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#       Global Precipitation Climatology Centre (GPCC) Raingauge-Only        #
#      Global Precipitation at 0.5 and 1 degree resolutions (1986-1995)      #
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File Name Convention
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The GPCC raingauge-only precipitation data set provided here is made up of four
sets of *.zip files on two common Earth grids with spatial resolutions of 0.5 
and 1.0 degree in both latitude and longitude, and a pair of *.asc files. 
1) monthly precipitation rates (mm/month) based on raingauge measurements
   (ASCII map file)
2) the number of rain gauges per cell used to produce the values in #1 (ASCII
   map file)
3) "difference" files (with an extension of ".dif") that hold all the points
   from each precipitation that does not match the ISLSCP II land/sea mask,
   and were either removed or added through interpolation 
4) monthly correction factors for systematic gauge-measuring errors
   (unitless), based on Legates (1987)
5) a "change map" showing which points were removed or added (ASCII map file).
"change map" showing which points were removed or added (ASCII map file). 

These five sets of files are named using the following naming convention:

1) gpcc_precip_xx_YYYYMM00.asc: Gridded monthly precipitation rates, in 
   mm/month where:
   xx can be either hd or 1d for spatial resolutions of 0.5 and 1.0 
      degree respectively.
   YYYY is the year from 1986 to 1995.    
   MM is the month from 01 to 12, and "00" means it is a monthly average. 

2) gpcc_numgauges_xx_YYYYMM00.asc: Gridded number of rain gauges used in the 
   production of the gridded monthly precipitation rate data set above.

3) gpcc_precip_xx_YYYYMM00.dif: "difference" files (with an extension of 
   ".dif" that hold all the points from each precipitation that do not
   match the ISLSCP II land/sea mask, and where removed or added.

4) gpcc_corr_fact_xx_MM.asc: Gridded monthly gauge correction factors 
   (unitless), after Legates (1987), where:
   MM is the month from 01 to 12. 
   Note that these factors do not vary on a yearly basis. 

5) gpcc_precip_xx_changemap.asc: Gridded ASCII map showing the results of 
   applying the land/sea mask: all points added (positive number), all 
   points unchanged ("0"), and all points removed ("-1"). There is one 
   file per spatial resolution. 

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ASCII Data Format
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All of the files in the ISLSCP Initiative II data collection are in the ASCII, 
text format. The file format consists of numerical fields of varying length, 
which are delimited by a single space and arranged in columns and rows. The 0.5 
degree files in this data set all contain 720 columns by 360 rows. The 1.0
degree files all contain 360 columns by 180 rows. The precipitation rates and
correction factor file are written as real numbers. The files with the number
of gauges are written as integer values. Cells with no data values over water
are assigned the value of -99 and cells with no data over land are assigned 
the value of -88 on all data layers. 

The ASCII map files (with the extension of ".asc") have all had the ISLSCP II
land/water mask applied to them. All points removed from the original files 
are stored in "differences" files (with the extension ".dif"). These ASCII 
files contain the Latitude and Longitude location of the cell-center of each 
removed point, and the data value at that point. At the bottom of these files 
are also a list of all points added to the file through "nearest neighbor 
averaging" interpolation, where the land/sea mask indicated land but there 
was no data in the original file. There is also a column called 
"Interpolation_Level" that contains the number of times the interpolation 
routine was run to get a value for that point. The higher the number, the 
less reliable the value is. There is one ".dif" file for each ASCII map file.

The "change map" files show the results of applying the land/sea mask, as 
a viewable ASCII map: all points added (positive number, containing the 
"Interpolation_Level", see above), all points unchanged ("0"), and all 
points removed ("-1"). There is one file per spatial resolution.

All files are gridded to a common equal-angle lat/long grid with spatial 
resolutions of 0.5 and 1.0 degree lat/long, where the coordinates of the 
upper left corner of the files are located at 180 degrees W, 90 degrees 
N and the lower right corner coordinates are located at 180 degrees E, 90 
degrees S. Data in the files are ordered from North to South and from West
to East beginning at 180 degrees West and 90 degrees North. 

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Reference:
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Legates, D.R. 1987. A climatology of global precipitation. Publ. in 
Climatology, 40 (1), Newark, Delaware, 85 pp.