Surveying GIS: The Great GPSathon of 1991
January 3, 2012
On March 19, 2011, surveyors across the United States participated in Surveying USA. This momentous event was coordinated by NSPS and overseen by NGS by way of its Online User Positioning Service (OPUS). Although this was the first attempt at a truly national exercise, it was not the first event of its kind; an earlier pioneering venture coordinated a large number of GPS receivers over a large area to achieve a high accuracy network.
In 1991, the County of San Diego developed what became known as the San Diego County High Precision Geodetic Network (HPGN).1 There were a number of contributing factors that led to a then first-of-its-kind cooperative High Accuracy Reference Network, but high on the list was a framework for the county’s rapidly developing geographic information system.
The 1989 Mission County Statement contained this directive:
The purpose of establishing and maintaining a GPS Network in San Diego is to provide a uniform horizontal and vertical datum to be used in support of the Regional Urban Information System (RUIS)2; in the assessment and management of infrastructure, Demographics and Environmental policies, and in the anticipation of future widespread use of GPS by public and private industry.
In 1991, contemplating an exercise the magnitude of the “GPSathon,” as it was dubbed, was daunting to say the least. The Internet use was still in its nascent stages. The graphical user interface (GUI) hadn’t entered the mainstream. Only a small number of people regularly used cell phones.
With the best technology available at the time, the GPSathon Triduum took place on April 5-7, 1991. Thirty-five strategically placed GPS receivers occupied candidate passive stations and collected data simultaneously in three coordinated 10-hour sessions.3 Putting the project together took several months, and it took more than a year to complete the data processing and get the data published by NGS.
What is significant about the development of the HPGN that resulted from the 1991 GPSathon was how it was promoted. Many people had an interest in developing a statewide HPGN. The geodetic community was leading the pack, but they lacked sufficient funding for anything so ambitious. The transportation sector had both the vision and a big share of the funding. The education group had expertise with spatial networking but had not addressed financial networking at the required levels.
It was the land development community that provided the “glue” that held this project together. The need to tie the land base to a truly robust geodetic network was important enough to be codified in the Public Resources Code.
Section 8817 of the California Public Resources Code requires that all new surveys and new mapping projects that use State Plane Coordinates must use the California Coordinate System of 1983 (CCS83). CCS83 is based on NAD83.
CCS83 is the coordinate system used for all mapping, planning, design, right-of-way engineering, and construction on Caltrans-involved transportation improvement projects including special-funded State highway projects. The physical (on-the-ground) reference network for CCS83 is the CA-HPGN.
There had been local requirements to tie subdivision maps and some records of survey to the state grid for some time, but it was unevenly applied, and the accuracies for existing passive stations varied. To ensure everyone was actually “on the same page,” so to speak, a firm definition of which NAD83 would be used for local ties needed to be specified. The GPSathon produced the 1991.35 Epoch.
What is an epoch? At the risk of sounding a bit poetic, an epoch is a place in time and space. In geodetic terms, it means the date for which published coordinate values are valid. Where ongoing crustal motion occurs, GPS values for passive stations change over time. To keep integrity in the land base positions, relative positions for reference points must remain fixed.
Initially a goal of A-Order of relative accuracy was proposed as the desired end result. But that was diluted a bit because the county required the stations to be set at approximate 20-kilometer intervals to use the network effectively for land base ties. The shorter base lines couldn’t meet the minimum accuracy requirements for A-Order, so in the end the network was published with B-Order values.
The big event took three days. But months of planning and organization were required to bring it to culmination. There was a substantial amount of “pre-testing” required. At the time, simultaneous sessions using different models of GPS receivers were largely undocumented. Experiments were performed under the supervision of Dr. Yehuda Bock of The Scripps Institute of Oceanography. There were some small issues that turned out to be discrepancies with antenna phase center variations, but they didn’t rise to a level that would have compromised the validity of the project.
Multiple agencies contributed both personnel and equipment to this event. The dual-frequency receivers were Ashtech LD XII and MD XII, and Trimble 4000 SST. The County of San Diego, City of San Diego, Orange County, California Department of Transportation, National Geodetic Survey and the Scripps Institute of Oceanography all actively participated.
I had the good fortune to play a small role in this momentous event. I spent three days occupying Station Junction Azimuth for 10 hours each. There was a small observation log (whereabouts unknown) to complete each day.
After the returns from the GPSathon were published, I got involved in readjusting a large number of existing county control points to the HPGN and creating a new database to tie subdivision and parcel maps to the GIS land base layer. Eventually, I moved from my position as supervisor of the CADD and calculations group to full time survey advisor to the geographic information system, and then to departmental GIS manager. The GPSathon was a turning point in my career.
Twenty years after the first GPSathon, a large part of its legacy in San Diego County is a full-featured, georeferenced online survey records system. It will be interesting to see what sort of fruit Surveying USA bears.
1. HPGN (High Precision Geodetic Network) was the original nomenclature applied to this project. HARN (High Accuracy Geodetic Network) was an acronym NGS developed later on. The San Diego data was already published as HPGN and not re-named to eliminate possible confusion.
2. RUIS was the original name for the countywide Geographic Information System for San Diego County.
Thirty-four stations were completed. There was one failure.
For more details about Surveying USA, see http://oceanservice.noaa.gov/news/weeklynews/mar11/surveyor-day.html
Author Acknowledgement: Special thanks to Steve Martin, PLS, chief of the Geodetic Branch of the California Department of Water Resources, for the photos and insight. For more details on the origins of the California HGPN, see California Surveyor Issue #167, Fall 2011, online at www.californiasurveyors.org/calsurv.html.