Technology Profile- Pristine Boundaries
December 1, 2008
Nestled in the Gruppo del Baldo mountains halfway between Venice and Milan lies Lake Garda, the largest lake in Italy. Surrounded by towering pines and lush olive and lemon trees, the Alpine lake is 32 miles long and 10 miles across at its widest point in the south. Dozens of exclusive hotels and resorts dot its shorelines, and thousands of people are drawn to the lake each year to immerse themselves in its grandeur and beauty.
Alessandro Prudenzi stands on the verdant hills of the west bank, but he’s not there to relax and enjoy the scenery. As one of the most well-respected cadastral experts in the Brescia region of Lombardy, Prudenzi has been tasked with establishing a new boundary line that will determine how much property the real estate firm Sellero 3000 s.p.a. of Milan will surrender to surrounding land owners as part of a purchase agreement. The rugged hills, numerous valleys, steep cliffs, abundant vegetation and densely built zones in the region create a number of surveying challenges, including spotty access to satellite signals, which makes using GPS technology difficult. But Prudenzi is a master of speed and accuracy, thanks in large part to the tools he uses to accomplish the job.
A Practical GPS SystemPrudenzi, who was trained as an architect, spent 10 years in the Brescia Office of Cadastre before opening his own office 15 years ago to conduct appraisals and verifications of local real estate property and provide technical consulting to the Court of Brescia. Land surveying is just one aspect of his practice and, until recently, Prudenzi had relied solely on a total station for land survey work. While he was aware of the benefits of GPS technology, he viewed it as too expensive and impractical for the amount of land surveying his business handled.
Still, he continued to research and study new products so that his firm could stay on the cutting edge. When he heard about Magellan’s ProMark3 RTK survey system in mid-2007 from Giorgio Viaggi, of Italian distributor Guido Veronesi SRL, Prudenzi decided to take the leap. The system included a base station and rover, and it promised high accuracy and simple operation at an affordable price. “I believed it would improve and simplify my job,” Prudenzi says.
Before using the instrument on a jobsite, Prudenzi ran it through extensive tests in his backyard on different days. He discovered that the ProMark3 RTK consistently outperformed Magellan’s own published accuracy standards. For example, with the instrument in static mode, he used a local Leica SpiderNET network with reference stations approximately 56 to 81 miles away (90 to 130 kilometers) to measure several known points. While Magellan’s published variations range from 9.5 to 12.5 centimeters (0.005 m + 1 ppm), Prudenzi found variations ranging from 2 to 4 centimeters. “The true values are decidedly better than those expected,” Prudenzi says. He experienced similar results in different RTK tests he conducted. “Using previously measured points, the verifiable error is always, in my experience, less than 0.01 meter,” he says.
Prudenzi was also impressed with the system’s ability to simultaneously collect raw data during RTK sessions. “When I collect in RTK using the base station and simultaneously collect raw data, I can achieve a precision up to 10 times better than the rover only with differential corrections,” he says.
Accuracy in the AlpsIn the Lake Garda project, the level of precision provided by the ProMark3 RTK proves especially beneficial. Italian cadastral law requires the use of three reference points that are not positioned using an RTK rover, so Prudenzi must first use more conventional surveying tools-a Sokkia SET4 total station, a Psion XP data recorder and a Leica DISTO D3 laser distance meter with an integrated inclinometer, which allows him to measure the distances of any points that are not directly accessible. With the help of Geometra Maurizio Gaioni, a local land surveyor and technical adviser for one of the property buyers, Prudenzi makes 12 measurements to obtain the required reference points. “I only need three points of reference, but I always take at least three or four different measurements on each point to ensure precision,” Prudenzi notes.
After verifying the reference data with the cadastral office of Brescia and other local measurements, Prudenzi places the ProMark3 RTK base station just inside the newly defined border and connects the rover to Enri-brand UHF radios with high-gain antennas that are capable of operating at 436 MHz at a range of about 1,500 meters. (The ProMark 3 is supplied with a basic plug-and-play PDL radio that operates at frequencies of 869 MHz in Europe and has a range of 300 to 400 meters. While this range works well in many regions, Prudenzi discovered that he needed stronger signals to ensure a continuous connection in extreme terrain conditions such as those found at Lake Garda.) With this setup, Prudenzi measures 21 points, observing each point for at least 15 seconds to pick up enough data to ensure accurate post processing.
To calculate the coordinate values with high accuracy and in the coordinate system used locally, Prudenzi acquires RINEX data from the fixed GPS stations in the neighboring province of Trentino for the same time periods in which he measured every point in his survey. This step ensures that he won’t have to repeat the measurements to verify the reference points.
Within a matter of hours, Prudenzi has collected all of the data necessary to complete the survey. Using SOGEI’s Pregeo software (a commonly used Italian cadastral software) and Magellan’s GNSS Solutions post-processing software, Prudenzi saves the solved, adjusted positions of all the points to a PDF file, which he immediately transmits to Toscolano Maderno town hall. This information is then sent to the cadastral office of Brescia, where it is used to update the maps.
“If I had only used a conventional total station, this job would have taken me several days,” Prudenzi says. “I also would have had to move the total station to different locations, which would have degraded the survey precision. Using the ProMark3 RTK, I was able to generate measurements that are accurate to within a few millimeters.”
Prudenzi points out that there is no single perfect tool for every survey project. “To be economically competitive, every surveyor must know how to choose the instrument and method that will guarantee the level of precision required for [a particular project] while also allowing him to use his time to the best advantage,” he says. He adds that in some cases, such as lowland cadastral surveys without obstacles, total station measurements will continue to be sufficient. Often, however, Prudenzi’s work carries him into difficult terrain such as the rugged landscape surrounding Lake Garda. In these situations, he says, “I’ll surely use GPS.”