Earth curvature and other additional corrections

Earth curvature corrections are applied to minimize distortions resulting from the usage of a non-cartesian coordinate system, such as a tangential coordinate system like UTM. In many Aerotriangulation solutions, a shift is estimated in image space to correct this effect. This allows to use ground control points in UTM coordinates without transferring them to a local Cartesian coordinate system. The correction equations in image space were designed for Nadir images with relatively flat terrain and large ground resolution (>20cm). For higher resolution datasets, undulating terrain like mountains or cities and oblique imagery the common equations should not be applied during Aerotriangulation.

SURE relies on a Cartesian coordinate system, where the classical pinhole camera model maps from a global coordinate cartestian coordinate system to a Cartesian camera coordinate system. Additionally, various distortion models are supported. When using SURE, it should always be ensured that a consistent camera model definition is in use. Please make sure, that the input data should be derived without earth curvature compensation and other additional corrections, in order to get correct data.

Otherwise, shifts in the data of up to multiple decimeters can occur in particular in undulating terrain. When having an Aerotriangulation result to be used with such corrections, please re-perform the bundle adjustment step without the corrections. Remaining effects resulting from the usage of non-cartesian GPS orientations or ground control points will be shifted to the exterior orientation – leading to a consistent result when using SURE. As shown in the following alternative, a fully consistent result can be achieved by carrying out the Aerotriangulation using Cartesian coordinates only from the beginning.

Using ellipsoidic heights generally simplifies transformation steps to the tangential coordinate system and for compatibility with subsequent software packages. This is also required for SURE functions that require a coordinate system transformation, e.g. Cesium export.

In order to get an optimal result with full consistency, a cartesian coordinate system should be used through all processes including the following steps:

  1. Choose Cartesian coordinate system (recommended: local tangential coordinate system approximating the target system if 2.5D products such as a DSM or True Orthophoto are desired or geocentric coordinates EPSG:4978 for 3D products only (point clouds, meshes etc.), which is supported in all SURE functions that perform a coordinate system transformation and works with Cesium)
  2. Transfer all GPS/INS as well as all gound control point measurements to this coordinate system
  3. Set up the Aerotrianguation specifying the used coordinate system
  4. Execute Aerotriangulation
  5. Pass results of Aerotriangulation to SURE
  6. If local coordinate system is close to desired result, it can be used right away
If an optimal transfer to a projected coordinate system like UTM is desired for the DSM, you have the option to transform the raw point cloud (3D_Points) with tools like lastools before executing a custom DSM step using the 2.5D tool or the command line. Please note, that for the True Orthophoto also the orientations need to be in the same coordinate system to yield to a correct result.

If the bundle adjustment was carried out without such corrections, but the Inpho/Trimble Match-AT project file is created with having the respective check boxes being activated, please edit the first lines of the Inpho/Trimble Match-AT project file with these options by using a text editor ($REFRACT_CORR_DEFAULT : off | $CURV_CORR_DEFAULT : off).

General note: the earth curvature shift correction is depth depending. Consequently, no camera-constant image correction methods such as distortion grids or radial distortion parameters should be used to compensate effects – in particular in undulating terrain and for oblique imagery. 

By specifying an EPSG coordinate system (e.g. in the Inpho Match-AT project file), the coordinate system Information WKT will be transferred to point clouds, orthos and DSMs. This is also required for SURE functions that require a coordinate system transformation, e.g. Cesium export.