Vermont's high accuracy reference network.
GPS allows for more precise positioning in less time than conventional methods. It does not require a line-of-sight between ground points, eliminating the need to use mountain tops and towers. GPS does require a line-of-sight to the satellites and easy access to the survey monument, which means that open spaces near good roads make good sites for GPS survey disks.
To furnish a new set of GPS survey disks or reference monuments, Vermont initiated a High Accuracy Reference Network (HARN). The Vermont HARN upgrades the reference monuments and geographic reference system. These HARN monuments have a more exact geographic position and are consistent with the relative precision that GPS surveying provides. About half of the states have or will soon have a HARN.
Vermont's initiative came from Wisconsin. Paul Hartzhiem, Geodetic Survey Engineer with the Wisconsin Department of Transportation, spoke at the Annual Meeting of the Vermont Society of Land Surveyors in spring 1990 about the HARN in Wisconsin, which was among the first states to implement a HARN. In June 1990, Harry Roush of the Vermont Mapping Program received a proposal from Photo Science Corporation titled "A Rational Approach to the Establishment of a Control Network for Vermont," calling for a Vermont HARN.
These two incidents stimulated interest in establishing a Vermont HARN, and later that summer, the Vermont Geographic Information Systems Advisory Board unanimously carried a motion supporting the idea for Vermont. The 1991 Vermont General Assembly approved funding for the Agency of Transportation (VAOT) to establish a HARN. The budget designated state dollars to match federal funds from the Federal Highway Administration. A total of $60,000 was allocated to finance the project.
Soon after the funding was approved, the Vermont Geodetic Network Committee was established to promote the Vermont HARN and provide local liaison. The committee initiated three informational meetings held in different parts of Vermont. In September 1991, a forum regarding the Vermont HARN was held in Montpelier. William Strange, Chief Geodesist of the National Geodetic Survey, was the principal speaker at the forum.
A cooperative agreement between VAOT and the National Geodetic Survey (NGS) was developed to carry out Vermont's HARN. In Vermont, the Green Mountain National Forest joined in, and other states were contacted to see if they were interested in participating. The New Hampshire Department of Transportation joined the effort. In Canada, the Quebec Geodetic Survey also joined the HARN campaign. As the project progressed, what was once the Vermont HARN had grown to be the North East HARN (NEHARN). Vermont and New Hampshire were most active in achieving a sufficient number of HARN survey monuments. Currently Connecticut and Quebec are planning to establish additional HARN points, and New York is considering it.
The VAOT effort started with the designation of Chris Reed as project manager responsible for the reconnaissance of existing monuments and for selecting new HARN sites. The most stable monument is desired. The first choice is a disk in bedrock; 17 of the 29 monuments in Vermont are in bedrock. Second choice is a stainless steel rod monument; 6 of the HARN monuments are rods. In addition, 5 HARN monuments are traditional poured in place concrete monuments, and one is set into a large boulder.
The principal requirements for the HARN survey monuments are:
* Open sites with no obstructions above 15 degrees on the horizon.
* Easy access by car or truck.
* Located on public property.
* Stable monument.
In general these requirements were met. Only one site has any significant obstructions and these are minor. Seven are on private land and were used with the land owner's consent. There is one site, Killington, that is not so easy to access, but it can be reached year round and provides an important link to historical surveys.
Ten of the monuments are new points, of these 5 are epoxyed into bedrock and were set by VAOT. The remaining new marks--deep set stainless steel rods--were set by NGS with VAOT personnel assistance. At one of these sites, the rod was driven over 140 ft. Steve Randal of NGS's field party did an excellent job of demonstrating the setting of the deep rod mark for VAOT and the Green Mountain Forest.
During the mark setting and the reconnaissance, arrangements were made by VAOT to rent GPS receivers. Rental estimates were obtained for three different types of dual frequency GPS equipment. After review of the estimates, Ashtech, Inc. (Sunnyvale, California) receivers were selected by VAOT, however some Trimble Navigation (Sunnyvale, California) units were also used.
The final schedule for the NEHARN was issued in May 1992. VAOT was scheduled to start observing on June 18. The NGS field party started observing earlier in New Jersey and worked its way towards Vermont. The schedule consists of two parts, the first part is the A order observations and the second part is the B order observations. The A order points are extensions of the fundamental global reference sites for geographic position. The fundamental sites are the Very Long Baseline Interferometry stations and the Cooperative International GPS Network stations. From the A order positions, additional B order high accuracy reference monuments are established.
The first observing was done for the A order points. This took place during the first four days of the campaign and involved 17 stations in five states and the province of Quebec. The remainder of the campaign was to establish B order points. The Vermont portion lasted through July 2, 1992. Each station in the HARN was occupied for at least two days and each occupation lasted about five hours.
The success of the campaign was attributed to the patient, diligent work of the data reduction team both at the field office and at NGS's Space Geodesy office. NGS's field office was located in White River Junction and headed by Claude Stevens. Dan Callahan was in charge of the GPS field reductions, which was done using NGS's OMNI software and the broadcast ephemeris. Dan Callahan did an outstanding job managing the field reductions. Dan Martin of VAOT was assigned to the field office and was trained on NGS office procedures and reductions.
Once the data for entire NEHARN project was sent from the field office to NGS's Space Geodesy Branch, the final reductions took place. Thomas Soler lead this effort. The final reduction of the GPS data has been made using a precise ephemeris of the GPS satellite orbits. This computation ensures the most exact relative positions and the highest accuracy geographic positions.
NGS's Horizontal Branch is performing a rigorous least squares adjustment of the High Accuracy GPS data to determine the final HARN coordinates. These new coordinates will be part of a modern geographic coordinate system that can be used by all. These HARN coordinates were expected to be available in April or May of this year. They will be in the North American Datum of 1983 (1993 adjustment).
Modern geographic reference positions will support the growing use of GPS and GIS into the 21st century. These few geographic positions are the cornerstones for modern satellite surveying and future computerized mapping of our nation. They provide a uniform and globally accurate reference network, which all users of geographic information and spatial data can share. The mountain monuments will remain as sentinels to the historic surveys and maps of the past. The new HARN monuments will be used to make the next generation of modern maps. If your town, county, or state is considering new mapping or implementing a GIS, you should use the new HARN monuments to coordinate your project--to coordinate with coordinates.
J. MILO ROBINSON, Geodesist, Vermont Society of Land Surveyors, Montpelier, Vermont
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|Title Annotation:||global positioning network|
|Author:||Robinson, J. Milo|
|Date:||May 1, 1993|
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