Integration of Real-time Kinematic GPS Technology into Everyday Surveying
Hewlett-Packard establishes new benchmark
By John V. Mettee, III

In July 1996, Frederick Ward Associates, Inc., of Bel Air, Maryland, purchased three Trimble GPS units, 4400 Site Surveyor models. Since delivery of the equipment and crew training, the receivers have been enthusiastically used on a daily basis. The RTK GPS technology has been integrated into everyday surveying activities such as construction stakeout, topography, as-built surveys, and planimetric locations for ALTA Land Title Surveys.
     Accuracy has been increased and productivity has doubled. Projects are executed in a more timely manner and the manpower savings is actually paying for the equipment investment.
     One reason the implementation was so successful was that I already had prior knowledge and experience with GPS technology. We are convinced that GPS technology is for the everyday surveyor and should already be included in his or her "toolbox."

Site Selection
The effective implementation of GPS technology should be a key issue addressed at the initial project planning stages. Considerations need to be given to satellite geometry, site obstructions, and location of existing horizontal and vertical control. Of course, where to establish the base station during the survey operations-to keep it out of harms way and safe from theft-is always a concern. Each potential site for RTK GPS applications is closely evaluated in detail. First, the site is investigated for potential obstructions, such as tall structures and tree canopy. Buildings as well as tree foliage always contribute to a high RMS problem. Occasionally, this task can be accomplished by examining existing maps, USGS quad maps, or previous survey maps. Frequently, this investigation will require a drive to the survey site.

Project Calibration
At Frederick Ward Associates, we have been fortunate in our use of RTK GPS surveying applications. This is partly because we are located in Harford County, Maryland, where there are plenty of first order NGS horizontal and vertical survey control monuments. If a project site does not have adequate control, generally there are enough NGS control monuments within a six-mile reach of our 35-watt radio to calibrate the project.
     However, if additional control is needed for the calibration of the project, we will use static GPS surveying applications to establish the required control. Sometimes, we even supplement the process with conventional methods, namely leveling. We typically use a minimum 10-minute occupation time at each control point and occupy more than five, if available. Upon completion of the initial GPS observations on the control monuments, our skilled party chiefs perform the calibration calculation in the TDC1 data collector right on the site. Furthermore, we prefer to have even more control points on-site than those that are required for calibration. Once the calibration calculation is complete, as an added quality control measure to double check the calculations performed in the field, we enter the stakeout mode on the TDC1 data collector and navigate to an additional control point to verify how close we strike.

Project Implementation
RTK GPS surveying is ideal for any construction stakeout project provided the site is open woodlands, of course. However, it requires good organization. Though the TDC1 collector permits stakeout in a line and along a curve with offsets, the most productive execution is accomplished by loading pre-calculated points.
     This means to COGO the locations of where you intend to place the various layout stakes, and then navigating to those points. Good care and judgment must be exercised in this process in order to alleviate the misfortune of computing offset stakes on the opposite side of utility pipes from what the contractor actually wanted. This simply means more effort needs to be given to coordination with others.
     We accomplish the COGO computations (generating points, coordinates, and elevations) with a combination of C&G and Geopack software in conjunction with our Microstation CADD drawings. ASCII coordinate files are generated of the respective design points and are imported into the Trimmapª project file. From Trimmap, the points are loaded into the TDC1 data collector. Once loaded, its off to the job site for construction stakeout.
     We have used this procedure on many types of projects, ranging from single and multiple family housing developments, to commercial development projects, to warehousing and distribution centers. We have noticed no inability to maintain the desired accuracies. In fact, in several projects, both GPS and conventional methods were used with no problems whatsoever.

Recapturing the Cost
Needless to say, as shown herein, RTK GPS surveying dramatically increases, if not doubles productivity. However, too many people are quick to just give away the technology at no cost. How can they? Yes, receiver prices have come down over the years, but it still costs more than pocket change. Our man hours will be cut almost in half. Does this mean we need to settle for less money for performing our survey? I think not! After all, we are making a sizable investment in equipment. Again, why just give it away?
     Therefore, we have developed an hourly premium for the equipment usage. We did this in such a way that we can also give a 10 to 20 percent discount to our clients in the overall project cost. This was done primarily to squelch the skepticism that existed in some of our client's minds about the new stuff called GPS. And to this date, we have recaptured a third of our investment into GPS technology. In the final analysis, the technology is going to pay for itself and we have happier clients because work is being accomplished for them faster and more accurately, with a cost savings on top of it all. They tell us, "GPS is pretty good stuff."

About the Author:
John V. Mettee, III, Prof. L.S., is the chief of surveys at Frederick Ward Associates, Inc., in Bel Air, Maryland.

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