GPS Processing Methods: Comparison with Precise Trilateration

Since 1982, the Geodetic Survey of Canada has been carrying out precise trilateration by electro‐optical distance measurements (EDM) incorporating atmospheric data recorded along the EDM ray path. These networks have been established in seismically unstable areas of Vancouver Island, British Columbia, and the Charlevoix region in Quebec. The three‐dimensional adjustments of EDM measurements show standard deviations of the order of 1 cm in horizontal position or relative accuracies of 0.5 parts per million on average. Replacing trilateration with GPS observations, to monitor these networks, would provide substantial savings in time and money, as well as eliminating many logistic constraints inherent to terrestrial survey methods. In 1986, in order to evaluate the GPS measurement as a potential alternative to precise trilateration, ten stations of the 16‐station network near Port Alberni, on Vancouver Island, were positioned using four TI‐4100 GPS receivers immediately following the precise EDM survey. This network covers an area of $50\mathrm{km}\times 75\mathrm{km}\text{,}$ with interstation distances varying from 15–45 km. The data collected were processed using DIPOP, software developed at the University of New Brunswick, and with PHASER developed by USNGS. Solutions using triple, double, and nondifferenced phase observations in baseline mode and network mode are compared. Details of the observing procedures are discussed. Solutions from the different processing methods are compared against each other and against the terrestrial values.