Custom geodetic datum and orthometric height

The branch feature/proj of the ROS driver (located at https://github.com/fixposition/fixposition_driver/tree/feature/proj) presents a new configuration model to manually select the desired geodetic datums to convert ECEF coordinates to LLH.

The updated configuration file contains the following section:

proj:
  enabled: true               # Use PROJ for ECEF/LLH conversions
  ecef_crs: "EPSG:4978"       # Source CRS for ECEF coordinates
  llh_crs: "EPSG:4979"        # Target CRS for LLH coordinates

where:

• enabled indicates whether to use the proj library to use the custom geodetic datum when converting coordinates from ECEF to LLH.

• ecef_crs is the source CRS for ECEF coordinates.

• llh_crs is the target CRS for LLH coordinates.

Instalation procedure

First, install the proj library. To do this, you must ensure all the required libraries are installed (see https://proj.org/en/9.4/install.html#build-requirements for more information). For example, here is a short snippet with some of the required libraries:

sudo apt update
sudo apt install curl sqlite3

Then, head to the root of the fixposition_driver repository and run the following script:

cd fixposition-sdk/docker/scripts/
sudo bash install_proj.sh

Source the setup.bash from the ROS2 distro repository (/opt/ros/{ROS_DISTRO}/setup.bash). For example:

source /opt/ros/$ROS_DISTRO/setup.bash

Then, build the ROS2 driver node and all its dependencies with:

colcon build --symlink-install --cmake-args -DBUILD_TESTING=OFF -DFPSDK_USE_PROJ=ON

Lastly, source the development environment:

source install/setup.bash

Known transformations

For documentation on the definition of each coordinate frame, please refer to https://epsg.io/. For example, for WGS84: https://epsg.io/4979.

• WGS84 (Used on VRTK2 for FP_A-LLH, NMEA-Gx-GGA, etc. outputs):

  • ecef_crs: "EPSG:4978" (https://epsg.io/4978)

  • llh_crs: "EPSG:4979" (https://epsg.io/4979)

• For example, if correction data is in ETRS89, and you want LLH coordinates in WGS84:

  • ecef_crs: "EPSG:4936" (https://epsg.io/4936)

  • llh_crs: "EPSG:4326" (https://epsg.io/4326)

• NAD83(2011) - US coverage:

  • ecef_crs: "EPSG:6317" (https://epsg.io/6317)

  • llh_crs: "EPSG:6319" (https://epsg.io/6319)

• NAD83(CSRS) - Canada coverage:

  • ecef_crs: "EPSG:4954" (https://epsg.io/4954)

  • llh_crs: "EPSG:4955" (https://epsg.io/4955)

Orthometric height

The geoid height, N, can be used to convert a height above the ellipsoid, h, to the corresponding height above the geoid (roughly the height above mean sea level), H, using the relation:

h = N + H; H = -N + h.

To calculate the height above the geoid (normal altitude used in sectors such as mining), use the following equation:

Height above the Geoid = Height above the Ellipsoid - Geodetic Height

Where:

• Height above the Geoid is the orthometric height.

• Height above the Ellipsoid is the ellipsoidal height in Fixposition's output.

• Geodetic Height is the height above the geoid model.

To enable this output, please follow these steps:

1. Download the geoid grid into a known PROJ data dir:

projsync --file us_nga_egm08_25.tif --user-writable-directory

2. Ensure PROJ can see that directory (run everytime or set it in your launch environment):

export PROJ_DATA=/usr/share/proj:$HOME/.local/share/proj

3. Use a compound CRS for LLH + orthometric height. For example, EGM2008 vertical CRS is typically EPSG (line 3855), so the compound CRS is:

EPSG:4326+3855

4. Update the config.yaml file accordingly:

proj:
  enabled: true
  ecef_crs: "EPSG:4936"
  llh_crs: "EPSG:4326+3855"

5. Verify the grid is picked up:

projinfo -o PROJ -s EPSG:4936 -t "EPSG:4326+3855"

If this step fails, copy the geoid grid into the system PROJ data dir:

sudo cp ~/.local/share/proj/us_nga_egm08_25.tif /usr/share/proj/