City GPS System to Receive a Significant Upgrade

City Survey, Water and GIS departments will be provided an upgrade in GPS equipment, following the discussion and approval by City Council Tuesday of adding a new Leica Geosystems GPS base station to the top of City Hall.

The City has been using a similar unit from Trimble up until now, however that station functions by continually transmitting a radio frequency, which is limited in range to a four to five mile radius around City Hall. The Leica system operates on a data link, extending the range to areas where a cell phone signal is available.

Surveyors and other City departments use the GPS units for a variety of purposes, including locating water and sewer lines, roadway surveys, project design data collection, and for mapping features to update GIS maps. The units used by the City differ from commercially available handheld devices, cell phones and car navigation systems in terms of accuracy.

“Most devices that are used for public GPS functionality are only accurate to 3 meters…vertically and 1 meter…horizontally,” City of San Angelo Drafting and Survey Supervisor Ryan Ward said in an email. “This type of accuracy is still far advanced of what was capable just 10 years ago with these types of devices. Survey and construction work requires a higher level of precision, centimeter accuracy. By accessing more satellites, having known positions that are static (fixed) such as base stations, survey-grade equipment can provide these accuracies.”

Ward explained that units such as the Leica and Trimble models are able to achieve this accuracy by collecting data from approximately 26 orbital satellites, whereas cell phones traditionally use three or more. As high-end units are collecting information, they are simultaneously doing hundreds of thousands of calculations in “real time”, Ward said.

“The station then communicates with the field rover unit, which is also linked to those satellites, giving this type of surveying a more precise triangulated network, one that lends itself to centimeter accuracy, both horizontally and vertically.”

For the City, the cost of the unit is free. Leica will provide and maintain both the unit and an indefinite single user license agreement, allowing the City to utilize the service free of charge. Public access will be granted for a monthly fee similar to a charge for XM Radio, which is paid directly to Leica and not the City, Ward said. Those who wish utilize the services of the network are further granted access to all units in west Texas, including Midland, Lubbock and Big Spring, as well.

Because the City will be given a single user license, only one City operator may use the service at any given time. “However, at this time, we only have two survey grade pieces of equipment, so the license can be shared in accordance with survey needs. In most cases, 80 percent – 90 percent of the time we will still be doing work in the range of the Trimble base station, which will continue to provide close range survey data. Also, the Trimble base station will stay operational and free for public use,” Ward said.

A foreseeable hurdle for the future includes gaps in cell service that would interfere with the use of the Leica system. Ward says that in the survey industry it is common to use portable base units to combat these issues, a practice the City will continue to use.

City Council voted unanimously to approve the new system. Plans for installation have as of yet not been announced.