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#        Global Gridded Gross Domestic Product (GDP) at 0.25, 0.5           #
#                and 1 degree spatial resolutions for 1990                  #
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File Name Convention
--------------------
The GDP Density data set provided here is made up of three types of files on 
three common Earth grids with spatial resolutions of 1 degree, 0.5 degree, 
and 0.25 degree in both latitude and longitude: 1) estimates of the global 
gridded Gross Domestic Product (GDP) (ASCII map file), 2) "difference" table 
files (with an extension of ".dif") that hold all the points from each GDP 
map that didn't match the ISLSCP II land/sea mask, and were either removed 
(one for each map), and 3) a "change map" showing which points were removed 
(ASCII map file). The three sets of files are named using the following 
naming convention:

1. gdp_1990_XX.asc: Gridded map estimates of the global gridded Gross Domestic 
   Product (GDP), where XX can be either "1d", "hd", or "qd" for spatial 
   resolutions of 1 degree, 1/2 degree, or 1/4 degree, respectively.

2. gdp_1990_XX.dif: ASCII tables of "differences", points in the original file 
   that didn't match the ISLSCP II Land/Sea mask, and were removed from the
   ASCII map files (see section 8.4 for more details).

3. gdp_1990_XX_changemap.asc: Gridded ASCII map showing the results of applying 
   the land/sea mask: all points added ("1"), all points unchanged ("0"), and 
   all points removed ("-1"). There is one file per spatial resolution.

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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 degrees
W, and 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 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.