In most cases, when 911 is dialed from a landline phone (a phone that is connected through a physical line or cord, not a cellular/mobile phone), the call is passed through an enhanced system that connects more than just the caller's voice. A database is queried that matches the phone number you are calling from with the address associated with that phone number.
The information obtained and passed through the telephone lines for the emergency dispatcher to reference contains an Automatic Number Identification (ANI) and an Automatic Location Identification (ALI). When the call is connected at the 911 Communications Center (PSAP), the ANI/ALI displays your callback number and address on a monitor at the emergency dispatcher's workstation. In the event that a caller is unable to speak, or loses consciousness after phoning for help, the emergency dispatcher can reference the ANI/ALI and still send help. This is the value of using landline phones. In an enhanced system, the emergency dispatcher can "see" the address associated with the phone used to call for help.
Calling 911 from a Cell Phone
When 911 is dialed from a cell phone the dispatcher does not receive the caller's address because a cellular phone is mobile and a caller need not be at home, or even at a physical address, to initiate the 911 System. Examples are: phoning from a park, a boat, or a vehicle that is in motion. In these situations, the 911 dispatcher only receives a limited amount of information through the cellular switching network and must work with the caller to obtain, and then verify, the true location of the emergency.
Cellular usage continues to increase each year and more and more emergency calls are initiated from mobile devices - even from the caller's home. There is an increasing trend to not even maintain a landline phone in the home and rely completely on mobile devices. In these situations, the caller can provide their address verbally for the dispatcher.
What happens then when the caller does not know the address? This situation is more common than one would expect, especially around holidays when people travel or visit relatives and may not have those addresses committed to memory. When obtaining the address verbally from the caller is not possible, the 911 Dispatcher will then utilize the cellular signal to triangulate the approximate location of the caller.
How About GPS?
As mobile devices have continued to improve over the last decade, it is now standard for phones to be manufactured with Global Positioning System (GPS) functionality. These receivers connect to GPS satellites orbiting the earth and cross-reference data from the cellular towers transmitting the cellular signal (the phone call). The strength and direction of the signal is triangulated and translated into a Latitudinal and Longitudinal Coordinate which the 911 Dispatcher can plot on a map. The accuracy of these coordinates depends on a number of different variables and is not 100% foolproof.
As the accuracy of latitude/longitude triangulations can vary, a margin for error will always exist. For example, consider a variance of 30-50 meters in plotting the location of the caller. If that caller was lost on a hiking excursion or stranded on a watercraft, a 30-50 meter radius of accuracy would be negligible as responders would get "close enough" to eventually locate the caller. If the location of the emergency was within an apartment complex, a large office, or the mall, 30-50 meters may encompass more than one apartment, locations within a business, or several different stores. Quickly locating the caller in these situations proves more problematic.
Technology Isn't Everything
While technology continues to improve, there is still the need for a live (human) 911 Dispatcher to interpret the data with their computer aided dispatch (CAD) system and apply it within their given geographic area. There is no substitution for a caller verbally providing their exact location. But when a caller cannot provide, or does not know, the address of the emergency, the 911 Dispatcher has the skills and technology to utilize the cellular data to approximate the location and "see" where the caller is phoning from.