WingtraOne PPK—UK coordinate system

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 a the workflow with coordinate systems and WingtraOne PPK.

WiH_CoordinateSystemOverview

In the following tutorial example, the optional coordinate conversion of WingtraOne PPK aerial data to local projected British coordinates OSGB36/British National Grid (OSTN15/GM15) is detailed.

Basically, after WingtraHub PPK processing, another coordinate conversion has to be applied, which deviates from the default setup.

UK_coordinates

Step-by-step instructions: 

1. The base station location is provided by the CORS network and should be given in the ETRF89 coordinate system, which is a geographic Earth-center, Earth-fixed coordinate system. Since it is ITRF based, these coordinates can be used directly in WingtraHub as base location. If you work with, for example, Trimble VRS, the base location is already in ETRF89. If you know it in another ITRF-based system, you may use Epoch Conversion (www.epncb.oma.be) to transform to ETRF89 with the correct epoch.

2. Open WingtraHub. Make sure to use Agisoft Photoscan .csv file type settings defined in the "Settings" tab of WingtraHub. Follow the WingtraOne Helpdesk article about PPK geotagging to proceed with PPK processing of your data.

3. Define the base station location in WingtraHub. For clarity, indicate coordinate system you are using with the base station name. In the United Kingdom, it's recommended to use ETRF89 directly. As visualized in the flow chart above, the image geotags will be in the base station coordinate system—which is in this case ETRF89—while the altitude will be ellipsoidal. To save space and avoid errors, we recommend using the option "CSV file only."

4. Load the .csv file to GridInQuest and convert the coordinates by following these steps:

i) Download Grid InQuest software available here Grid InQuest (www.ordnancesurvey.co.uk) in order to transform the horizontal coordinates to planar projected OSGB36 (OSTN15/GM15).

ii) Use the functionality "File/Load Data Point" to open the .csv file in the OUTPUT folder of the WingtraPilot project.

iii) In the popup window, define the required transformation, ETRS89 as the input and OSGB36 TN15 as output coordinate system.

iv) Select "Has Vertical Data" and select the respective columns for latidude, longitude and altitude from the drop downs.

v) Once the settings are correct, click "OK."

Grid InQuest II_settingsforcsv

5. Convert the *.csv file by clicking "File/Transform Data" and save the output by using "File/Save Output" to the OUTPUT folder of your WingtraPilotProjects. We recommend using descriptive filenames that include the coordinate systems used to distinguish different files. Each image geolocation in the *.csv file has now projected OSGB36 as coordinates and OSTN15/GM15 as a height reference.

6. In your post-processing software, define a new project and import the images from the IMAGES folder within your WingraPilotProjects folder.

7. Import the converted .csv to load the image geolocation.

8. Define image geolocation accuracy of all images using the accuracy values specified in the WingtraHub processing report, which you will find in the OUTPUT folder of your WingtraPilotProjects folder.

9. Define the image / input coordinate system. Make sure to select OSGB36 as a horizontal and OSTN15 as a vertical coordinate system. In Pix4Dmapper, select the vertical coordinate system named "Arbitrary," which will maintain the image coordinate system.

11. Process the project and verify the results using check points defined in OSGB36 and vertical OSTN15.