Stereoscopic observation of NOAA AR 2678 with two spacecraft. The positions of the two spacecraft and any point in the solar corona to be triangulated define a plane called the epipolar plane (see e.g. Inhester 2006). Because LOS1 and LOS2 must lie in the same epipolar plane, their intersection in this plane is unambiguously defined by the 3D point. For convenience, the 3D coordinates of the points are reconstructed in HEEQ heliocentric coordinates (the x-axis is directed towards the Earth). Calculation of the 3D point coordinates is the first step in the velocity vector construction.