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| In 2004, the Wireless Association reported that more than 170 million Americans owned cell phones. By 2005, that number had exceeded 187 million, with more than 65% of the US Population owning cell phones. In the year 2000, when an estimated 30% of 9-1-1 calls were from cell phones, predictions were that half of all 9-1-1 calls would come from cell phones by 2005. With more Americans using cell phones exclusively, the 9-1-1 world faces increasing challenges in providing the same quality emergency service to cellular 9-1-1 callers as is available to those using traditional land-line phones. The ability to accurately route cellular and land-line calls to the appropriate dispatch agency, generate and display the most accurate location information, determine the appropriate emergency responders and map that location on an in-house call location mapping system are all integral to high-quality land-line and wireless E9-1-1. All of these factors rely on databases and mapping, the essential ingredients of a Geographic Information System (GIS).
Enhanced land-line 9-1-1 returns address-based location and emergency responder information to the dispatch agency when a call is answered. The address describes the location of the telephone that placed the call. A traditional land-line phone is tied to a specific address location-a house or structure number, a road name and a community, such as "1644 Cottonwood Rd, Willow Springs" A cellular phone, however, moves with its owner. Location information transmitted with an "enhanced" cellular 9-1-1 call differs according to the type of wireless 9-1-1 service in the caller's area: none, Phase 1 or Phase 2. An emergency dispatcher handling a Phase 1 cellular 9-1-1 call receives a callback number, a tower address and the sector of the tower that received the call. Phase 2 includes a latitude/longitude coordinate location of the phone at the time the call was placed along with the Phase 1 information. (see Figure 1). 9-1-1 calls in areas with no wireless E9-1-1 service are usually routed to administrative lines in dispatch centers and nolocation information is available at all. Land-line E9-1-1 relies on several databases. The MSAG (Master Street Addressing Guide) contains road names forthe area covered by the dispatch agency. The MSAG also specifies the high and low addressing for each road segment and pairs this information with an Emergency Service Number (ESN) (see Figure 2).
The ESN links to the ELT table (English Language Translation), which lists the fire, EMS and law enforcement responders associated with each ESN. (see Figure 3) The resident data is stored in the ALI (Automated Location Information) database. The ALI database contains each land-line phone number paired with an MSAG-valid 9-1-1 address, resident name, ZIP code community and ESN (derived from the MSAG). A 9-1-1 call triggers a database search - the record tied to the phone number that dialed 9-1-1 is returned to the PSAP and displayed on the dispatcher's monitor.
While land-line E9-1-1 provides a street address and community name as a call location, fully-implemented (Phase 2) Enhanced Wireless 9-1-1 provides the location of the cellular 9-1-1 caller as a latitude/longitude coordinate along with the tower address, IDs for the tower and sector that received the call and the wireless caller's call-back number. Each cellular tower array is divided into sectors, often three sectors that combine to form a 360-degree circle. The wireless 9-1-1 call is routed to the appropriate PSAP based on routing information for each cellular sector, stored in routing tables. Sectors from the same tower can be routed to different PSAPs (see figure 4 need graphic of cell tower on county line). There are no addressing databases as the phone's location changes constantly and cannot be tied to a specific physical address. The accuracy of the coordinate location can vary greatly as it is provided either from a GPS chip in the phone itself or a mathematical triangulation method based on the receiving and nearby tower locations. Both systems ultimately use distance measurements (from phone to towers or from phone to GPS satellites) to pinpoint location, and GPS location is more accurate outdoors than indoors. One of the main benefits of Enhanced 9-1-1 for both land-line and wireless calls is proper and intelligent call routing. It's important-and can be life-saving-that the call be directed to and answered by the appropriate PSAP. Each PSAP has a geographically-defined boundary, typically following a county or city boundary. For example, all land-line 9-1-1 calls originating from within Douglas County are routed to the Douglas County Communications Center. However, PSAP boundaries are sometimes defined by local telephone exchange boundaries, Native American land boundaries, geographic features such as rivers, or groups of counties. An E9-1-1 call made from a land-line phone is routed to the appropriate answering point by a PSAP routing code associated with each ESN. In essence, a land-line phone number is associated with a physical address, a physical address is associated with an ESN, an ESN is associated with a routing code, and a routing code is associated with a specific primary and backup PSAP. Routing wireless calls relies on PANI (Pseudo-ANI). This is a fictitious non-dialable phone number assigned to each cell sector. Since wireless 9-1-1 calls are processed and routed within the land-line telephone 9-1-1 network, a Pseudo-ANI provides the same routing number for all cellular calls that originate in a single cellular sector. There will be many different cellular phone numbers with different prefixes accessing each sector of a cell tower at any given moment. In essence, a cellular phone that dialed 9-1-1 from a specific geographic location is associated with the tower face (sector) that received the call, the sector is associated with a Pseudo-ANI and the Pseudo-ANI is associated with a specific primary and backup PSAP. While land-line location information (address and community) can be readily displayed in a digital mapping program at the dispatch site, the address itself, mapped or not, has meaning, especially to dispatchers and responders who are familiar with the PSAP's response area and especially in areas with consistent, logical 9-1-1 addressing. Wireless E9-1-1 location data, however, is intrinsically linked to mapping. A coordinate displayed as a string of numbers on a computer monitor is essentially meaningless. If voice communication is lost, which is not uncommon with wireless calls, mapping allows the dispatcher to send emergency services based on the caller's location at the time of the call, even if the caller doesn't answer or can't be reached on a call-back. A coordinate must be plotted on a map with additional map layers for reference such as road centerlines, corporate or political boundaries, structure locations or aerial imagery in order to attach meaning to those coordinate numbers, locate the caller based on known reference points, and dispatch the appropriate emergency services (Figure 5).
With the nationwide push to provide enhanced 9-1-1 to wireless callers, more and more PSAPs are moving into the GIS world as they purchase dispatch mapping software to display land-line and wireless 9-1-1 call locations. All of the suitable call mapping software packages rely on good and appropriate GIS data, just as E9-1-1 relies on appropriate routing and good, complete addressing, MSAG and ALI databases. The PSAP with enhanced land-line and wireless 9-1-1 will need, at a minimum, a spatially-accurate and properly-attributed road centerline GIS layer. Each centerline must be linked to a road name, synchronized with the MSAG, and contain high/low, even/odd address ranges.
For Phase 1 wireless, which identifies the tower and sector that received the call and the tower address, a cell tower sector coverage layer is helpful. This layer depicts the mapped wedge- or circle-shaped areas associated with each sector on a cellular tower (See Figure 6). Additional layers enhance the quality and usefulness of emergency call mapping. Though centerlines are critical, a cellular call can originate miles from a known, named road. Thus, natural features and outbuildings depicted on aerial imagery take on importance, as do addressed structure points or footprints, parcels, driveway centerlines, streams and rivers, bodies of water, corporate limits, emergency service zones and landmark points. All of these additional layers add intelligence to the mapping by providing points of reference for locating emergencies. A pair of coordinates translated onto a map as a precise point is only meaningful and useful for location when that point is displayed along with named roads and other known landmarks and features.
Land-line 9-1-1 calls are mapped by physical address. Most dispatch call mapping software uses a process called "geo-coding", which interpolates an address location based on the high and low address range for each road segment (see Figure 7). However,
many software In
summary, mapping 9-1-1 |
