Thousands of owners of private wells in Fairfax County should not be surprised this summer to see a person wandering in their back yards who appears to have lost his bearings on the way to a "Star Trek" convention.

Wearing a vest with a large antenna fastened at his back and carrying a cryptic-looking hand-held electronic device, the person will in fact know exactly where he is.

This mysterious visitor is using the latest in global-positioning system (GPS) technology to precisely map the location of the county's 13,000 private drinking water wells. More than 50,000 residents get their drinking water from such wells, county health officials said.

The information will be used to create a geographic information system (GIS) database that county health officials can use to identify groundwater sources that could be affected by future road construction, development and sewer system installations or in emergency situations such as contamination from a spill.

The field work is under way -- in the Dranesville/Great Falls area -- and is scheduled to be completed in October. Residents in the affected neighborhoods are supposed to receive letters from the health department about two weeks before a well mapper arrives at their homes. County officials said residents do not need to be home or take other action.

The mapping work is being done by employees of the engineering and energy company Michael Baker Jr. Inc. of Moon Township, Pa. They operate in much the same way as a Dominion Virginia Power meter reader. The mapper spends about 90 seconds at each well, using the equipment to download the GPS coordinates from orbiting satellites. The wellheads themselves are not opened or otherwise impacted. Each mapper will be clearly identified and will carry credentials, officials said.

Dennis Hill, a county environmental health director, said in a statement: "Ultimately, this GIS-enhanced ability will help the health department provide improved service and protect more than 50,000 county residents who derive drinking water from a private well."

The information collected from the project will offer county officials a quick reference that will allow them to see how proposed road improvements, construction activity and sewer system installations could have an impact on groundwater sources.

Officials said that the GIS database could also potentially be useful in the event of a hazardous material spill or even a terrorist attack that targeted the local water supply.

For more information, including a map of the areas where wells will be mapped, go to www.co.fairfax.va.us/service/hd/wellproject.htm.

Fairfax County Health Department contractor Charlie Milewski uses a GPS receiver strapped to his back and a laptop while mapping wells in a Great Falls neighborhood. A laptop with mapping software will help form the database.