Authority Establishes Countywide GPS Control Network

July 1, 2002
Nestled at the southern end of the Blue Ridge Mountains, Cherokee County is one of the fastest growing regions in Georgia, realizing double-digit population and economic growth for the last decade. The constant influx of people and business from nearby Atlanta has kept local utilities like Cherokee County Water and Sewerage Authority (CCWSA) hopping, as they look to manage what they have and prepare for what they need.

Nestled at the southern end of the Blue Ridge Mountains, Cherokee County is one of the fastest growing regions in Georgia, realizing double-digit population and economic growth for the last decade. The constant influx of people and business from nearby Atlanta has kept local utilities like Cherokee County Water and Sewerage Authority (CCWSA) hopping, as they look to manage what they have and prepare for what they need.

"Industry convention points to the use of GIS to plan, manage and maintain infrastructure. That works - if you have data. Finding a way to easily and affordably get repeatable, accurate digital data is always the catch," said Jeff Hooper, GPS/GIS & Sewer Model Coordinator for the CCWSA.

The CCWSA has assembled the nation's first Active GPS Control Network specifically designed for water/wastewater management. Only in its first year of operation, the network consists of a series of Global Positioning System (GPS) reference stations that provide real-time data for countywide water resource management.

GPS Network

First introduced in the late '90s, GPS reference stations have been used by large public and private entities as a point-to-point real-time method of extending control for urban development. Each GPS reference station includes a dual-frequency GPS receiver, GPS antenna and communication equipment. The combined unit acts as one of the two receivers - the fixed known position - required for differential GPS operation. These stations track visible GPS satellites, then receive, store and even transmit RTK (real-time kinematic) and differential GPS (DGPS) data while providing centimeter accuracy in real state plane coordinates.

The CCWSA modeling group adapted cellular phone-capable GPS receivers as an alternative to improve communication between the roving GPS receiver and the reference station.

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Since these stations are permanent, surveyors and engineers are able to reduce the need for physical control point monuments. A roving GPS receiver linked to a standard 2-watt radio modem can typically use the reference station to gather data with two-centimeter accuracy in a six mile radius.

Dwayne Fowler and David Kubala from the CCWSA, along with Gil Puffer and Jeff Hooper, then part of the engineering staff at Welker and Associates Inc., (Marietta, GA), a CCWSA consultant, collectively came up with the idea of establishing a network of countywide base stations specifically for water/wastewater management. In a county the size of Cherokee (approximately 423 square miles), a few of these stations conveniently located throughout the area could significantly improve the ongoing GIS sewer/water model development, while supporting countywide planning activities. The utility currently serves 48,000 water customers and 15,000 sewer customers.

"With a GPS receiver, we can pinpoint manholes, trunklines, pipelines and much more in minutes. We've eliminated the need to set up control points to maintain accuracy," Hooper said.

During the first two weeks of operation, Cherokee County staff located around 500 manholes including pipe types and depths.

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The CCWSA executive management agreed and invested $250,000 in an Active GPS Control Network, which to-date includes four Leica Geosystems RS500 Reference Stations and two GPS rovers with two-way radio connection.

During the first two weeks of operation, Hooper and his staff located around 500 manholes including pipe types and depths. Once gathered, the data goes into the InfoWorks v4.0 (Wallingford Software), an integrated water/wastewater modeling package, where it can be analyzed and modeled. Linked with the CCWSA's SCADA system, this emerging sewer and water model is expected to be the heart of real-time planning and maintenance activities throughout the agency.

"We know this is just the beginning - after all the county has some 15,000 manholes alone. Our goal is to first model the most active development areas so that we can effectively support near-term growth," Hooper said.

Modeling for Growth

Once a month the CCWSA Board meets to review proposed development projects. One particular proposal came out of the small community of Macedonia where a local developer looked to construct a new school, commercial development and 300 residential homes. The CCWSA Board was asked by the developer to determine the impact of this development on the existing water and sewer system. The initial GIS output showed little or no impact - but Hooper and his staff opted to verify these results with the support of the newly operational GPS network.

"We spent about two hours locating the outfall lines using RTK GPS techniques. With the nearby reference station as the base, we simply walked the lines shooting positions every few feet and keying details about size," Hooper said.

They returned to the office by early afternoon, imported the data into InfoWorks and within an hour a far different picture emerged. As it turned out, the impact of the new construction would place a significant burden on the system, causing possible overload. The subsequent damage and overflow of raw sewage from a surcharged manhole would in turn be costly to fix. The result: CCWSA Board looked to research safer development options.

"It really gave us credibility with the CCWSA Board. We were able to show a highly visual model depicting where pipes are surcharging. In this case, we were able to show the CCWSA Board the existing sewer system capacity," Hooper said.

Dialing for Distance

As with any GPS operation, particularly RTK GPS, the key to success is constant communication between the satellites, the base station and the roving receiver. The GPS Active Control Network takes care of the satellite tracking problem. However, communication between the base station and the rover has limitations. Once a base reference station has been established, a GPS receiver linked to a standard 2-watt radio modem will operate within a six mile perimeter.

"The radio is pretty much line of site. Get too far away, and you can't get accurate readings," Hooper said. Like most surveyors and engineers, he often requires a much wider perimeter of operation.

The CCWSA modeling group adapted cellular phone-capable GPS receivers as an alternative to improve communication between the roving GPS receiver and the reference station. The telephones and calling plans are from Verizon.

Once in the field, a surveyor turns on the receiver prompting it to dial the dedicated phone number at the base reference station. Within 10 seconds from satellite lock, the reference station resolves ambiguities at a distance up to 50 kilometers at 1cm+1ppm. The rover receiver can also be configured to immediately produce state plane coordinates and surface elevations, which cuts back on headaches of "calibrating" or "transforming" local coordinates to do a job with RTK.

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