soils: 
  dataset: CONUS-Soil (or derived from)
  contents: ussoilsuborder - 0, 1, 2... 65 for the 65 major subgroups
            ussoildomtex - 0, 1, 2... 16 for no data, sand, loamy sand, sandy loam,
                           silt loam, silt, loam, sandy clay loam, silty clay loam,
                           clay loam, sandy clay, silty clay, clay, organic, H2O, bedrock, other      
            usrockdepth - 0, 1, 2... 15 for depth to bedrock in decimeters. 15 includes soils
                          in which depth was not observed in the top 1.5 meters 

 parent material: 
  dataset: Surficial geology of the U.S. (Hunt, 1979)
  contents: usparentmat - 0, 1, 2... 6 for other, stream alluvium, fan alluvium, colluvial, 
                          glacial, eolian, lacustrine  

 climate: 
  dataset: temp: ERL-C monthly data, precip: PRISM monthly data
  contents: ustemp - mean temp in degrees C
            ustempsimple - 0, 1, 2 for cold (<10), medium (10-20), hot (>20) 
            usprecip - mean precip in mm
            usprecipsimple - 0, 1, 2 for arid (<250 mm), semiarid-subhumid (250-1000 mm),
                             and humid (>1000 mm)
            usprecipseas - seasonal precipitation index 
            usprecipseassimple - value of 0, 1 for nonseasonal (1-2), and seasonal (>2)

 slope: 
  dataset: 1 km resolution elevation data from GLOBE project analyzed for slope data 
  contents: usslope - values of slope x 1000 
            usslopesimple - value of 0, 1, 2 for low (0-1%), med (1-5%) and high (>5%) slopes

 vegetation: 
  type: 
   dataset: Land cover type gridded at 1 km resolution
   contents: usvegtype - land cover type with legend 
             usvegtypesimple - 0, 1, 2, 3, 4, and 5 for ice/water, desert, grass & shrub & crop, 
                               deciduous, evergreen, marsh
  density:
   dataset: NDVI for Spring, 1997 at 1 km resolution
   contents: usndvi - values of ndvi x 100
             usndvisimple - value of 1, 2, 3 for 0.0 to 0.2 (barren), 0.2-0.5 (med), 
                            and 0.5-1.0 (high)