5/2/02
readme.txt

SAFARI Subset - GRIDDED SURFACES of SELECTED SOIL CHARACTERISTICS (IGBP-DIS)

This subset was taken from the Global data set :

Global Soil Data Task. 2000. Global Gridded Surfaces of Selected
Soil Characteristics (IGBP-DIS). International Geosphere-Biosphere
Programme - Data and Information Services. Available on-line
[http://www.daac.ornl.gov /] from Oak Ridge National Laboratory
Distributed Active Archive Center, Oak Ridge, Tennessee, U.S.A.

The Global Gridded Surfaces of Selected Soil Characteristics (IGBP-DIS) data
set contains 7 data surfaces:  soil-carbon density, total nitrogen density, 
field capacity, wilting point, profile available water capacity, thermal 
capacity, and bulk density.  

All the surfaces were generated by SoilData, at a resolution of 5arc 
minutes by 5arc minutes, and a soil depth interval of 100cm.  These data
surfaces were generated by the SoilData System, which was developed by the 
Global Soil Data Task of the International Geosphere-Biosphere Programme (IGBP)
Data and Information Services (DIS).  The SoilData System generates soil
information and maps for geographic regions at soil depths and resolutions 
selected by the user.  Derived surfaces of selected soil characteristics are
suitable for modeling and inventory purposes.  The data surfaces are also
distributed as part of the Global Soil Data Products CD-ROM.

The SoilData System uses a statistical bootstrapping approach to link the
pedon records in the Global Pedon Database to the FAO/UNESCO Digital Soil Map
of the World.  It can generate maps and output data sets for a range of 
original and derived soil parameters, such as carbon and nitrogen density,
thermal conductivity, and water-holding capacity, for any part of the world
at user-selected depth ranges.  The digital output can be at any resolution
(in increments of 5').

Creating the subset:
-------------------

The original (global) data were downloaded from the Oak Ridge National 
Laboratory (ORNL) Distributed Active Archive Center (DAAC).
http://daac.ornl.gov 

Using GRID ( a raster- or cell-based geoprocessing toolbox that is integrated
with ArcInfo) the SETWINDOW command was used to define the subarea of interest.

This subarea was defined by identifying the bounding coordinates as follows:

x_min 5     y_min -35     x_max 60     y_max 5

The "snap_grid" option of the SETWINDOW command was used.  This snaps the lower-
left corner of the specified window to the lower-left corner of the nearest 
cell in the snap_grid and snaps the upper-right corner of the specified window 
to the upper-right corner of the nearest cell in the snap_grid.  In this case 
the snap_grid is the original data grid.  The purpose of this is to ensure the
proper registration of the newly set analysis window.  The command format 
used is as follows:

SETWINDOW x_min y_min x_max y_max original_grid

Once the window was set, creating the new grid was simply a matter of setting
the new subset grid equal to the original grid.

subset_grid = original_grid

An ASCII array was created from the new subset grid using the GRID command
GRIDASCII.
file = GRIDASCII(subset_grid)

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Data files are provided in ASCII GRID format for ARC INFO.  Each file
contains a single ASCII array in a geographic (lat/long) projection.  The
ascii files consist of header information containing a set of keywords, 
followed by cell values in row-major order.

The first six lines of the ascii files (header) should read:
ncols 660 
nrows 480
xllcorner 4.999926
yllcorner -35.0000086
cellsize 0.0833333
NODATA_value -9999

To import the *.dat files into ARC INFO as GRIDS use one of the following 
commands:

At the ARC prompt:
Arc: ASCIIGRID <in_ascii_file> <out_grid>  FLOAT

OR

At the GRID prompt:
Grid:  (*) = ASCIIGRID (<in_ascii_file>,  FLOAT)

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There are seven data files included in this data set: (0 - 100cm depth)

1. bkldens.dat  - Bulk Density (g/cm3) 

2. fieldcap.dat - Field Capacity (mm) (PsiFC = -10 kPa) 

3. pawc.dat     - Profile Available Water Capacity (mm) (PsiFC = -10 kPa) 
               (PsiWP = -1500 kPa)  

4. soilcarb.dat - Soil Carbon Density (kg/m2) 

5. thermcap.dat - Thermal Capacity (J/m3/k) (Theta = 0.00 %v/v) 

6. totnitro.dat - Total Nitrogen Density (g/m2) 

7. wiltpt.dat   - Wilting-Point (mm) (PsiWP = -1500 kPa) 

Note:  SoilData's default settings were used in the generation of these
surfaces. (i.e. - bootstrap sample size = 500; Field Capacity water 
potential = -10 kPa;  and Wilting Point water potential = 1500 kPa.  Missing
soil unit data was patched from the default data set.)