS altitude".

 

[AMann]

Mavic 2 Pro flight and pictures excellent, but image EXIF data shows altitude was a bit over 115 million meters...which would be impressive if true. Lat/Lon also consistently off by about 3/4 mile. This matters when feeding pics into Pix4D photogrammetry software. Anyone else check their EXIF yet?

What accuracies are you expecting from the drone’s GPS? Its not good enough for orthophotos anyways, especially if you want submeter accuracy. Altitude would be even worse, depending not only on satellite numbers, but their geometry. If they’re too close together, your errors get worse. If it were used for true altitude, I think it would still affect your results. I have been depending on using ground control points as reference and put a good GPS unit on them, and adjust later during post processing.

 

[queeg730]

"This is so so so bad for dji image..."

To maybe 10 people worldwide.

Make that 11 people..... I have been following this post, and have exactly the same issue.....
This is not imagination.... there is a problem here, and it has been going on for months (although I must admit that I haven't tested it since the last firmware update).
This is a DJI issue on quite an expensive machine..... it MUST be fixed

 

[sar104]

Make that 11 people..... I have been following this post, and have exactly the same issue.....
This is not imagination.... there is a problem here, and it has been going on for months (although I must admit that I haven't tested it since the last firmware update).
This is a DJI issue on quite an expensive machine..... it MUST be fixed

But you haven't actually read the thread - right? There is absolutely no mystery here - the contents of the EXIF file are perfectly clear and, other than the "GPS" label, are correct. You have relative and absolute barometric altitudes. There is nothing to fix. DJI could add the actual GPS absolute altitude - is that what you are asking for?

 

[AMann]

Make that 11 people..... I have been following this post, and have exactly the same issue.....
This is not imagination.... there is a problem here, and it has been going on for months (although I must admit that I haven't tested it since the last firmware update).
This is a DJI issue on quite an expensive machine..... it MUST be fixed

These drones do not have a survey grade GPS in them, and even though they are WAAS enabled, you will get errors measuring altitude as much as +- 10m to 15m depending on satelite geometry.

 

[sar104]

These drones do not have a survey grade GPS in them, and even though they are WAAS enabled, you will get errors measuring altitude as much as +- 10m to 15m depending on satelite geometry.

Actually that's not generally the case. Having finally got around to testing vertical accuracy it turns out that the inherently good sky view that the aircraft tends to have results in vertical errors of only a couple of meters, most of the time. And, in any case, that's not the issue in this thread, because the GPS elevation data are not in the EXIF file - only barometric data.

 

[AMann]

Actually that's not generally the case. Having finally got around to testing vertical accuracy it turns out that the inherently good sky view that the aircraft tends to have results in vertical errors of only a couple of meters, most of the time. And, in any case, that's not the issue in this thread, because the GPS elevation data are not in the EXIF file - only barometric data.

I meant if gps was used in EXIF, also depending on PDOP (satelite geometry), 3D altitude measurements can vary that much. You are right thogh, the use of barometeic pressure relative to the takeoff point is much more accurate for the drone to navigate vertically.

 

[AMann]

I meant if gps was used in EXIF, also depending on PDOP (satelite geometry), 3D altitude measurements can vary that much. You are right thogh, the use of barometeic pressure relative to the takeoff point is much more accurate for the drone to navigate vertically.

Just for those who dont know - Sky view and number of satellites is only part of getting a good GPS fix, if the satellites are close together during a pass, they are more difficult for the GPS to triangulate its position, this is known as Position Dilution of Precision (PDOP).

 

[sar104]

I meant if gps was used in EXIF, also depending on PDOP (satelite geometry), 3D altitude measurements can vary that much. You are right thogh, the use of barometeic pressure relative to the takeoff point is much more accurate for the drone to navigate vertically.

Actually I didn't mean that either - it's pretty clear from the DAT files that the GPS altitude data are far more accurate than the absolute barometric altitude data (not surprising) and at least as accurate as the relative barometric altitude change data - and more accurate when the atmosphere is locally different than a standard atmosphere.

