Systems and methods including a mobile device determining the coordinates of its location using a location determination system, such as a global positioning system. A database stores the identifiers of cells representative of predefined regions in a hierarchical grid system. A server computer or the mobile device converts the coordinates of the location to one or more cell identifiers at different grid resolution levels and searches the database to find a match between the cell identifiers representing the regions and the cell identifiers representing the location of the mobile device to determine one or more regions in which the mobile device is located. For example, the hierarchical grid system can be constructed in a longitude latitude space of location coordinates, with resolution levels aligned with decimal precision levels of the location coordinates; and the cell identifiers can be constructed from the digits of the longitude and latitude coordinates.