These monthly maxima of the Gallo experimental GVIs were computed by taking 
the maximum values of biweekly GVIs for each month, then reprojecting the 
original mercator-projected data to latitude-longitude projection.  The 
compilation was produced from Gallo's data by the U. S. Geological Survey's 
EROS Data Center.

Inspection of these data at the National Geophysical Data Center showed that 
the computed data were internally consistent to within one grid cell (the 
locational accuracy usually attributed to the NOAA Polar Orbiting 
Environmental Satellites that house the AVHRR sensor).  However, the data were 
misregistered to the Earth by 1 grid cell (to the south).  The data were re-
registered at NGDC by removing the northernmost row of data, and inserting a 
new row at the bottom of each data file.  As these rows contained no GVI 
values, no data were lost in the process.

This problem, and a similar problem encountered with another large data base, 
suggest that users of projection software should ensure that the software is 
performing as documented.  Our initial interpretation of this problem is that 
there is an undocumented incorrect "roundoff" of spatial coordinates in some 
projection packages under certian (as yet undefined) circumstances.  Based on 
characteristics of only two (albeit very large and significant) data sets, 
this interpretation should be considered somewhat speculative.  Nevertheless, 
the problem could be significant for multivariate spatial analysis, as in 
global environmental study.

This collection of data is sorted into several subdirectories:

The ADOC subdirectory contains IDRISI 3.X .doc files derived directly from the 
data, using the IDRISI function DOCUMENT.  These data have been placed adja-
cent to the file allocation table, to optimize the use of the IDRISI function 
LIST, which accesses both the file allocation table and the .doc files.  

The BDOC subdirectory contains IDRISI 3.X .doc files containing identical ma-
xima and minima.  This version of the .doc files will ensure consistent color 
representations when using the IDRISI function COLOR A (which autoscales the 
8-bit values to fit the 16-color VGA display).

The CDOC subdirectory contains IDRISI 4.0 .doc files derived directly from the
data, using the IDRISI function BATCH324.

The CELL subdirectory contains simple 8-bit raster grids.  These data are 
identical to GRASS CELL files, to IDRISI 8-bit image files, and are a de-facto 
standard for raster data interchange.  Such data have been proven to be rela-
tively simple to import into many spatial analysis (image analysis and geo-
graphic information) systems.

The CELLHD3 subdirectory contains GRASS 3.X cell header files.  Depending on 
how you organize your global latitude-longitude data you may want to change 
the "north:" "south:" "east:" "west:" "n-s resol:" and/or "e-w resol:" 
entries, particularly if you want to register raster and vector data, such 
as the vector data included in the Global Change Data Base.  Note that the 
CELLHD3 entries are of the format FILENAME.IMG.  The .IMG extension is 
required for compatibility with the naming of the CELL files, which is a 
compromise allowing operation of this CD-ROM in both GRASS and IDRISI.

The CELLHD4 subdirectory contains GRASS 4.0 cell header files.  GRASS4 
supports latitude-longitude projections in degrees and minutes, so this
format is used here.  If you prefer to use unprojected columns and rows
of imagery, you might want to adapt the CELLHD3 files for use in GRASS4,
or use them unmodified.

=============================

To use these data in GRASS, you have the minimal CELL and CELLHD directories.
Remember that the naming convention for GRASS is FILENAME.IMG.  You will
probably want to copy data that you want to use often to your hard disk,
and to run the GRASS support program (support in GRASS3.X, r.support in
GRASS 4.X) to add appropriate color tables, build histograms and legends, 
and add documentation to the file(s).

You may want to link the CELL and CELLHD3 or CELLHD4 subdirectories on this
CD-ROM to the CELL and CELLHD subdirectories, respectively, in a global
data base that you create on your hard disk.  This will allow the output
of r.support to be written to your hard disk.

==============================

To use these data in IDRISI, use the DOS APPEND command as follows:

APPEND /E
APPEND DRIVE:\1MONTH\CELL; DRIVE:\1MONTH\ADOC       or

APPEND /E
APPEND DRIVE:\1MONTH\CELL; DRIVE:\1MONTH\BDOC

If you want to use standardized colors when autoscaling (COLOR A), use the
second line.  If you want to do computations using the actual value ranges
of each GVI file, use the top line.  Notice that the only difference between
these two commands is in the last "word" which is ADOC or BDOC, depending
on which .doc files you want to use with the .img files in the CELL 
subdirectory.

===============================

Both of these procedures may appear slightly different from the way you
may use GRASS or IDRISI.  But they allow the same CD-ROM to be used as
a (albeit slow) form of mass storage for both of these widely available 
analysis systems, both designed for global environmental analysis.

=================================