Climatological station elevation
From AMS Glossary
climatological station elevation
The elevation above mean sea level chosen as the reference datum level for all climatological records of atmospheric pressure in a given locality; not necessarily the same as station elevation.
This term is denoted by the symbol Hpc in international usage. The reduction of pressure to this standard elevation is necessary only if a station has changed its specific location but has stayed within the general area, so that the two (or more) sets of records remain comparable.