Basic 911 System: A telephone system which automatically connects a person dialing the digits "9-1-1" to an established PSAP (Public Safety Answering Point) via a dedicated phone line. A trained 911 operator asks the caller for their location and dispatches the appropriate emergency personnel.

Mapped ALI: When ALI is downloaded to an enhanced 911 system, the location of the caller is also displayed on a map. A computerized map in a GIS (Geographic Information System) helps identify the closest available responders and can calculate the fastest route to the scene, incorporating relevant information such as road closures and traffic patterns. Navigational instructions can be automatically formulated and downloaded. Wireless E911 systems require ALI to be delivered in mapped format due to fact that cell phones are by nature mobile and are not associated with one fixed location or address.

Wireless E911 (WE911): Due to the rapid increase in 911 calls generated from cell phones, the Federal Communications Commission issued Orders in 1997 to phase in enhanced wireless 9-1-1 service. Phase I, referred to as the "network solution," calculates a 911 caller's approximate location by measuring the angle and strength of signal reception at a cell tower. Because Phase I is dependent on more dense cell tower coverage than currently exists in Montana and locates the cell phone caller with only approximate accuracy, Wireless E911 in Yellowstone County will be deployed using Phase II technology, the "handset solution." Phase II relies on GPS (Global Positioning System) technology that is now being incorporated by cell phone manufacturers. GPS transmitters embedded in the cell phone are activated when 911 is dialed. Dispatchers utilizing CAD (computer aided dispatch) systems with mapped ALI (automatic location identification) can observe the caller's location as the GPS latitude/longitude coordinates are plotted on a basemap. The map contains local features such as street lines, structure points, mileposts, jurisdictional boundaries, and other useful references. The basemap for Yellowstone County is maintained by the GIS Department and is utilized by the Sheriff's Department in their AVL (automatic vehicle location) application. The map is also attached to the CAD system in the 9-1-1 Center. It's a good idea, though, to know your location so that you can confirm what the dispatcher is seeing on the map and also, so that you can give more detailed instructions when necessary such as which floor, which entrance, or other details to expedite the arrival of public safety personnel.

Reverse 911: Where 911 is the number you dial in an emergency to contact authorities, reverse 911 allows the authorities to contact you. Because GIS technology has made it possible for phone companies to link your phone number to a mapped location, officials can "draw a circle" around an area under threat and initiate an automatic call campaign to only those residences. A recorded message is delivered to landline phones (not to cell phones) in the affected area, relaying instructions on what to do. A reverse 911 situation may be triggered by a wildfire, severe weather, industrial accident, a terrorist attack, or even to help locate a missing child. In Yellowstone County, Reverse 911 can be activated for any areas where E911 service for landline phones is in effect.