GPS Drives Freeway Project

April 20, 2000
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As monstrous bridges and concrete structures loom over the horizon east of booming metropolitan Phoenix, the Price Road Freeway comes to life. Since the first contract was approved in November 1998, the five-mile Arizona Department of Transportation (ADOT) project-which will be part of the expanding Phoenix outer belt-is taking shape. But what really makes this project stand out is how surveyors are using GPS for extensive construction staking-and more.

Sunrise Engineering (Phoenix, Ariz.) surveyors selected GPS for the Price Road Freeway project because of their past experience with the technology. The 20-year-old engineering firm learned the GPS ropes while working on a tailings project in Salt Lake City, Utah. Karey Rowley, Sunrise survey manager, says survey crews quickly learned how to use GPS and to prove its capabilities. According to him, although GPS is a relatively new technology-especially with regard to construction staking-it is extremely accurate and fast, which increases productivity and saves the customer money.

"With GPS, one crew can do as much work as three conventional crews," he says. "In fact, a crew can complete in one day a survey that would take a week with conventional methods."

Sunrise surveyors position the base receiver of a Trimble 4800 GPS Real-Time Kinematic (RTK) system (Trimble Navigation Ltd., Sunnyvale, Calif.) near the center of the project each morning. From that position, Dale Robinson, chief of surveys for Sunrise, has his GPS crews move freely throughout the five-mile project, setting points accurately to within 1 cm horizontally and 2 cm vertically. The setup also eliminates line-of-site problems, he says. Three survey crews using GPS and conventional equipment will complete the Price Road Freeway project by the end of this year.

On the burgeoning Price Road project, surveyors are performing slope staking (1,000 m a day) with one crew. Two other crews are staking water, sewer and storm drains-up to a couple of miles a day. In addition, surveyors are staking subgrades (1,500 m a day)-at 20 m intervals-then taking a differential level to establish grade, curb and gutter, and road alignments.

The survey includes traffic detour roads, frontage roads, exit and entrance ramps, a three-by-three lane separated freeway, sanitary and storm sewers, three retention ponds, retaining walls, sound walls and two sewer lift stations.

"Communication is key to the success of this project," Robinson says. "We are in constant communication with the contractor's project managers and foremen to facilitate the numerous survey requests every day. The combination of experience, survey techniques and equipment allows the team to respond to requests almost immediately."

The Choice to Go With GPS

Rowley says when the project first got underway, the main focus for surveyors was to verify control. To get started, the team obtained a list of control from ADOT, then started a differential level loop using a Zeiss digital level (Carl Zeiss, Oberkochen, Germany). In the meantime, surveyors searched the National Geodetic Survey (NGS) database at www.ngs.noaa.gov for High Accuracy Reference Network (HARN) stations or other control in the area. They found a HARN station 2 km west of the south end of the project along with two first-order horizontal points to the east.

"We were concerned about providing good latitude, longitude and height (LLH) values to coordinate the seed post-processing," Rowley says. "Using a strictly RTK system, and obtaining a beginning LLH by letting the unit secure them autonomously can introduce errors as great as 10 parts per million in the horizontal and 20 to 30 parts per million in the vertical. Beginning with good values eliminates this error source from the start." On-the-fly initializations are gained quicker and lost less often when good LLH values are used as part of the calibration process, he adds.

After the level loop was closed and found to be in agreement with ADOT elevations, the surveyors began fast-static GPS observations on stations opposite to each other throughout the corridor.

During the effort, two Trimble 4400 and two Trimble 4800 systems and TSCI data collectors were used. Once observations were completed, the data was run through the WAVE processor in Trimble's GPSurvey software package. The processor computes the vectors from field observations using static, fast-static or kinematic data collection procedures and generates values that can be used to determine the quality of the observations.

"Looking at the solution summary for the ratio and reference variance values indicate baselines that might create problems during the least squares adjustment," Rowley says. "We were satisfied with the results, so the data was passed to Trimble's Trimnet adjustment software to begin the process of constraining to the horizontal and vertical control points provided by ADOT."

Sunrise surveyors first imported the raw baselines into the GPS network modules for a pre-adjustment analysis. Rowley says it is a good place to check for field errors such as different names for the same station and duplicate baselines. Once surveyors are satisfied with the data, it is sent to the network adjustment module where most of the work is done. By constraining to one station and running a minimally constrained adjustment, the quality of the observations can be checked against each other. Station weighing errors and scalars can be applied until the adjustment passes Chi square and Tau. Once the adjustment passes this phase, it can be passed to the geoid module to begin the process of building a residual model of the local geoid.

