Look again in the EXIF data using the Microsoft Notepad App to open the .JPG file. Look in the top block of mostly unreadable lines. Look at the last line of this block. Scroll over to the right and you will find human readable text added by DJI. There you will find RelativeAltitude and AbsoluteAltitude fields showing altitude in meters. The RelativeAltitude field is used in the photogrammetry calculations by DroneDeploy. The AbsoluteAltitude field is used to set the elevation you see when clicking on a location of your map in DroneDeploy. The GPS Altitude field in the EXIF is a copy of the DJI AbsoluteAltitude field. It is not a real GPS altitude. The GPS altitude is NOT used by DroneDeploy at all. You can zero it out and it will have no effect. I did an experiment, by changing the data in the EXIF, to verify this.
The RelativeAltitude and AbsoluteAltitude fields are based upon the on-board barometer in your (and my) P4P. Take a picture with the P4P while it is setting on the ground. Then move it up to a table top (say 3' higher). Take another picture. Look at the EXIF data of the 2 pictures with Notebook. You will see the RelativeAltitude and AbsoluteAltitude fields have increased by about 1 meter (P4P stores altitude values in meters).
If the P4P stays in one place for some time and the barometric pressure changes, the recorded RelativeAltitude and AbsoluteAltitude values may not change because the on-board IMU says there was no movement during that time and thus it keeps the same altitude and resets its barometric reference. This is what I discovered from my experiments.
Because the P4P uses the barometer, the recorded AbsoluteAltitude can change by 1000' for the same takeoff location over time because the barometric pressure could change by 1", a typical variation during the changing of the seasons or weather. Thus the absolute elevation reported by DroneDeploy when you click on a Location is useless. Mine have varied by 1000' for the same takeoff location over a 1 month period. But the relative elevation can be very good; I have seen less than 6" error for spots on concrete surfaces separated by 150'. The concrete has an aggregate surface so this may help the photogrammetry do better.
Let me know if you have any more questions about this. I am working with DroneDeploy to get an option into the workflow that allows you to set the absolute elevation so you can get sea-level correct elevations on your map (at least for one location you choose). You can do this now with my Auto-Calibration Technique described in an earlier post to the Forum: Accuracy problems in Oct 2017.