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#   GTS Gauge-Based Analyses of Daily Precipitation over Global Land Areas   #
#        (Version 0103) at 0.5 and 1 degree resolutions (1986-1995)          #
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File Name Convention
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The GTS raingauge-only precipitation data set provided here is made up of four 
sets of files on two common Earth grids with spatial resolutions of 0.5 and 1 
degree in both latitude and longitude: 1) daily precipitation rates (mm/day) 
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 map that don't match the ISLSCP-2 land/sea mask, and were 
either removed or added through interpolation, and 4) a "change map" showing
which points were removed or added (ASCII map file). The four sets of 
files are named using the following naming convention:

1) gts_precip_xx_YYYYMMDD.asc: Gridded daily precipitation rates, in mm/day, 
   where xx can be either hd or 1d for spatial resolutions of 1/2 and 1 degree, 
   respectively. YYYY is the year from 1986 to 1995, MM is the month from 01 
   to 12, and the DD is the day of the month from 01 to 31.
2) gts_numgauges_xx_YYYYMMDD.asc: Gridded number of rain gauges used in the 
   production of the gridded daily precipitation rate data set above.
3) gts_precip_xx_YYYYMMDD.dif: "difference" files (with an extension of ".dif") 
   that hold all the points from each precipitation map that don't match
   the ISLSCP-2 land/sea mask, and were removed or added.
4) gts_precip_xx_changemap.asc: Gridded ASCII maps 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, 
or 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 degree 
files all contain 360 columns by 180 rows. The precipitation rates are written 
as real numbers. The files with the number of gauges are written as integer 
values. Cells with over water are assigned the value of -99 while cells with no 
data over land are assigned the value of -88 on all data layers. 

All files are gridded to a common equal-angle lat/long grid with spatial 
resolutions of 0.5 and 1 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. 

The ASCII precipitation map files (extension of ".asc")  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 
table files contain the Latitude and Longitude location of the cell-center of 
each point from the precipitation map that don't match the ISLSCP-2
land/sea mask, and were removed. At the bottom of each file is 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 (if there were no "neighbor" pixels with data, the missing 
data was kept at -88). 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 (there are no ".dif" files 
for the number-of-gauge maps)

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.

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Reference:
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Rudolf, B., 1993: Management and analysis of precipitation on a routine basis. 
Proc. Internat. WMO/IAHS/THE SYMP. On Precip. And Evap., Slovak Hydromet. Inst., 
Bratislava, Sep. 1993, 1,69-76.

Xie, P., and co-authors, 1996: Gauge-based monthly analysis of global land 
precipitation from 1971 to 1994. J. Geophy. Res., 101D14, 19023 ­ 19034.

Shepard, D., 1968: A two dimensional interpolation function for regularly spaced 
data. 23rd National Conference of American Computing Machinery, Princeton, N.J.