  Uncertainties and errors in magnetic equilibrium reconstructions are a
wide-spread problem in interpreting experimental data measured in the tokamak
edge. This study demonstrates errors in EFIT++ reconstructions performed on the
COMPASS tokamak by comparing the outer midplane separatrix position to the
Velocity Shear Layer (VSL) position. The VSL is detected as the plasma
potential peak measured by a reciprocating ball-pen probe. A subsequent
statistical analysis of nearly 400 discharges shows a strong systematic trend
in the reconstructed separatrix position relative to the VSL, where the primary
factors are plasma triangularity and the magnetic axis radial position. This
dependency is significantly reduced after the measuring coils positions as
recorded in EFIT input are optimised to provide a closer match between the
"synthetic" coil signal calculated by the Biot-Savart law in a vacuum discharge
and the actual coil signal. In conclusion, we suggest that applying this
optimisation may lead to more accurate and reliable reconstructions of the
COMPASS equilibrium, which would have a positive impact on the accuracy of
measurement analysis performed in the edge plasma.
