probability surface from sites?

Grassnetters:

     How can I generate a lightning probability density surface from a
(huge) site file containing every lightning hit for the last 7 years? I'd
like the surface to return the probability *100 of lightning hitting that
spot.

     Any ideas?
                         Thanks,

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   ____ Oak Ridge National Laboratory
    / ) / / / / Environmental Sciences Division
   /--< o / / /--/ P.O. Box 2008, M.S. 6038
  /___/__<__/__/_ / / _ Oak Ridge, TN 37830-6038
William W. Hargrove (615) 574-1902
hnw@mtqgrass.esd.ornl.gov (615) 576-8646 (fax)
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