It is impossible to do terrain following without GPS being involved. Period. The SRTMs that are used are not +/- values, they are relative to local mean sea level. The only negative values you would receive at that point are under the water which don't apply.
Negative Altitude Data
- Thread starter pcc01
- Start date
It is impossible to do terrain following without GPS being involved. Period. The SRTMs that are used are not +/- values, they are relative to local mean sea level. The only negative values you would receive at that point are under the water which don't apply.
I'm sorry - I've lost track of what you are arguing. My post was specifically on the EXIF altitude data fields. What does terrain following have to do with that?
Terrain following references the fact that in cases the altitude can be derived including GPS. This relates to the OP. We are determining how they received negative values. I am pretty sure that they are talking about the map, because there is no way to write negative altitude data in the exif.
The OP already confirmed in post #10 that he was referring to EXIF data. And it's perfectly possible to write negative AMSL altitude in the EXIF data. @Meta4 even provided an example of that in post #11.
That's great. We must be talking about different things. Is DJI just using a static barometric pressure? How are they deriving the GPS value if they are apparently always the same. Shouldn't there be at least an occasional variance? You would have to be fairly close to MSL to get a negative value?
There are two variants in the DJI firmware. The older aircraft and older firmware on newer aircraft (such as the Mavic 2) did not use any GPS altitude data in the EXIF record, even though one of the fields was labeled GPS altitude. That field contained the same barometric absolute altitude that was in the other absolute altitude field. That's calculated from the pressure measured by the onboard barometric sensor related to a standard atmospheric model. The actual atmospheric conditions often deviate significantly (especially the altitude-temperature profile) from the standard model, which is the major source of error, leading to discrepancies of up to a few hundred meters.
The newer firmware corrected this, and actually uses GPS altitude in the GPS altitude field and the absolute altitude field, as explained here.
So yes - you would have to be within a couple of hundred meters of sea level ever to see negative values.
Similar threads
