RE: USGS Web address for determining IBC SDC and Base Shear

To continue with Dennis's question regarding how to determine the mapped spectral acceleration values for a specific location, I have found there are two sources, one is the USGS website using the Java Ground Motion Parameter Calculator, and the other is the FEMA 450-CD which is available free from FEMA, (800-480-2520). They are both based on 2003 information, BUT, interestingly they each give different results for a specific latitude and longitude, (not much difference- but definitely different). Again, they both say they are based on 2003 FEMA?USGS information, and the output "format" appears to be similar, (i.e. USGS output).
 
Any ideas on which is "more" correct, and why?
 
Thanks in advance,
 
Larry Hauer S.E.

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