Hi Robert and Sam,
The images that are saved on the drone’s SD card are geotagged by DJI.
Regarding the planar coordinates, the drone’s GPS save reliable information in the image EXIF. There is no problem from that side.
However regarding the altitude there might be some inaccuracies depending on the location when you are mapping.
According the latest news on our side that refer to the firmware v.220.127.116.11 and v1.7.0060 for the Inspire I and Phantom 3 Pro respectively, DJI measures the absolute elevation above sea level using EGM 96 as the reference.
They used to only record the elevation above ground level but this is no longer the case.
However the vertical coordinate is still not fully reliable. Indeed, we made testing in our office here and we found that the vertical coordinate is off by several meters that can reach an error of 100 meters. Our users noticed the same.
Therefore our developers suspect something wrong in the DJI EXIF regarding the vertical coordinates.
As a consequence, we always recommend to process with ground control points (GCPs) in order to fix these uncertainties.
About using GCPs: https://support.pix4d.com/hc/en-us/articles/202558699
Note that DJI recently released new versions of the firmware v1.8.1.00 and v1.8.80 for the Inspire I and Phantom 3 Pro respectively.
We do not know so far if they did change something with the vertical coordinate but perhaps they effectively did I fixed the issue.
For more information about the firmware:
We hope this is clear and helps in understanding the situation.