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#         Global Gridded Population of the World at 0.25, 0.5 and            #
#                1 degree spatial resolutions (1990, 1995)                   #
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File Name Convention
--------------------
These data sets are provided at three spatial resolutions of 0.25, 0.50, and 
1.0 degrees in both latitude and longitude. The following files are included 
in the compressed zip archive:

1) Population Counts (gpw_population_counts_xdeg.zip)
   a) gpw_pop_counts_XX_YYYYa.asc: Gridded ASCII maps of the global gridded 
      population counts (persons per cell). XX can be qd, hd, and 1d, 
      denoting a spatial resolution of 1/4, 1/2 and 1 degree, respectively. 
      YYYY is the year (1990 or 1995), and if there is an "a" after the year, 
      the population counts have been adjusted to match United Nations (U.N.) 
      population totals at the national level. Both U.N. adjusted and 
      unadjusted data are provided.
   b) gpw_pop_counts_XX_YYYYa.dif: ASCII tables of "differences", points in the 
      original file that didn't match the ISLSCP-2 Land/Sea mask, and were 
      removed from the ASCII map files (see documentation for more details).
   c) gpw_pop_counts_XX_changemap.asc: See 3a) below

2) Population Density (gpw_population_density_xdeg.zip)
   a) gpw_pop_density_XX_YYYYa.asc: Same as 1a) above but for global gridded
      population density (persons per square km per cell).
   b) gpw_pop_density_XX_YYYYa.dif: Same as 1b) above but for population
      density.
   c) gpw_pop_density_XX_changemap.asc: See 3a) below.

3) Land Area (gpw_land_area_xdeg.zip)
   a) gpw_land_area_XX.asc: Gridded ASCII maps of the global gridded land area 
      (square km of land per cell) used to calculate the population density.
   b) gpw_land_area_XX.dif: ASCII tables of "differences" (see 1b) above).
   c) gpw_land_area_XX_changemap.asc: Gridded ASCII map showing the differences
      between the ISLSCP2 land/water mask and the original data set: All points
      with negative values ("-1") are those where the ISLSCP2 mask showed water
      but where the original data set showed land (i.e. Land Area>0). All points
      with a value of zero are those points where the two land/water masks
      agreed and all points with positive values were land or water points with
      no data over land or water in the original data set which were filled in
      with a "missing data" value (see documentation for more details). There is
      one file per spatial resolution. Duplicates of these files are included in
      the archives for both population counts and density.


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ASCII Data Format
-----------------
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. All 
values in these files are written as integer numbers. In all layers areas with 
no data over oceans are assigned the value of -99. Areas with no data over land 
(i.e. Antarctica) are assigned the value of -88.

The files at different spatial resolutions each contain the following numbers
of columns and rows:
   One degree: 360 columns by 180 rows 
   1/2 degree: 720 columns by 360 rows 
   1/4 degree: 1440 columns by 720 rows 

All files are gridded to a common equal-angle lat/long grid, where the 
coordinates of the upper left corner of the files are located at 180 degree W, 
90 degree N and the lower right corner coordinates are located at 180 degree E, 
90 degree 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 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 each removed point (center
of the pixel), and the data value at that point. 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 ("1"), all points unchanged ("0"), and 
all points removed ("-1"). There is one file per spatial resolution.