An improved technique identifies risky transactions by mapping raw user location data to a particular cell in a fixed grid. Along these lines, when a user initiates a transaction with a service provider over a mobile device, the service provider collects raw location data such as a latitude and longitude for the user and transmits the location data to an adaptive authentication server. The adaptive authentication server then accesses a fixed set of geographical areas overlaid on a map of the Earth. For example, the geographic areas can correspond to square cells whose corners are defined by selected latitudes and longitudes. The adaptive authentication server finds a particular geographical area which contains the latitude and longitude for the user. Based on an identifier of the particular geographical area, the adaptive authentication server assigns a risk score to the transaction.