WingtraOne PPK per default gives data in WGS84 with ellipsoidal height. However, for surveying applications, local coordinate systems are usually used.
The figure below illustrates the workflow with coordinate systems and WingtraOne PPK.
In the following, the optional coordinate conversion of WingtraOne PPK aerial data to any local State Plane Coordinates (SPC) with orthometric heights based on geoid 12b is detailed.
After WingtraHub PPK processing is complete, another coordinate conversion has to be applied, which deviates from the default setup.
1. The base station location is provided by the OPUS solution and is given in the NAD83(2011)(EPOCH:2010.00) reference frame, which is a geographic Earth-centered, Earth-fixed reference frame. Since it is ITRF based, these coordinates can be used directly in WingtraHub as a base location.
2. (Optional) In case your coordinates are given in WGS84, you can transform them to NAD83(2011) using the tool provided here: https://www.ngs.noaa.gov/TOOLS/Htdp/Htdp_transform.html).
3. Define the base station location in WingtraHub. For clarity, indicate the coordinate system you are using to the base station name. In the US we recommend using directly NAD83(2011) delivered by OPUS. The image below shows parts of the OPUS report. Choose the left column. You can either use XYZ or LatLongELheight.
As visualized in the flow chart above, the image geotags will be in the base station coordinate system—in this case NAD83(2011)—while the altitude will be ellipsoidal. To save space and avoid errors, we recommend to use the option "CSV file only."
Coordinate conversion (manually using NGS tools)
1. Open the *.csv file in the OUTPUT folder of your WingtraPilotProjects folder and remove the orientation information to get four columns with "filename, longitude, latitude, altitude" in decimal degrees (and meters for the altitude).
2. Use the *.csv created during WingtraHub geotagging, and add the four columns as shown in the screenshot below to match the desired input format for the conversion tool. Use NGS Coordinate Conversion and Transformation Tool (NCAT) in order to transform the horizontal coordinates to the your State Plane Coordinate System.
3. The output *.csv contains spcNorthing and spcEasting in meters and feet (be careful with units) and the spcZone. The SPC zone depends on your location, it is automatically detected by the conversion tool. Copy the image names and Northing and Easting coordinates to a new csv file (use excel or google sheets).
4. To transfer the ellipsoidal heights to orthometric heights based on the commonly used geoid 12b, you need to calculate the geoid height at the mission location. Check out the figure below for a better understanding. If you have an OPUS report of your base station, you can calculate the geoid height as follows: geoid height = ellipsoid height - orthometric height.
In case you do not have an OPUS report, the geoid height can be calculated using the conversion tool provided here: https://www.ngs.noaa.gov/GEOID/GEOID12B/GEOID12B_data.shtml.
As an output you will get the geoid height in meters. To transfer the ellipsoidal heights that are given in the .csv of WingtraHub to orthometric heights, use the following formula for every image: orthometric height = ellipsoid height - geoid height.
5. Add the derived orthometric heights to the *.csv containing the SPCs.
6. Add the columns containing the horizontal and vertical geotaging accuracy to the *.csv. The accuracies can be found in the original WingtraHub output file.
Make sure to only use one unit for all columns. The excel formula =convert(value, "m", "ft") helps to perform fast conversion.
Coordinate conversion (automated using convert2planar.py)
1. Install python 3.7.: https://www.python.org/downloads/release/python-370/ (Windows x86 executable installer).
2. Open the command prompt (Click on the search function in Windows and type cmd). Copy the lines below to the command prompt to install two more modules (copy the first line and run it by pressing enter, then copy and run the next line):
py -3 -m pip install --upgrade pip
py -3 -m pip install requests
4. In the command prompt, change to your WingtraPilotProjects directory as follows:
5. To run the conversion tool, run the following command (adapt the specified settings to fit your case):
py -3 convert2planar.py --inputCoord "NAD83(2011)" --outputCoord "NAD83(2011)" --outputHeight 13 --inputCsv "Flight Name\OUTPUT\WingtraOne geotags.csv" --outputUnit m
6. To get help on the required inputs of the program, run the following command:
py -3 convert2planar.py -h
Processing in Pix4D
1. In your post-processing software, import the images and the created *.csv.
2. Define the image / input coordinate system. First, select the unit you are using (m/ft). Select the horizontal datum NAD83(2011). The coordinate system depends on your location (e.g. Louisiana South (ftUS)). In Pix4Dmapper select the vertical coordinate system: "Arbitrary," then it will maintain the image coordinate system (geoid 12b).
3. Define the output coordinate system to match the input coordinate system. In case the input is in meters, you can specify to use feet as your output.
4. Process the project and verify the results using checkpoints defined in your local coordinate system.