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