Stereoscopic observation of NOAA AR 2678 on 19 November 2017, shown in the 3D HEEQ coordinate system. The X-axis is directed towards the Earth and the Z-axis is aligned with the solar rotation axis passing through the solar north. The two observing satellites are separated from each other by 37°. The blue-shifts at point 4 are measured along their respective LOSs and are represented by the vector magnitudes v_LOS1 and v_LOS2. The velocity vector v_ep, measured in the epipolar plane, is calculated from v_LOS1 and v_LOS2 following the geometry depicted in the figure. In this case the true velocity vector of point 4 is directed along the coronal loop, showing the plasma flow confined by the magnetic field, and it is calculated directly from the deprojection of V_3D on the coronal loop. If the plasma flow is perpendicular to the epipolar plane, its velocity cannot be measured, i.e. V_3D cannot be derived when it makes an angle of 90° degrees with the epipolar plane.