PUTTING GPS TO THE TEST

Field tests of an innovative global positioning system (GPS) technique show how the technology can be put to effective use in construction layout. Two-base station RTK is a new real-time kinematic (RTK) GPS technique in which two base stations are used simultaneously. Each base station has a radio modem to transmit the RTK information to the rover. The base station radio modems are set to operate on different frequencies to prevent unwanted interference between the bases. The RTK method differs from static GPS in that the rover moves from point to point across the network, much like traditional RTK measuring techniques. Field test results revealed that RTK would be acceptable for clearing limits, rough grade, centerline, and slope stakes. Although 2-mm accuracy was achieved, it was not consistent enough to be used for the high-accuracy requirements. Hardware costs can be a drawback, if the user has not already invested in the necessary dual-frequency receivers required for such a sophisticated operation. The method, although more hardware-intensive, can be learned quite easily--in approximately 2 hours for most people. In terms of efficiency, RTK GPS does not offer quicker results, but it does offer the possibility of using a smaller crew. Even though consistent 3-mm accuracy was not achieved in field tests, two-base station RTK offers an advantage that most surveyors do not incorporate into construction layout: an independent check.