IMAPP MODIS Direct Broadcast Snow Mask
           Product Description
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  The IMAPP Snow Mask software produces a map of the location of snow
  covered ground or snow covered frozen water bodies under cloud 
  free conditions during the daytime.  The inputs to the software are
  the MODIS 1 km resolution L1B files and the MODIS 1 km Cloud Mask file.
  The software was developed by William Straka III, UW-Madison, Space
  Science and Engineering Center (SSEC), and Dr. Jeffrey Key 
  NOAA/NESDISand and integrated into IMAPP by Kathleen Strabala, 
  kathy.strabala@ssec.wisc.edu.

  The algorithm used to determine the Snow Mask, is the same technique
  used by the MODIS cloud mask to determine surface snow cover, and is
  described in:

  Ackerman, S. A., K. I. Strabala, W. P. Menzel, R. A. Frey, C. C. 
   Moeller, and L. E. Gumley, 1998: Discriminating clear-sky from 
   clouds with MODIS. J.  Geophys. Res., 103, D24. 32141–32157.

  Salomonson, V. V., & Appel, I. (2004). Estimating fractional 
   snow cover from MODIS using the normalized difference snow index. 
   Remote Sensing of Environment, 89(3), 351-360. 

  NOTE: This algorithm is a very simple implementation, and may be 
  replaced in the future by a more sophisticated technique.  The 
  output has not been rigourously validated, and should be used 
  with this in mind.

  Output for this product is a flat binary file and an HDF4 file.

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Binary Output File
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  The *.snowmask.bin file consists of one band containing 2 byte integer 
  values. The size of the file will depend upon the size of the pass that 
  is being processed (1354 x number_of_lines x 2 bytes).

  The values in the file are:

  1000  -  Snow
     9  -  Land or sea with no snow cover found
 -1000  -  No retrieval (Clouds or missing data)

   Example file name:  t1.11012.1633.snowmask.bin
   2 byte integers
   1 parameter  -  Snow Mask
   1 km-pixel resolution

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HDF4 OUTPUT FILE
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  The HDF4 output file (*snowmask.hdf) contains 3 arrays.  Latitude, 
  Longitude, and the Snow_Mask. The Snow_Mask array is stored as 2 Byte
  Integers, with 1000 being snow found, -1000 meaning no retrieval
  because of missing data and cloud, and 9 meaning land or sea with 
  no snow found.

  The Latitude and Longitude arrays are stored as 4 byte float values
  at 5 km resolution.
  
  Example filename:  t1.11012.1633.snowmask.hdf


 (Example ncdump -h of an IMAPP Snow Mask HDF file)

netcdf t1.11012.1633.snowmask {
dimensions:
        Cell_Along_Swath_5km = 942 ; (example)
        Cell_Across_Swath_5km = 271 ; 
        Cell_Along_Swath_1km = 4710 ; (example)
        Cell_Across_Swath_1km = 1354 ;

variables:
        float Latitude(Cell_Along_Swath_5km, Cell_Across_Swath_5km) ;
                Latitude:_FillValue = -999.98999f ;
        float Longitude(Cell_Along_Swath_5km, Cell_Across_Swath_5km) ;
                Longitude:_FillValue = -999.98999f ;
        short Snow_Mask(Cell_Along_Swath_1km, Cell_Across_Swath_1km) ;
                Snow_Mask:scale_factor = 1. ;
                Snow_Mask:add_offset = 0. ;
                Snow_Mask:valid_range = -1000s, 1000s ;
                Snow_Mask:_FillValue = -32768s ;
}