The Met Office Hadley Centre produced the HadSLP1 dataset which replaces the Global Mean Sea Level Pressure (GMSLP) data sets, and is a unique combination of monthly globally-complete fields of land and sea pressure observations a 5 degree latitude-longitude grid from 1871 to 1998.

The advantages of HadSLP1 over GMSLP2 are an improved land station data base, new interpolation scheme and the incorporation of local detail while safeguarding against random errors. Like GMSLP2, HadSLP1 was developed by Tracy Basnett and David Parker, in collaboration with R.J. Allan (previously at CSIRO and now at the Met Office) and M.J. Salinger (NIWA).

Marine observations were taken from the Met Office Marine Data Bank (MDB) and from the NOAA Comprehensive Ocean-Atmosphere Data Set (COADS) (Woodruff et al, 1987). The MDB data were given priority in the blend, which is described in Basnett and Parker (1997). Land observations are a combination of data obtained from CSIRO (Australia), NIWA (New Zealand), CRU (University of East Anglia), GHCN, and operationally-received "CLIMAT" messages. The latter were used mainly for updating station series and for the Arctic.

HadSLP1 pressures are reconstructed using a reduced-space optimal interpolation procedure, followed by superposition of quality-improved gridded observations onto the reconstructions to restore local detail.