The previous pages have already indicated the basic application of GPS in engineering surveying such as in control surveys, topographic surveys and setting-out on site. Indeed, much three-dimensional spatial data normally captured using conventional surveying techniques with a total station can be captured by GPS, even during the night, provided sufficient satellites are visible. However, GPS is not the solution for all survey data capture problems. Where GPS works, i.e. in an open skies environment, it works very well.
However, once the skies become obstructed as they often are on the construction site, then conventional survey techniques may be more appropriate. GPS does not, at present, work well or at all close to buildings, indoors, underground or underwater.
On a national scale, horizontal and, to a certain extent, vertical control, used for mapping purposes and previously established by classical triangulation with all its built-in scale error, are being replaced by three-dimensional GPS networks. In relation to Great Britain and the Ordnance Survey, this has been dealt with in previous pages. The great advantage of this to the engineering surveyor is that, when using GPS at the local level, there is no requirement for coordinate transformations. Kinematic methods can be used for rapid detailing, and Real-Time Kinematic (RTK) for setting-out. Whilst the above constitutes the main area of interest for the engineering surveyor, other applications will be briefly mentioned to illustrate the power and versatility of GPS.
Machine guidance and control:
Earth moving is required to shape the ground prior to construction taking place. In this context machine guidance is concerned with guiding the operator to move the blade or dozer, scraper, excavator or grader using GPS as the reference. On the other hand, machine control is when this information is used directly
to control the machine’s hydraulics to automate the blade. Initially a GPS base station is established on site.
The base station collects the code and phase data and this is broadcast by radio throughout the whole site. Rover units working in RTK mode get corrections from the base station. Shortly after, position at the machine is determined with 20–30 mm precision at a data rate of at least 0.1 second with an even smaller latency. Usually there are two GPS antennae attached to each end of the blade so that the grade profile may be determined precisely. The machine operator needs to be guided in terms of what to do with his/her machine, therefore the GPS derived coordinates and site design components need to be transparent to the operator in the cab. The site DTM and project design are loaded into the control unit aboard the machine. As the task progresses.
and changes are made to the plan, these can be added either with updated physical media or by transferring the data by radio link. The operator may be guided on the task by light bars telling him/her whether to raise, lower or angle the machine’s blade or there may be a visual display of the task so that the ground ahead can be compared with a design visualization. With machine guidance there is no need for conventional setting-out because no profile or batter boards need to be established.
The role of the surveyor is now more concerned with setting up the GPS system and ensuring that the correct coordinate systems are in use, that the GPS is functioning correctly as well as ensuring the design data is correctly formatted for the task in hand. As well as using the system to make the process of cut, fill and grading more efficient the GPS can be used to do a survey of the work achieved so far on a daily basis. This can be used to calculate payment for the task and to determine whether the specification for the work has been achieved. The same design file can be used for the initial rough cuts and moves of mass of dirt as well as the finished grades. Where there is automatic control of the blade this helps with more accurate grading with fewer passes. When the GPS is interfaced to the machines’ hydraulic valves there can be automatic control of elevation and cross-slope.
Plate tectonics is centred on the theories of continental drift and is the most widely accepted model describing crustal movement. GPS is being used on a local and regional basis to measure three-dimensional movement. Locally, inter-station vectors across faults are being continually monitored to millimetre accuracy, whilst regionally, GPS networks have been established across continental plate boundaries. The information obtained adds greatly to the study of earthquake prediction, volcanoes and plate motion.
The concepts are similar to those used in machine guidance and control; however, DGPS levels of accuracy are often sufficient. The unit steers the agricultural machine along parallel, curved, or circular evenly spaced swaths taking account of the swath width. Automated steering may be used to free the operator from steering the equipment until he/she comes to the end or the corner of the field. Yield monitoring may also be incorporated into the system. The system may also be used to vary the levels of seed or fertilizer distribution across the field to ensure there is a minimum of waste.