Recommended coordinate system
The geolocation accuracy of aerial data after WingtraOne PPK processing depends mainly on the distance to the base station, the accuracy of the base station location, and the realization of the coordinate system.
It is critical to know the base location in an ITRF-based, global, Earth-centered, Earth-fixed (ECEF) coordinate system. The coordinates can be provided in cartesian coordinates (X, Y, Z) or geodetic coordinates (lat, long, h). In the latter case it is important that the heights are in ellipsoidal heights.
We recommend to use WGS84 but other coordinate systems like ETRFXX/ETRSXX, NAD83 (ellipsoidal) or other ITRF based systems work as well. It is important that the realization of the coordinate system is based on a reference frame that is up-to-date (not older than 2000). For the height reference, common ellipsoids are WGS84 or GRS80. The following coordinate systems are supported and can directly be selected in WingtraHub:
- WGS84 (EPSG: 4326) with ellipsoidal height
- NAD83 (EPSG: 6318) with ellipsoidal height
- ETRF (EPSG: 9059) with ellipsoidal height
- CHTRF95 (EPSG: 4151) with ellipsoidal height
- Other ITRF based, custom with ellipsoidal height
Depending on your region and the availability of conversion tools, it might be better to use local reference frames to avoid unnecessary conversions of coordinates in the photogrammetry software.
Implications of selected coordinate system
The base station coordinate system defines the coordinate system of the flight images after PPK processing. As you see in the figure below, the image coordinate system is always the same as the base station coordinate system.
Global ECEF coordinate system workflow
For default processing, the image coordinate system (which is always the base station coordinate system) has to be selected as the input coordinate system when post-processing flight images in any third-party software. Step-by-step instructions on how to process geotagged images in Pix4Dmapper in WGS84 can be found in Process WingtraOne images with Pix4D.
Local projected coordinate system workflow
Most of the popular photogrammetric software packages are able to perform the transformation from WGS84 to national projected coordinate systems. However, local projected systems are usually not included in the default workflow of these software packages. Also, the list of geoid models might not be complete. In these cases, we recommend to transform the image geotags in the .csv to your local coordinate system prior to importing the data into your desired photogrammetry software (optional route in the figure above).
Usually, there are web-services or programs provided by governmental agencies that allow high-precision conversion from global ECEF coordinates to projected coordinates.
Please check out all articles in the WingtraOne PPK section to see if we already provide country-specific instructions. If you cannot find a tool to convert the geotags to your required coordinate system, we are happy to advise you when you submit a new support request.