GPS/GIS: Puerto Rico Establishes Geodetic Network
A Northern California survey-engineering firm uses GPS to establish a GIS-based cadastral system
By Bill McGarigle

Andregg Inc., a Northern California survey-engineering firm, recently completed the GPS survey phase of a high-accuracy, geodetic network for the commonwealth of Puerto Rico. The purpose of the survey was to establish a reference for an eventual GIS-based cadastral system and all future mapping in the commonwealth. Overall, the project encompassed the islands of Puerto Rico, Vieques, Culebra and St. Croix, and included extensive monument (survey marker) construction, field documentation, data processing, and preparation of reports and files for the National Geodetic Survey (NGS). The network has since been approved by NGS for incorporation into the U.S. National Spatial Reference System.

Background
Establishment of a precision geodetic reference network was prompted by the need to replace a cadastral system that had its roots in Spanish Colonial mortgage laws of the late 19th century, when sparsely-populated Puerto Rico had a single-crop economy based on sugar. From the mid-20th century on, however, population growth, economic development and the influx of high-tech industry accelerated land transactions, sub divisions, and mapping applications beyond the ability of the old system to track. Mapping based on the loosely-defined geodetic network established in the 1960s further complicated issues of ownership and land classification. As a result, courts became mired in real-estate litigation, and government's ability to collect property progressively diminished. As one member of the Supreme Court observed, the situation reached a point where "the physical reality of the land no longer corresponded to the legal."

Proposal to Improve Cadastre
A Chamber of Deputies resolution in 1992, calling for a cadastre based on modern technology, cited numerous loopholes in the existing system: "The law does not require the buyer to register his purchase of the property. However, until he does so, the property is still recorded as belonging to the seller. If the seller sells the property again, and the second buyer registers the property, the property is legally his." In addition to changes in property laws, the resolution proposed building additional survey points, or stations, in the existing geodetic network as a means of achieving greater accuracy. "A cadastre tied to a high-accuracy network will aid the government in identifying its properties, resources, uses, and rights of way. It will serve citizens and private institutions by guaranteeing property rights - land will not be registered by more than one person - and it will facilitate the buying and selling of property and associated banking transactions."
     Map projections, coordinates, land surveys, and boundary determination are referenced to geodetic network control points whose coordinates are based, in turn, on a mathematical model of the Earth as an ellipsoid. The model provides the datum used to measure the size and shape of the Earth's surface in the area to be surveyed. The most accurate model of the Earth to date is the Geodetic Reference System of 1980 (GRS80), having been determined through extensive satellite measurement. For geodetic surveys on the North American Continent, the datum used is the North American Datum of 1983 (NAD83).
     "The more precise and extensive the network," explained Andregg Geodetic Engineer Mark Bardakjian, "the more exact the subsequent data tied to the network. In Puerto Rico, the old network was based on a less precise ellipsoid, dating back to the 1960s. Upgrading and enhancing that network into the GRS80 and NAD83, would serve to tie Puerto Rico into the latest, precise geodetic network in the Continental U.S."
     In 1995, the legislature approved plans for a geodetic control network to be surveyed using U.S. Federal Geodetic Control Committee Standards for 'B' Order (1 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.

Planning
Prior to field work in Puerto Rico, Andregg secured NGS approval of plans for an observation scheme and network station locations. Through professional association with Marel Bayamon Inc., a public-land survey company in Puerto Rico, the firm secured transportation, communications, local assistants, clearances for airport surveys, a base of operations, and living quarters for a crew of six. Despite an unexpected contract timeline of 11 weeks for the entire project, field crews were able to complete their work on schedule - most of it on an island with few roads and highways. "The deadline," Bardakjian recalled, "was not open for negotiation."

Reconnaissance and Construction
On Nov. 2, 1995, an advance crew of two set up the operations base in San Juan, and hired three assistants for translation, driving, and field operations. The next priority was determining the driving time needed to reach the various station sites on the main island. Although Puerto Rico is only 133 miles E-W by 41 miles N-S, a complete circuit of the island took up to 10 hours. At certain times of the day, traffic congestion in and around cities could stretch that out even longer. Stations away from the coast required the most time to reach, roads to the interior being the poorest. To reach Vieques, Culebra and St. Croix, the crew used air taxis.
     At existing network stations, the advanced crew with field assistants replaced monuments (survey markers) that were damaged or missing, and constructed monuments for each of the 22 new stations. Monument construction involved driving stainless-steel, threaded rods 10-32 feet into the ground, depending on soil holding power, and enclosing the upper section of the rod and inscribed brass cap in a ground-level cement casing. The cap is accessed by removing the metal plate secured over the top. A bright-colored "witness post" driven into the ground nearby signals the presence of the survey station. Fieldnotes provided coarse GPS coordinates and directions to each site, and a description of the monument. Despite road conditions and difficulties in securing air transportation at the height of the Caribbean tourist season, the crew completed this phase of the project in three weeks.

Observation Phase
On December 2, four crew members from Andregg brought equipment needed for observations and data processing - six geodetic receivers and antennas, barometers, thermometers, a computer, printer, and other support equipment. In the field, the six-member team carried out between three and seven, four-hour GPS observations at each station, concurrently with observations at 'A' Order (1 part per 10 million) and 'B' Order control stations. Each observation included field notes on geographic, meteorological and temporal conditions. The observation scheme assured optimum network geometry and 100 percent station-occupation redundancy. Travel between most stations required two hours. Including air travel time, observations on each of the smaller islands took an entire day. Bardakjian pointed out that the most distant sites were scheduled for observation first, followed by stations successively closer to San Juan. "The strategy worked well because by the time the long hours were wearing on us, the commute was down to a half hour."
     The team made all observations with Trimble 4000 SSE and 4000 SSI, dual-frequency, dual P-Code GPS receivers, using L1/L2 geodetic ground plane antennas. SECO fixed-height antennas, and tripods with bubble levels were used to minimize setup errors.
     At operations in San Juan, crews backed up and post processed observation data at the end of each day using broadcast ephemeris and Trimble GPSurvey post-processing software. Validity checks included loop closure confirmation and vector solution analysis. All data produced reasonable vector solutions, and loop closure exceeding 1 part per million (1ppm) relative positioning accuracy.
     "The schedule was grueling," Bardakjian recalled. "To be at our stations by 0630, some of us had to leave the house at 0400. 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 around 1700 put returning crews in the middle of rush-hour traffic. After reaching the house, unloading equipment and debriefing the crews, it was 2100. 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 0330." Difficulties notwithstanding, the team completed the observations in nine days.

Data Preparation
Andregg conducted a final post processing at their California headquarters, using the precise satellite ephemeris (available two weeks after observations). In accordance with NGS requirements, final results were processed and adjusted with OMNI and ADJUST (NGS software), respectively, and submitted in Bluebook format. For comparative purposes, the firm also processed and adjusted the data using Trimble GPSurvey and Trimnet Plus software. Results from both sets of minimally-constrained and fully-constrained least-squares adjustments showed better than 'B' Order precision achieved for the entire survey. 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 8 mm (0.025 ft., or in.). Bluebook results were submitted to NGS, Jan. 22, 1996. The data have subsequently been approved for inclusion in the NSRS.

Benefits
The new, high-accuracy network gives Puerto Rico a foundation on which to develop a GIS-based cadastral system that will serve all the islands in the archipelago. It will provide precise geodetic control for future photogrammetry, surveying, land-use planning, construction and engineering, and contribute to a better understanding of the islands' natural resources.

About the Author:
Bill McGarigle is a freelance writer specializing in communication & information technology. He may be reached at [email protected]

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