Global Positioning Satellites (GPS):

                       GPS (Global Positioning Satellites) is quickly becoming one of the more frequently used surveying instruments today. With GPS, a surveyor can cover more ground in less time than more conventional methods. However, there are risks associated with GPS if not used correctly.

Gamba and Associates, like most surveyors who use global positioning satellites, rely primarily on differential GPS for our GPS surveying needs. Differential GPS uses a base receiver at a known and previously checked control point, often a government monument, and a roaming receiver or rover. The base receiver and rover record concurrent satellite data which is used to determine the rover's position relative to the base. The base and the rover are connected via a radio modem allowing instant access to location information at the rover.

GPS accuracy is dependent on numerous factors including tree canopy, number of satellites available to the receiver, and ionosphere and sun spot activity, but the average accuracy of differential GPS is 1 cm (0.394 inches) plus 1 ppm for horizontal measurements and 2 cm (0.787 inches) plus 1 ppm for vertical measurements. It should be noted that these accuracies are only achieved with survey level differential GPS units and not a common hand held unit purchased at the store. The documentation accompanying the hand held unit should address the accuracies of that particular unit.

          As mentioned above, GPS surveying has to be used correctly and is sometimes limited in it's application. Since GPS surveying relies on the ability to use satellites, one must be able to "see" enough of the sky to collect data from a minimum of four satellites at the same time. Areas with dense overhead vegetation (i.e. trees) or with steep adjacent hillsides, all of which block the view of the sky, often are not suitable areas for GPS surveying and other techniques must be utilized.