Google an EXIF reader and you will find free ones to download. Then you just use the program to open your image and you can see all of this metadata and even change it.
The elevation data recorded for our photos are inaccurate with the elevations being recorded as 'Below Sea Level'. It's hard to explain the specifics as I'm trying to figure it all out myself but I've been in contact with Pix4D Support about the negative elevations and they gave me details on how DJI algorithms store the elevation data.
Here is what Pix4D sent me:
"In the latest public DJI firmware (FW_V01.05.0030), it seems like DJI changed the georeference of the vertical coordinate.
They used to provide information above ground level (AGL) using the altimeter of the drone, but by default in the EXIF of the images that DJI writes, they define the datum as "Mean Sea Level". In the latest version they changed this setting to "Below Sea Level", which might be confusing. Apparently, they tried to fix the relative elevation and retrieve the absolute elevation but they probably made a mistake with the reference name.
There is a DJI forum post related to this issue:
DJI Forum|Photo,exif Altitude incorrect? Possible Firmware bug?
The consequences are the following:
- When Pix4Dmapper reads the geolocation contained in the EXIF, the software is misled and interpret "below" as a negative value. If the EXIF is edited, meaning "below" is replaced with "above", this fixes the problem.
- The vertical coordinate is still not fully reliable. Indeed, we tested it and even if the previous AGL altitude value is indeed smaller than the MSL altitude value (discarding the minus sign), we noticed a offset in the vertical coordinates.
Our developers still 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."