LV_Forestry
 First Officer
Flight distance : 4726654 ft
Latvia
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Following recurring requests on this subject, You will find these few lines that will potentially get you out of this problem.
I am not the first to comment on this problem, others have done so before me, I am thinking of @patiam who regularly intervenes to bring solutions, thanks to him. The idea being to centralize the response on a single thread, since it is always the same.
When you collect RAW data with a DJI drone/sensor. JPG/TIFF/Point cloud/P1/L1/L2/H20/M3/M2/mini... You must know that at least until today, DJI devices are geotaging produced data in WGS84 format, whether horizontal or vertical.
WGS84, What is that ?
We'll keep it short here, the earth is not round. That's it, I'm sorry to break certain beliefs.
If you survived this first step, consider that despite the fact that it is not round, its center is perfectly defined:
image source : BESTXYZ (novatel.com)
The WGS (World Geodetic system) coordinate system was created to standardize and simplify the display of coordinates at the global level (GNSS navigation system).
It has two big flaws:
-The first is that its "horizontal" unit of measurement is in arc second, it is an angular measurement, which makes it more difficult to manipulate than values in meters.
-The second is that its standardization does not at all allow us to respect sea level, which is level zero in a lot of "local" coordinate systems.
In the image below:
-The green curve is the one which represents the zero level of what we call the WGS ellipsoid. This green curve is the reference used by DJI to indicate the altitude in the EXIF of the images which is denoted "h"..
-The red curve represents the mean sea level (MSL) Which we call "Geoid". It is the reference, the zero level, of local coordinate systems based vertically on mean sea level.
The geoid (Mean sea level) is a function of gravity, remember the earth is not round, so the level of the oceans/sea on earth does not represent a perfect sphere. On the image below The altitude of the points referring to these local MSL coordinate systems is denoted "H".
image source : (7) (PDF) Evaluation of Open Digital Elevation Models: estimation of topographic indices relevant to erosion risk in the Wadi M’Goun watershed, Morocco (researchgate.net)
Unlike DJI equipment, GNSS measurement tools such as Trimble, Leica, Emlid, etc. are configurable and therefore have the ability to choose the reference system.
Be careful to configure your receiver correctly when marking GCPs!!
Let's take an example of point on earth.
N[size=11.088px]56°56'54.46246" E[size=11.088px]24°03'30.95078"
At this location there is a fundamental geodetic landmark whose coordinates are known and serve as a reference to the local system.
The altitude of this point is 8.49m AMSL (H) taking as reference the mean sea level as defined in the local system LKS92 LV14.
If you land your DJI drone on this point and take a photo, the geotag will indicate +/- 29.338m (h) (WGS84 vertical datum)
There is therefore a difference of 20.84m (N) between these two measurements, but physically it is indeed the same place! In the image above, this difference is noted "N". This is the value of the geoid LV14 at this location.
To align the rasters from DJI data, all you need to do is remove the geoid value corresponding to the coordinate system you want to use. H = h - N
You can confirm this result by watching the geoid value :
These geoids are computer files readable by "GIS" software (Metashape, QGIS, GlobalMapper. ArcGis...). You can download them from the websites of the geographical institutes in your country.
And finally there is a geoid taking as reference the average sea level which covers the entire globe, it is the EGM (Earth gravitational model) available here:
EGM96: The NASA GSFC and NIMA Joint Geopotential Model
Please note, this model does not offer the precision of local geoids, and therefore cannot replace them if you need ultimate precision.
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