Calculating Elevations

Rowley says that when a network includes control with differential and GPS observations, it can be combined in Trimble's Trimnet adjustment software. After the minimal constraint has been accepted and any additional horizontal points held (two minimum), the observations then can be passed on to the geoid module, and the separations between ellipsoid and geoid elevations can be estimated using GEOID96. Error estimates are applied (a priori) at this point so they can be used in the combined adjustment in the network module.

To begin this process, surveyors first combined GPS (static) observations with differential elevations placed strategically throughout the project site. Once a minimal constraint was performed (using a minimum of two horizontal control points), the data was transferred to the geoid module.

"GEOID96 is an excellent starting place to generate good error estimates," Rowley says. "Once these were applied to the observations, then transferred to the terrestrial module, the elevations were calculated using the computations utilities menu. The information then was passed to the network adjustment module, where vertical constraints were applied and residuals generated." At this point, elevations may not be accurate even though the differences in elevations between the points are precise. They must be constrained within the network module.

Pre-analysis of the vertical control was performed by fixing three known vertical benchmarks surrounding the project, then comparing the rest to see how closely they fit. In a small area, they should match closely; large projects still could have significant differences.

Rowley says it is crucial to ensure the points used do not fall in a straight line. "Bad elevations can be eliminated using this method," he says. At least three vertical control points must be used to define a reference surface. A minimum of four vertical control points (the more the better) must be held in order to calculate residuals to transfer back to the geoid module so new and better error estimates can be applied to further redefine the model. Five benchmarks are needed in order to apply the residual model generated. Points to be constrained are placed in key areas, such as the foot and crest of hills, to ensure proper modeling of the terrain. Rowley says surveyors on the freeway project have been fortunate to have a consistent geoid that gradually slopes toward the southeast because in hilly and mountainous areas, the geoid can contain significant undulations. "Using this modeling technique vastly improved the quality of the model used," he adds.

Once the vertical control, consisting of nine benchmarks, was fixed and adjusted, a local geoid model was exported to the geoid module and converted to a GGF format so it could be used in Trimble's TSOffice to perform a site calibration. Rowley says the calibration process in TSOffice is fairly straightforward. Once the LLH, northing, easting and elevation (NEE) and values have been imported, the points can be selected on the screen and the calibration computed. The residuals should be within a few millimeters. "Accept the calibration, then create a DC file that can be exported to the data collector," Rowley advises.

First, the points should be edited on the properties screen, then changed to control class so the DC file can be used as the master control file for several projects on the same site. The information then should be exported to the DC.

"Our team went to the jobsite and used RTK to see how well this process fit the site control," Rowley says. "Our errors were a few millimeters horizontally and under 1 cm vertically. The vertical errors remain under 1 cm, with most around a few millimeters. In addition, the elevations established by RTK are checked routinely using digital levels." Some of the baselines are two to three miles long. These values are attained anywhere on the site with the base receiver at the same central location, giving surveyors the ability to respond to the contractor's problems or questions at a moment's notice. Any questionable locations or grades can be checked within a few minutes after the crew arrives on the site. Points set by other crews can be shot and checked within minutes.

Rowley says that with a little ingenuity, the surveyor has no problem staking anything that is required. Mapping for as-builts or volume calculations is accomplished easily with one surveyor, although staking is quicker with two. When setting subgrades or blue tops for paving, three surveyors are very efficient. One locates the point with GPS, while the other two come behind with a level to set grade. Rowley says 1,500 to 2,000 m can be staked per day at 20 m intervals (both of shoulders and centerline). He adds that 1,500 m of slope stakes with offsets (both sides) can be done easily in a day as well.

Knowing when to use the right tool is essential in surveying. Greg Sanders, Sunrise's project manager on the Phoenix outer loop jobsite, decides when and where to use GPS versus conventional equipment. Sanders says that on this project, crews not only are surveying dirt work such as slope staking, but also are surveying pipe runs, curb, gutter, retaining walls and sound walls. While not surveying bridges and structures with GPS, the Sunrise team is using GPS to set control on the structures.

For example, once the slope stakes have been set, the contractor brings in equipment to build the pad. The survey crew uses GPS to stake the footings. The contractor then builds the footings, and GPS is returned to the site so the walls can be staked-all within a moment's notice.

Sunrise surveyors say that by using Trimble's guidelines and methods, good elevations are being produced. "We are confident our methods produced results consistent with the manufacturer's claim of 1 cm horizontally, and 2 cm vertically," says Rowley.

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