Although legal and physical aspects of this archaic system have been modified over time, its continued use in the rapidly expanding industrial and high-tech economy of Puerto Rico seriously impedes public- and private-sector land-use transactions.
Continuing losses in property-tax revenue and interminable litigation over property disputes finally prompted government action. In 1993, to correct the situation, the Legislature called for a modern system of land-use identification and classification, beginning with the establishment of a high-accuracy geodetic reference network to support a GIS-based cadastral system and all future mapping applications in the commonwealth. The network was to be surveyed using Federal Geodetic Control Committee Standards for "B" Order -- one part per million, relative positioning accuracy -- GPS surveys, and approved by the National Geodetic Survey (NGS) for incorporation into the U.S. National Spatial Reference System. Plans for establishing the network were approved in July 1995.
GEODETIC NETWORK
A geodetic network is a system of survey points, or stations, whose locations are determined in accordance with national accuracy standards, such as those established by NGS. All land surveying, boundary determination, map projections and coordinates in a given geographic area are referenced to stations within the network.
The network itself, said Geodetic Engineer Mark Bardakjian, "requires measurements based on a mathematical model of the Earth as an ellipsoid. The closer the model is to the actual shape of the Earth, the more accurate will be the surface on which to relate measurements and spatially reference geographic features. With the advent of satellites and ground-tracking systems, we have been able to get a better understanding of the true size and shape of the Earth."
The ellipsoid, in turn, provides the datum, or set of parameters and control points used to define the three-dimensional shape of the Earth in the area to be surveyed. The most precise ellipsoid to date is the Geodetic Reference System of 1980 (GRS80). The datum for map projections and coordinates on the North American Continent is the North American Datum of 1983 (NAD83).
"The more precise and extensive the network," Bardakjian added, "the more exact will be the subsequent data tied to the network. An older geodetic network based on a less precise ellipsoid existed in Puerto Rico. The upgrading and enhancement of that network into GRS80 and NAD83 would serve to tie Puerto Rico into the latest, precise geodetic network in the continental U.S."
PROJECT SCOPE
Puerto Rico's existing network, established in the 1960s, consisted of a mix of survey stations of different order -- varying accuracy standards -- loosely scattered across the main island and the smaller islands of Vieques, Culebra and St. Croix in the U.S. Virgin Islands. NGS requirements for the high-accuracy network called for adding 22 new "B" Order stations to the six previously existing "A" and "B" Order stations.
The reconnaissance phase of the project required locating and assessing the physical condition of monuments -- survey markers -- at existing stations, replacing older monument caps where needed, and locating sites and constructing monuments for the 22 new stations. Brass monument caps are connected to stainless-steel threaded rods screwed together and driven 10 to 32 feet into the ground, depending on the holding power of the soil. The upper portion of the rod with the station's inscribed cap is enclosed in a ground-level cement casing with a metal plate screwed down over the access hole. A red-orange "witness post" is driven into the ground nearby to indicate presence of a survey station. Field notes accompany station construction and monument replacement, providing a description of the station and directions for locating it.
The survey phase called for "B" Order GPS observations at each station, accompanied by photographs and field notes on the location, times and meteorological phenomena recorded during each observation period. The process required data backup, post-processing and validity checks at the end of each day's observation.
FIRM SELECTED
The firm selected by the prime contractor to conduct the survey was Andregg Inc., a 16-member survey-engineering firm from Auburn, Calif., with an extensive background in public- and private-sector projects in the United States and Latin America. The firm specializes in applying GPS survey systems to large, complex computer-based projects. On notice of the forthcoming contract, Andregg drew up plans for the network and submitted them to NGS for approval. The firm also worked out the logistics of getting a six-member survey crew and all equipment to Puerto Rico; establishing housing and a base of operations in the capital, San Juan; and getting around on an overpopulated island with few highways -- at the height of the tourist season.
By the time the contract was signed in November 1995, the firm had received NGS approval for the proposed survey, and through Hector Sanabria Valentin & Associates of Bayamon, Puerto Rico, secured a base of operations and combined housing in San Juan, along with cellular phones and transportation for each crew member. The detailed preplanning enabled the firm to handle an unexpected contract stipulation requiring the project be completed in 11 weeks, with data submitted to NGS by Jan. 22, 1996. According to Andregg's Bardakjian, "the deadline was not open for negotiation."
FIELD WORK
On arrival in San Juan, Andregg's advance crew of two set up the base of operations and hired three local people to assist with field operations, translation and driving. The location of the base allowed the crew to be close to San Juan for business purposes, while providing easy access to the few available highways.
A day of orientation showed that road and traffic conditions required six to 10 hours to complete a circuit of the island -- 133 miles E-W x 41 miles N-S -- and considerably longer over much poorer roads to reach sites in the interior. On an island with an average of 520 persons per square kilometer -- compared to 44 persons per square kilometer on the U.S. mainland -- and with most of the people in San Juan, traffic at certain times of the day doubled or tripled the amount of time it took to reach various sites.
The crew began by locating and assessing the condition of stations in the existing network and replacing monuments where needed. They located sites for the 22 new stations and constructed 18 new monuments, with GPS coordinates and directions for reaching each site. Island-hopping planes provided access to Vieques, Culebra and St. Croix. By late November the first phase of field work was completed.
