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In the PVsyst sites database (Meteonorm source), basic irradiation values are usually defined for a free horizon. Nevertheless we observed in the DRY swiss data of very mountainous sites - as probably in other DRY or user's own meteo measurements - the horizon effect of the measuring site is usually already accounted for in the data since it is the result of a measurement. Therefore it is not always clear if  a horizon correction is to be applied or not, depending on the field horizon with respect to the measuring station horizon.
 

When the horizon correction is already included in meteo monthly value, one can think that the horizon correction should not be taken into account in the simulation.
 

This is not correct for two reasons:

- The horizon at the exact PV plant site may be very different than the one included in the meteo data (at the real measurement site),

- The synthetic model cannot "know" the horizon line, and will create dummy hourly values below the horizon.
 

In these cases the only way of dealing with such situation would be to generate an hourly file with the original monthly data, apply the horizon correction on these hourly values (using the "original" horizon line), recreate monthly values and applying back the horizon correction  for getting monthly values "without horizon correction".  After that the synthetic generation may be applied again on these new values and the horizon of the site should be used in the simulation.