Mavic 3E RTK (HAE) to orthometric coordinate system

Question

I am attempting to process imagery acquired with a DJI Mavic 3 Enterprise RTK using Drone2Map (v2023.1.1), and cannot figure out the correct combination of Image and Project vertical coordinate systems to get the DSM output in orthometric (ASL) elevation (NAVD88). I have reviewed multiple Drone2Map and DJI forums, support articles and blogs, and cannot come up with specific recommendations for the settings. For as popular as the M3E RTK is, I’m hoping someone has a specific workflow established, ideally inside of Drone2Map (no external manipulating of imagery coordinates or DSM). I have done several dozen D2M projects using GCP, but this is my first RTK project.

Specifications/Background:

DJI Mavic 3 Enterprise RTK (connected to Wisconsin’s WisCORS network).

ArcGIS Drone2Map Advanced v2023.1.1, with the ArcGIS Coordinate Systems Data (v.11.1.40209) installed.

Processing in Drone2Map with no GCP, and “Fix Image Location for High Accuracy GPS (RTK and PPK” selected.  (GCP were shot for the project, and could be used as checkpoints, but are not being used in the project. I would like to process using only the RTK for future projects.)

All attempts at processing have resulted in DSM elevations approximately 36.9 meters below the known/surveyed ground elevation. This exactly corresponds to the Height Above Ellipsoid (HAE) elevation. I have used Esri’s Sample_Heights_For_Earth_Terrain web mapping tool (as suggested in a blog) to confirm the HAE and ASL values for this position.

Inspecting the EXIF data for the imagery confirmed the altitude is indeed HAE (ground HAE plus 300 feet flight altitude), as is expected from DJI RTK imagery.

For the D2M Project (output) coordinate systems, I specified XY: WGS 1984 UTM Zone 16N (m) (WKID 32616), and Z: NAVD88 Height (m) (WKID 5703) to return orthometric/ASL elevations.

For the Image (input) coordinate systems, I specified the XY of WGS 1984 (WKID 4326).  Several postings said to select “Height Above Ellipsoid” as the vertical, but this appears to no longer be an option with v2023.1.1 and the Coordinate Systems Data program.

I initially tried the default vertical coordinate system of EGM96 Geoid (WKID 5773) under the “Gravity-Related / World” folder. This resulted in DSM elevations being returned in HAE values (36.9 meters lower than desired).

I then tried the WGS 1984 (WKID 115700) coordinate system under the “Ellipsoidal-based / World” folder, which produced the same results.

Has anyone come up with the magic coordinate system combination to get imagery from a M3E RTK (HAE) to NAVD88 (orthometric/ASL) in Drone2Map?

AlexPapiez · Answer

I use a DJI M3M with RTK and an Emlid RS3 which broadcasts corrections to the drone controller via Emlid's local NTRIP, and set up the RS3 base over a known point.

My GCPs (checkpoints) and Base Station Point were measured in NAD83(2011) State Plane WA South (WKID 6598) / NAVD88 Geoid18, unit=meters. In ArcGIS Pro I projected only the base station point to WGS84 Horizontal / WGS84 Vertical, then used the "add x-y data tool" to get WGS84 and ellipsoidal height coordinates for the base. These coordinates were entered into the base station broadcasting local NTRIP to the drone.

When I upload the images to Drone2Map and select a coordinate system I chose WGS84 / WGS84 as the Image Coordinate System.  For the GCPs and project coordinate system, I select NAD83(2011) WKID 6598 for the Horizontal Coordinate System, and NAVD88 (WKID 5703) for the Vertical Coordinate System.

However, in the drone2map processing report it shows a Coordinate System Transformation:

WGS_1984_(ITRF00)_To_NAD_1983 + NAD_1983_To_NAVD88_CONUS_GEOID12B_Height

My checkpoint X-Y RSME is 1-2 cm, however my Z RSME is 10 cm.  I am wondering if the higher Z RMSE is due to a vertical datum mismatch, potentially caused by ESRI's NAVD88 (WKID 5703) using Geoid12B instead of Geoid18? Since my GCPs were collected using Geoid18.

Also, is it necessary to enter the base coordinates as WGS84 and ellipsoidal height? Or could I use the same projected coordinate system as my GCPs: NAD83(2011) (WKID 6598) and NAVD88 Geoid18 orthometric height coordinates?

Thanks,

Alex

MarkBuie · Answer

RTK networks return corrections in a coordinate system defined by the RTK network provider. The reference system can usually be found by visiting the providers website or contacting them directly. I did a quick search and believe that WISCORS networks output their corrections in NAD83 (2011), but this should be confirmed with the RTK network provider.

Once the reference system is known, change the Image Coordinate System in Drone2Map to match the coordinate system of the RTK network (NAD83 (2011) / NAD83 (2011). The project coordinate system can be set to any coordinate system you would like your products to be in. However, we recommend staying within the same geographic coordinate system, as transformations may impact accuracy.