The remaining four members of the crew arrived at the beginning of December with six geodetic receivers, antennas, barometers, thermometers, a computer, printer and other surveying and mapping equipment. Following a two-day orientation, the team began two four-hour observation periods a day, allowing two hours for traveling between most stations. Observations on other islands required a full day. During the entire time on the island, the crews made between three and seven, four-hour GPS observations at each station, concurrently with observations at "A" Order -- one part per 10 million -- and "B" Order control stations. "The observation scheme," Bardakjian said, "assured optimum network geometry and 100 percent station-occupation redundancy."
During data logging, crews made field notes on geographic, meteorological and temporal conditions associated with each observation. Hector Sanabria Valentin & Associates assisted in obtaining security clearances for the crews to conduct observations at stations located on airport perimeters.
OBSERVATIONS
Observations were made using Trimble 4000 SSE and 4000 SSI, dual-frequency, dual P-Code GPS receivers and L1/L2 geodetic ground plane antennas. SECO fixed-height antennas, and tripods with bubble levels were used to minimize setup errors. Data from observations were backed up and postprocessed daily using broadcast ephemeris and Trimble GPSurvey postprocessing software. Validity checks included analysis of vector solutions and confirmation of loop closure. All data produced reasonable vector solutions, and loop closure exceeding one part per million relative positioning accuracy. Observations were completed in nine days.
The schedule was grueling, said Bardakjian. "To be at our stations by 6:30 a.m., some of us had to leave the house at 4 a.m. Before leaving, crews reviewed the day's schedule and routes, calibrated barometers and thermometers, checked cellular phones and loaded equipment. At that hour, there was little traffic. But completing observations at around 5 p.m. put returning crews in the middle of rush-hour traffic. After reaching the house, unloading equipment and debriefing the crews, it was 9 p.m. By the time we downloaded the data from all six receivers, finished dinner, showered and made it to bed, it was midnight. The next day started at 3:30 a.m."
Bardakjian pointed out that the most distant sites were scheduled for observation first, followed by stations successively closer to San Juan. "This strategy worked well because by the time the long hours were wearing on us, the commute was down to a half hour."
DATA PREPARATION
Final postprocessing of the GPS data was conducted in California, using the precise satellite ephemeris -- available two weeks after observations. To meet NGS requirements for submitting final Bluebook results, all data was processed and adjusted with NGS software, OMNI and ADJUST, respectively. For purposes of comparison, the data was also processed and adjusted with Trimble GPSurvey and Trimnet Plus software. Results from both sets of minimally constrained and fully constrained least-squares adjustments showed the survey achieved better than "B" Order precision. The final positions of all stations in the network were determined using measured baseline vectors ranging from 12 to 236 kilometers, with a precision of 8mm -- 0.025 feet, or one-quarter inch -- less than the diameter of a dime.
Data from the survey, now accepted and published, was presented in Bluebook format to NGS on Jan. 22, 1996, for review, analysis and inclusion in the National Spatial Reference System.
OTHER BENEFITS
The network will also facilitate photogrammetric applications, land-use planning and greater understanding of the island's natural resources, topography and coastal hydrography. In addition, the new, permanent monuments will provide precise geodetic control for a broad range of surveying, construction and engineering projects.
Bill McGarigle is a free-lance writer specializing in communication & information technology.
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Terminology
A Order precision: (for GPS surveys) One part per 10 million, relative positioning accuracy.
B Order precision: (for GPS surveys) One part per million, relative positioning accuracy.
Baseline vector: Also, simply baseline or vector. Usually, a line in
Cadastre: A public record of property ownership within a district describing the extent and value of the land for tax assessment purposes.
Datum: A mathematical model (ellipsoid) of the Earth's shape, as applied to a certain area of the globe.
Ephemeris: Predicted GPS satellite positions, continually broadcast from the satellite, along with positioning data.
Fully constrained: In a least-squares adjustment of observational data, known or accepted values are held "fixed" and not allowed to be freely adjusted; i.e., constraints are introduced to the adjustment. Constrained adjustments will use models which account for the reference ellipsoid, the orientation and scale differences. The constrained adjustment determines the appropriate orientation and scale corrections to the GPS baseline vectors so it will conform to the local network control. (Federal Geodetic Control Committee).
Least-squares adjustment: A method of adjustment of observational data by which the sum of the squares of the residuals is minimized. (Mikhail, Analysis and Adjustment of Survey Measurements).
Loop closure: A method of analyzing the internal consistency of a GPS survey network whereby a series of baseline vector components, forming a loop or closed figure, is added together. The error of closure is the ratio of the length of the line representing the equivalent of the resultant errors in the baseline vector components to the length of the perimeter of the figure constituting the loop analyzed. (FGCC).
Minimally constrained: In a least-squares adjustment of observational data, none or only one set of data is held fixed, or constrained, about which the remaining data is freely adjusted. The least-squares adjustment will indicate the maximum achievable precision for the geometric classification. (FGCC).
National Spatial Reference System: Also, National Geodetic Reference System.
SOLUTION SUMMARY
PROBLEM/SITUATION: Outdated cadastral system impedes land-use transactions and property taxation.
SOLUTION: Establish a high-accuracy geodetic network as a precision reference for a GIS-based cadastral system.
JURISDICTION: Commonwealth of Puerto Rico.
VENDORS: Andregg Inc., Hector Sanabria Valentin & Associates, SECO, Trimble.
VENDOR CONTACTS: Mark Bardakjian, Andregg Inc., 800/400-7072; Dennis Meyer, Andregg, Inc., 800/400-7072; Hector M. Sanabria, Hector Sanabria Valentin & Associates, 809/785-2240.
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