The installation of the GPS antennas on both the drill derrick and the cell and the subsequent measurements to determine their location are critical. If there are errors in the location of the antennas relative to the points on the structure being positioned then all subsequent measurements will be incorrect. Adding the inclinometer improved the results from the casing measurement. This could have been improved even further if it had been calibrated. This would have required that the drillers raise the derrick to vertical position while the values were read and recorded.
Calibration needs to occur only once during the project.
In addition, the cell placement results could have been improved if the antenna mounts had been place directly on to the cell rather than a box on the side of the cell where measurements may have been inaccurate. According to the technicians at Trimble, experience on other projects of this type has shown that including GPS into the positioning plans from the start will significantly improve the accuracy. Despite the results shown in the above table, the demonstration project was a success. The concept of using GPS to locate and position a large structure from a remote location was proved. The location of the cell as it was positioned was observed and recorded unobtrusively from a remote site.