| GIS: Emergency GIS for E911 Systems
A non-traditional approach to implementing accurate Enhanced 911 systems is saving Texas counties time and money.
By Fred Sedgwick and Leann Gilley San Jacinto County, Texas - An ambulance crew responds quickly and stops the flow of blood. The GIS crew of GeoInfo Inc. of Tyler, Texas; Sedgwick Associates of Houston; and Image Scans Inc. of Denver, Colo. is able to provide a countywide geocoded database in 30 to 40 weeks, and stops the county's flow of GIS data expenditures at a total as low as $80,000. According to John Nunn, 9-1-1 coordinator for San Jacinto County, "Using this imagery greatly improved the accuracy and productivity of my project. The savings in time alone allowed my project to come in on time and under budget."
As counties move toward a countywide Enhanced 911 (E911) method of coordinating all countywide police, fire, and emergency medical technicians (EMTs), they have to deal with a number of problems. First, both the E911 operator in the county seat, and the police or EMT personnel they direct may be unfamiliar with the other end of the county. Second, the home or address on the other side of the county may only have a rural route, mailbox address that is known to the postman. Third, new homes, new roads, entire new subdivisions may not appear on the latest maps available. First Steps
The county looks outside itself to the federal government, the state government, and the regional council of governments for funding that may be applied to the E911 project. In Texas, the 255 counties are organized into 24 councils of government averaging 10 counties each. The councils of government (COGs) are the primary conduit for funding E911 projects.
In theory, it would be possible for the county to look within itself for sharing GIS costs with other beneficiaries of GIS information, i.e.; the tax assessor, economic development, community development, etc. The county should also be able to look to the cities and towns it serves for participation in the GIS project.
More and more counties are expanding the range of funding participants and mutual beneficiaries; but in the main, the county emergency management organization will fund the GIS project first. After the GIS project is completed, then the other organizations want to make economic reuse of the project.
Once funding is determined, the county needs to determine whether or not a new phone system at the telephone company central office (CO) level and/or the county equipment level (PBX) is needed. The participation of the telephone company at the CO level, and perhaps the additional PBX level becomes important in using the telephone company as a prime source of populating the relational database.
Additionally, the county needs to determine what additional computer resources are needed for the E911, and whether or not they will choose to have imagery supported real-time on those computers. The recent capabilities of Pentium 133 MHz and 160 MHz have dramatically reduced the cost of the decision to include real-time imagery.
Whether or not the county opts for real-time computer based imagery, E911 operators will want or need to use books of "key maps." These books of maps are tied to coordinates in the relational database that is driven by the Automatic Number Identification (ANI) provided by the CO telephone system. The 911 operator is used to being able to have the telephone number automatically generate a "real" address, and the page of the book, and X/Y coordinates on the page of the book for that real address. The Imagery Step
Building the database, the key maps, and the real-time imagery all require cost effectively solving the problems mentioned at the start of the article. This is best done with digitized aerial film. Maps and images are coordinated, the information is readily reused by multiple personnel, and computer calculations needed for real addresses are done using the digitized aerial film as the source.
The counties in Texas using the GeoInfo, Sedgwick Associates, and Image Scans team benefit from having a team that is able to best weigh a number of factors. Included in those factors, is the currency of USGS film (most of Texas is only six to 18 months old at this point), how much of the county is missing from the government film repository due to unacceptable images, how to derive the ultimate amount of detail from scanning USGS film before the grains of the film become a psuedo-detail, other sources of existing film, and to what level of detail the other sources can be driven.
If the result of the above decision is that the film must be reflown, the team also has developed a proprietary methodology for driving the scanners to finer levels of resolution in concert with higher altitudes, fewer frames, and less cost for the entire project. Accuracy Appropriate to the Application
The team has also pioneered the use of simple rectification to dramatically lessen the cost of the project. In the recent past, the only alternative for the county has been to have the aerial film fully digitally orthorectified (DO). The scanned and rectified DO meets National Map Accuracy Standards for 1:12000 mapping. The cost for DO has averaged $1000 per image.
The average Texas county has been in the range of 80-90 images with each image encompassing 1/4 of a quad. These Digitally Orthorectified Quarter Quads (DOQQs) are the primary reason the DO method required an average of 18 months to complete a county, and $80,000 to $100,000 for DOQQ Imagery.
