Hi Eveyone
I set up my M210 V2 RTK up following the instructions but seem to have missed something as I have issues on the current positioning of the altitude. When the drone is still on the ground my altitude shows as a 100? Should this not show zero? Now I am also concerned about if the longitudes and lattitudes are true values?

I processed the info info and

I am flying to capture and pocess the data so that I can export the x y and z values and providing the information to the engineers / surveyors.

Thanks for reading

Hi,

we just bought the 210 RTK v2. If the base station is propperly connected (linked) there should be no problem. You can check the link status in the RTK tab on the DJI Pilot App. The image tag (EXCIF/XMP) has two height values - absolute and relative altitude.

The relative altitude is the height above ground (your starting position).

The absolut height is the GPS height meaning the geometrical height above a reference ellipsoid (as defined in WGS84). This height has absolut no relation to a height systems used for surveying purposes, because those heights do not have any physical properties, such as that water should not flow between two points of equal heights. So those heights need to be transformed to the height system your surveyor uses. In Germany for example we have the DHHN2016 Height System, which is based on GCG2016 Geoid Model.
Iam working with Agisoft (Pix4D is similar) for our applications and here is what I do, maybe this helps:

1. Get the Geoid Model for your country. Some are free to download, that depends on the country. In Germany it is not free of charge.

2. In Agisoft you can define coordinate transformations for horizontal positions (converting Latitude/Longitude into the official coordinate system of your country, in Germany we have ETRS89/UTM-projection (for x andy)  and here you can also add a geoid modell. This geoid model usually is a grid file and used to interpolate the geoid undulations at the Latitude and Longitude GPS positions. The geoid undulations are the "heights" of the geoid above the reference ellipsoid. Now every height (GPS measured) will be transformed to the proper height system by simply subtracting the undulation from the GPS height (Height_surveyor=height_gps - geoid_undulation). Off course you can also do that height interpolaton step with a simple script (matlab or octave).

3. I only see one slight problem you should be aware of. You can not change the coordinate system of your mobile DJI RTK station. So this base station will give geographical coordinates (Lat,Lon,H) in WGS84. On the other hand most downloadable geoid models are not in WGS84. In Germany for example the GCG2016 geoid model (which is pretty accurate) is referenced to ETRS89/DREF91 (Realization 2016, Europe) and this system has an offset of approximately 50-60cm to WGS84 (increasing a couple of cm every year due to continental drift) . In Germany this is not so much of a problem, because of the relativly small offset and regarding the fact that the absolut accuracy of the RTK base station is within meters anyway (not to mention the resolution of the geoid model). But this can look different in your country.

Alternativly you can use an official GPS correction service and enter this account into the DJI Pilot app under RTK options (in Germany this service is called SAPOS).  In this mode you actually dont need the mobile RTK base station at all as the drone will receive correction data from the provider. But I have not fully tested this option at the moment.

Most important thing at all: work with GCPs, so you are able to validate everything.

Hope this helps,
Felix