Because of the importance of spatial analysis in biosurveillance, the precision with which the spatial location of an Internet user can be determined affects the value of Internet utilization data for biosurveillance. There are numerous geolocation methods based on the IP address associated with queries.4 However, none of the methods claims 100% accuracy and their accuracy degrades as one tries to determine precise location information for reasons that include the use of dynamic IP addressing. Many companies that offer geolocation products will report accuracy rates for the country or city, but not for smaller geographic regions. Digital EnvoyTM, one of the leaders in geolocation, reports country accuracy rates of 99.4% and city accuracy rates of up to 94% (Digital Envoy). Digital Envoy also reports that they can obtain zip codes for IP addresses, but do not provide accuracy rates for this information, stating only "Accuracy rates vary greatly'' and "Accuracy rates are higher at the 3-digit zip code level.''

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