Precipitation gauge measurements over land from the GHCN-v2 (Global Historical Climatology Network version 2) are interpolated onto 3 grids: at 0.5, 1 and 2.5 degree resolutions via optimal interpolation. Note: As of May, 2011 the GHCN version 3 station data set is available.
National Center for Atmospheric Research Staff (Eds). Last modified 31 May 2016. "The Climate Data Guide: Precipitation Reconstruction Land (PREC/L): 1948-present." Retrieved from https://climatedataguide.ucar.edu/climate-data/precipitation-reconstruction-land-precl-1948-present.