This readme file will address the following:

	1) Contents of the zip file
	2) General overview
	3) Suggestions about "how to work" with these files


1) Contents of the zip file:

	Files in this zip file are:
		Geotiff - Full resolution of this file - Very Large  (~ 933 Megabytes)
		Geotiff - Lower resolution of the Full resolution image (~ 233 Megabytes)
		World header file (for the above GeoTiff files with a .tfw extension
				   - enables georeferencing)
		Readme file - Filename.txt

	The actual file names are:

		world_avg.tfw		 		(World header file)
		world_avg.tif.gz			(Full Resolution)
		world_avg_low_res.tfw			(World header file)
		world_avg_low_res.tif.gz		(Lower Resolution)
		world_avg.txt				(Readme file)


	The full resolution file contain byte images in a geographic
	projection with the following characteristics:

        	Image Size: 43200 columns X 21600 rows
       		Grid Spacing: 30 Arc Seconds
         	Center of Upper Left Pixel: 90 Lat, -180 Lon

	The lower resolution file (filename_low_res.tif) contain byte images in a
	geographic projection with the following characteristics:

		Image size: 21600 columns X 10800 rows
		Grid spacing: 1 Arc second
		Center of Upper left pixel: 90 North, -180 East


2) General Overview:

	world_avg - The final Radiance Calibrated world product.  Each pixel
	contains a DN value between 0 and 255.  The values between 0
	and 254 can be converted to radiance values with the equation
		  Radiance = DN^(3/2) * 10^(-10) watts/cm^2/sr.
	The value 255 signifies that no unsaturated pixels were found
	over that location for the duration of the reduced gain experiment.

	World Radiance Calibrated Images - March 1996, January - February 1997
	Derived from DMSP OLS Nighttime Imagery

	A radiance calibrated nightime lights image of the World has been
	assembled from Defense Meteorological Satellite Program (DMSP)
	Operational Linescan System (OLS). The satellite observation of the
	location and intensity of nocturnal lighting provide a unique view of
	humanities presence and can be used as a spatial indicator of other
	variables that are more difficult to observe at a global scale. Examples
	include the modeling of population density and energy related greenhouse
	gas emmissions.

3) Suggestions on "How to Work" with these files:

	TAR AND ZIP INFORMATION:

		UNIX:
		On Unix and Linux extract and unzip the files.
		The zip file has been zipped with gzip.
		(use Unix "man pages" if you're not fimiliar with "how to do" this)

		MICROSOFT WINDOWS
		On Windows use "Winzip classic" to untar and unzip the files.
		The zip file has been zipped with gzip.

	IMAGE SIZE:

		THE FULL RESOLUTION IMAGE FILES ARE VERY LARGE IN SIZE -
		MAKE SURE YOU HAVE ENOUGH DISK SPACE BEFORE UNZIPPING THEM.
		You will need almost a Gigabyte free on your hard drive before
		unzipping the full resolution tif file.

	IMAGE VIEWERS VS DATA VIEWERS:

		Image viewers have the ability to view the file, (one can
		see a pretty picture), but the end user will not be able to
		get data values.

		For viewing files and querying the data values use software
		such as listed below in the Data Viewing software section.

	IMAGE VIEWING SOFTWARE:

		Photoshop - If some images appear black in Photoshop, you will need
			    to increase the Brightness/Contrast to view the correct
			    representation of the image.
		Arc Explorer
		Other imaging software

		Image viewers might be unable to open up the full resolution
		Geotiff file. You will need alot of RAM or have	your image package
		set up to use all your computer resources to be able to open these files.

	DATA VIEWING SOFTWARE:
		(We recommend using these or similiar products to query the data)

		Erdas
		Envi
		PCI
		Idrisi
		ArcMap
		ArcInfo
		Arc View (Convert image to a grid in ArcInfo then use the Spatial
			  Analyst extension in Arc View - also the end user can use
			  the "Image analyst" extension to view the image files).

		How to load images in ArcView 3.X:
		___________________________________________________________________
		Start arcview
		File->extensions-> turn on tiff extensions (click tiff checkbox)
		(if you have Spatial Analyst turn that extension on too)
		Click the View "new" button
		Select the pull down "view"
		Select "add theme"
		Change the data source type from "Feature data source"
			to "Image data source" then select the tif.

___________________________________________________________________

If you have any questions regarding the data or the file formats,
please contact:

       	         Patrick Hayes
                 Solar-Terrestrial Physics Division
                 NOAA National Geophysical Data Center
                 325 Broadway E/GC2
	         Boulder, CO 80305
	         Tel: 303-497-6764   Fax: 303-497-6513
	         email: Patrick.J.Hayes@noaa.gov