In contrast, the team uses the text-based file on roads available from the Texas State Department of Transportation (DOT) as a starting point for the simple rectification. If the text file on the roads is not available, then Image Scans registers and rectifies a USGS Quad map and uses this corrected map as the starting point.
Around 12 to 20 or more image features, or vector convergence points, are chosen throughout the image to simple rectify the image. The resulting accuracy to the ground is the same as the vector layer. However, this simple rectification may be visually as appealing or more appealing with vector overlays than the DOQQ with vector overlays, because the DOQQs may accentuate any incorrectness in the vector overlay. Often the expense of the DO leads to the further expense of vector correction.
In counties that are more hilly, with greater changes in elevation, simple rectification may not work. In those cases, the team moves to DOs. Subsequent use of the imagery may be for applications that need the DOQQ accuracy and USGS certification. In those cases, the team moves to DOs. However, in the majority of subsequent image reuse scenarios, either simple rectification is sufficient, or DOs are done for cities and towns in the county on an as-needed basis starting with the already scanned images.
With simple rectified images overlayed with the vector information, GeoInfo Inc. is able to do "heads-up digitization" in the subsequent steps of finding the new subdivisions, new streets, new homes, and geocoding real addresses. Heads-up digitization is easier and faster than the traditional "heads-down digitization," of using a board digitizer to create road vectors from the simple photographs or dated maps. Heads-up digitization with simple rectification is half the workload of traditional heads-down digitization, because the text file from the DOT is used to create the geocoded image.
The expense for a scanned and simple rectified image is around $150. This portion of the project can be completed in less than a month for an average county. Being able to identify a house and provide a real address is easily done with + 15 foot accuracy. Accuracy appropriate to this application leads to 85 percent savings in cost and 50 percent savings in time. Identification and Addresses
With the simple rectified image, GeoInfo finds the roads and structures. The rural route and mailbox address for the house is converted to a real address through either the "matrix" approach for smaller counties, or the "go-down the road" approach for larger counties.
The matrix approach establishes an X and Y matrix point every 52.8 feet (or more) for the entire county. The problem with this approach is the number of six digit addresses. The go-down the road approach starts from the center of the county (or some convenient reference point), and allows + 100 addresses per mile of road. For example, 200 S. Colorado Blvd., might be the new real address for the former Rt. 2, Box 12 address for a house two miles from the center of the county.
The real address and the geocode of that real address are tied to the phone number of the house through the assistance of the telephone company. Generally, the telephone company is anxious to help because the completion of the relational database is a gating factor for the time frame of the installation of the telephone system.
However, if the portion of the telephone company that is excited about the new PBX or CO equipment is different (or a different company) than the portion of the company that can provide information, the back-up plans include using the Post Office, the power company, etc. for the tying together of the phone number, real address, and geographic location of the house. The Tax Man Cometh
When the relational database is complete and installed, the responsibility for the ongoing accuracy of the GIS database makes the most sense being turned over to the tax assessor. The tax assessor is also the most logical user of imagery.
In the hands of the tax assessor the E911 database and imagery can be used to make sure the tax base is correct. Correcting the tax base often pays for the entire GIS investment many times over. Wayne Dubose, 9-1-1 coordinator for the Deep East Texas Council of Governments, commented, "There exists a tremendous capability to use the imagery for a variety of other projects on the county level once the 9-1-1 is completed."
If the county is able to use the simple rectified imagery approach, five subsequent reflights for additional currency would still not equal the cost of one year's DO. The result is a brand new simple rectified image that is a far more accurate depiction of the county than a six year old DO missing subdivisions, roads, and houses. The County Thrives
Without the simple rectification alternative of the GeoInfo Inc., Sedgwick Associates, Image Scans Inc. team, many counties will never be able to afford the DO-laden cost of a GIS and the E911 system will die while waiting. Simple rectification gets the GIS in quickly and limits the flow of the county's blood...funding. About the Authors:
Fred Sedgwick is president of Sedgwick Associates, which has offices in Houston, Texas and Atlanta, Georgia. He may be reached at 712-493-2584. Leann Gilley is the cofounder and president of GeoInfo Inc. She may be reached at 903-592-4473. Back |
