Each of the above CD-ROM series is organized differently, but within each series the organization is consistent.  For example, the 1990 conterminous U.S. land cover characteristics data set CD-ROM consists of four directories and a <tt>readme.txt</tt> file:
<pre>
source/
lcclass/
derived/
attrib/
readme.1st*
</pre>
Each of the first 3 directories contains image data (.img files) in 8-bit or 16-bit format. There are also a number of text files (found in the <tt>attrib/</tt> directory) useful for describing the values found within each of these images. The images themselves are briefly described in the following table:
<pre>
FILE         TYPE     SIZE    RANGE            DESCRIPTION 
--------------------------------------------------------------------------------------------
	DIRECTORY: \SOURCE\IMAGES 
WATER.IMG    8-BIT  13251843  0-1 	WATER BODIES 
DEM.IMG     16-BIT  26503686  0-14018 	DIGITAL ELEVATION MODEL 
STLINE.IMG   8-BIT  13251843  0-255 	STATE BOUNDARY LINES 
STPOLY.IMG   8-BIT  13251843  0-255 	STATE FIPS POLYGONS 
CTYLINE.IMG  8-BIT  13251843  0-255 	COUNTY BOUNDARY LINES 
CTYPOLY.IMG 16-BIT  26503686  0-3112 	COUNTY POLYGONS 
NDVI90.IMG   8-BIT 106014744  0-200 	8-CHANNEL SET OF 1990 NDVI 28-DAY COMPOSITE 
NDVIMAX.IMG  8-BIT  13251843  0-200 	1990 MAXIMUM GREENNESS (NDVI) COMPOSITE 
MLRA.IMG     8-BIT  13251843  0-178 	MAJOR LAND RESOURCE AREAS 
ECOREG.IMG   8-BIT  13251843  0-76  	EPA ECOREGIONS 
LULC.IMG     8-BIT  13251843  0-91 	USGS LAND USE AND LAND COVER SAMPLE 
FROST.IMG   16-BIT  26503686  0-348 	FROST-FREE PERIOD 

	DIRECTORY: \LCCLASS\IMAGES
LCC71.IMG    8-BIT  13251843  0-71 	PRELIMINARY LAND COVER REGIONS CLASSIFICATION
LCC159.IMG   8-BIT  13251843  0-159 	FINAL LAND COVER REGIONS CLASSIFICATION

	DIRECTORY: \DERIVED\IMAGES
LCCUSGS.IMG  8-BIT  13251843  0-26 	MODIFIED USGS LEVEL 2 LAND USE AND LAND COVER CLASSIFICATION
LCCSIB.IMG   8-BIT  13251843  0-27 	MODIFIED SIMPLE BIOSPHERE MODEL CLASSIFICATION
LCCBATS.IMG  8-BIT  13251843  0-28 	MODIFIED BIOSPHERE ATMOSPHERE TRANSFER SCHEME CLASSIFICATION
ONSET.IMG    8-BIT  13251843  0-24 	ONSET OF GREENNESS CLASSIFICATION
PEAK.IMG     8-BIT  13251843  0-24 	PEAK OF GREENNESS CLASSIFICATION
LENGTH.IMG   8-BIT  13251843  0-255 	DURATION OF GREENNESS CLASSIFICATION
</pre>