The pDOP values for all the flights that I looked at easily reached 2, which means that in practice it's not really a limiting factor.

 

[Point Zero]

I have learnt a whole bunch reading this thread. You guys really know your biscuits lol.

So if someone wanted to get into some 3D mapping and having read this thread, is there any particular software package that does the job to a professional standard that you could recommend, or would the user always have to correct the data themselves to get the best results?

 

[AMann]

that is a good PDOP value for measuring it, I frequently had higher values with older units that did not use as many satellites as these do. Older Magellan and Trimble survey units were notoriously off with their 3D measurements due to this and we often had to schedule our fieldwork around it.

 

[AMann]

I have learnt a whole bunch reading this thread. You guys really know your biscuits lol.

So if someone wanted to get into some 3D mapping and having read this thread, is there any particular software package that does the job to a professional standard that you could recommend, or would the user always have to correct the data themselves to get the best results?

Ive used Agisoft’s Photoscan and ArcGIS, Photoscan has a less expensive lite version and a free 30day trial for the full and lite versions if you want to play with it.

I believe DroneDeploy also has a mapping tool, and there’s cloud based mapsmadeeasy.com that gives you a free trial and is a pay as you go based after the trial.

As for correcting GIS data, not sure- I use one called Pathfinder by Trimble. There are probably open source post-processing programs out there too.

 

[Point Zero]

Ive used Agisoft’s Photoscan and ArcGIS, Photoscan has a less expensive lite version and a free 30day trial for the full and lite versions if you want to play with it.

I believe DroneDeploy also has a mapping tool, and there’s cloud based mapsmadeeasy.com that gives you a free trial and is a pay as you go based after the trial.

As for correcting GIS data, not sure- I use one called Pathfinder by Trimble. There are probably open source post-processing programs out there too.

Thank you for this wealth of info! That will get me going just fine. I cant wait to try this out and see the results.
Thanks again @AMann

 

[AMann]

Thank you for this wealth of info! That will get me going just fine. I cant wait to try this out and see the results.
Thanks again @AMann

You are welcome. Just don’t let your jaw hit the floor too hard when you see the price of some of that stuff.

 

[Point Zero]

You are welcome. Just don’t let your jaw hit the floor too hard when you see the price of some of that stuff.

lol, gulp, is it really that pricey! Is the price tied to different levels of accuracy and stuff like that?

 

[AMann]

lol, gulp, is it really that pricey! Is the price tied to different levels of accuracy and stuff like that?

Yeah, I used it at work so they bought it. There are open source versions of similar software, I will post here when I can get to my home computer.

 

[Point Zero]

Yeah, I used it at work so they bought it. There are open source versions of similar software, I will post here when I can get to my home computer.

Nice one!

 

[kadras]

This from AMann, As for correcting GIS data, not sure- I use one called Pathfinder by Trimble, reminded me that Trimble also has GPS planning software to see what GPS conditions will be like when planning surveys. It could be used for flying conditions, also. The calendar goes back in time, but I'm not sure whether it loads old ephemeris data or just uses current data (more likely). I have used this for planning ground, not airborne, mapping projects. And on days when I've forgotten to check before going into the field, I've gone back afterwards (the same day) to look at the sat data to compare them to times when my results seem to be a bit off.

This site has a lot of parameters than have to be set to be useful. The results can be shown in "map form" as a sky plot or world view. There are charts showing time of day vs. satellite altitude, number of satellites, total and component DOPs, satellite visibility and ionospheric conditions. Under the information symbol in the upper right corner is a link to a page that explains what the ionospheric conditions are and what their values mean.

Links to online versions, for both desktop and mobile, are here: Support | Spectra Geospatial Scroll down to the links under this header "Trimble GNSS Planning Online Website" Both desktop and mobile versions (not an app, but a mobile version of the webpage) are available in multiple languages.