Mavic2: GPS accuracy

[Julmo01]

Hi,
I was expecting a "standard" GPS accuracy in terms of absolute accuracy (say ±5 m) , when I remarked once that the EXIF of my images was showing GPS Altitude "Below Sea Level" !!! ... OK, I live on an island next to the sea...
So, i did the following test:
I went in a place where there is an Elevation Benchmark from the National Geodesy Institute... The elevation is given to be +50m above sea level.
All my drone test were done in full open skies: no house, no trees around the takeoff point, at the summit of a small hill...
1st test:
I wait 2-3mn after drone & RC powering to make sure that the GPS sats are fully acquired (13 to 16 were visible) and elevation solution is OK, wait for "RTH point has been updated" message, I started the flight... once 20m above RTH, I took a photo, I was expecting the GPS altitude to be around 20+50= 70m AMSL
Surprise on the EXIF >> GPS Altitude: 0.2 m Above Sea Level instead of +70m AMSL
The Relative altitude (above RTH) was given at 20 meters, meaning that the DGPS from RTH was correctly calculated
Conclusion: The GPS initialisation after 2-3 mn was calculated below sea level, at around -70m Below MSL
2nd test:
I changed place and wait up to 4-5mn after drone & RC powering to make sure that the GPS sats are acquired (same number 15 were visible) . Again wait for "RTH point has been updated" message, I started the flight... once ±75m above RTH I took a photo , I was again expecting the GPS altitude to be 75+50= 125m AMSL
Surprise on the EXIF >>
Absolute Altitude : +43.90
Relative Altitude : +74.60
GPS Altitude : 43.9 m Above Sea Level
Conclusion: The GPS initialisation after 4-5 mn was calculated below sea level, now at around -80m Below MSL

I started to think that the so called "GPS altitude" was affected by the geoidal elevation... but in my area (35.87 14.56) the geoidal value is around 37m... nothing to see with 70m to 80m ....

So, I have no clue of what happened to my Mavic 2 GPS to be so poor in Altitude accuracy, it should be in a range of
5 -10 m in my experience!!

Some ideas/opinion or help?
Thanks

 

[Meta4]

I was expecting a "standard" GPS accuracy in terms of absolute accuracy (say ±5 m) , when I remarked once that the EXIF of my images was showing GPS Altitude "Below Sea Level" !!! ... OK, I live on an island next to the sea...
So, I have no clue of what happened to my Mavic 2 GPS to be so poor in Altitude accuracy, it should be in a range of
5 -10 m in my experience!!

Your "GPS Altitude" in the Exif info does not come from GPS at all.
It's a number derived from the barometric sensor data roughly converted to a very approximate height above sea level and is commonly out by 200 ft or more.

 

[Julmo01]

Your "GPS Altitude" in the Exif info does not come from GPS at all.
It's a number derived from the barometric sensor data roughly converted to a very approximate height above sea level and is commonly out by 200 ft or more.

Not 100% sure...
One can ask himself when you see in the EXIF (image) these 3 information together, I cannot imagine that the GPS Alt, GPS Lat, GPS Long are not from the same sensor.
In my example we are talking about nearly 300Ft error... As when the drone shows +43,90m (+145Ft AMSL) she is really flying at 125m (410Ft AMSL)
Anyway thanks for the comment. I'll dig a bit more... to make sure it's barometric one, which would then be acceptable for the range of error, but in that case DJI should write BARO Altitude, no?

 

[Meta4]

Not sure...
one can ask himself when you have these 3 information together in the EXIF (see image)
In my example we are talking about nearly 300Ft error... As when the drone shows +43,90m (+145Ft AMSL) she is really flying at 125m (410Ft AMSL)
Anyway thanks for the comment. I'll dig a bit more...

This is all well known.
You haven't uncovered anything new.

 

[nafisgal]

True ... the altitude that our drone shows us originates in the measurement of atmospheric pressure (barometric pressure) and logically it can be as exact as we would like ..
If it were even by GPS or altimetry (radio signal) the values would be very different and / or more precise ...
It is an estimated AMSL altitude value (No AGL ... if we had an altimeter ... it would be set at 1013 hPa barometric pressure).
In short ... it is a value that is not exact and varies according to atmospheric conditions ...

 

[Bigbird48]

so when it says we reached max alt 400 ft are we really 400 ft agl LOL

 

[MaxE1955!]

Your "GPS Altitude" in the Exif info does not come from GPS at all.
It's a number derived from the barometric sensor data roughly converted to a very approximate height above sea level and is commonly out by 200 ft or more.

Good to know, I was unaware of that!

 

[Meta4]

so when it says we reached max alt 400 ft are we really 400 ft agl LOL

The OP was asking about the altitude data stored in EXIF data for each image file.
It's quite different from the altitude displayed on your screen which is quite accurate.

 

[fisklask]

so when it says we reached max alt 400 ft are we really 400 ft agl LOL

In my experience relative altitude is precise enough, however I can’t support this claim, never have that properly tested.