The preferred method for obtaining GPS reference station coordinates is to occupy first-order (Class I) vertical, first-order horizontal monuments (bench marks) established by the National Geodetic Survey (NGS). However, it is seldom that first-order monuments are available near the study site and convenient to access by either land or boat. In addition, locating appropriate monuments without obstructions is often problematic.

To overcome the issue of locating high order bench marks, scientists at the USGS have developed a method for establishing new bench marks. It should be noted that this method does not conform to NGS "Blue Book" standards, but internal tests conducted by the USGS indicate this method is accurate to +/- 2 centimeters (cm). Novatel Waypoint Consulting, Inc., a GPS software development company, compared data using this method to traditional GPS methods and found similar results (McDonald, 2002).

When the USGS establishes a new bench mark, the location is carefully selected to avoid multipath errors. Signal multi-pathing errors occur when a GPS signal reflects off a nearby structure and is recorded along with the original unobstructed signal. Thus, two identical signals are recorded, each with different arrival times, thereby corrupting the signal. The general guideline applied is that the GPS antenna must be located where there are no signal obstructions above 10 degrees from the horizon and where it is easily assessible.

Industry standard survey components were used for physically building a new benchmark (fig. 46). When the location is over soil, the components consist of sectional 1 meter long by 1.905 cm diameter aluminum pipe, survey cap, 7.6-cm PVC, and bag concrete. The sections of aluminum pipe are coupled together and driven to refusal; the upper 0.3- to 0.6-m section is excavated and encased with the 7.6-cm polyvinyl chloride (PVC) pipe. The cavity is filled with concrete, and the survey cap is attached. If the location is situated on the tarmac pavement a or road surface, a hardened 3-inch Parker Kalon (PK) survey nail is driven into the surface.

The new bench mark is occupied by the reference GPS receiver for a minimum of three 8-hour occupations recorded at 30-second epoch intervals. The autonomous static GPS processing software service, NGS OPUS, was used to compute new benchmark coordinates for each daily occupation. OPUS uses the nearest three Continuously Operating Reference Stations (CORS) stations. The stated vertical accuracies of OPUS is between 1 to 3 cm Root Mean Square (RMS).

OPUS coordinates are provided in North American Datum 1983 (NAD83) horizontal coordinates and ellipsoid height vertical coordinates. NGS Horizontal Time-Dependent Positioning (HTDP) software was used to convert NAD83 to World Geodetic System 1984 (WGS84) coordinates. Positions derived from all occupations were then averaged. The derived position was applied when processing the appropriate kinematic boat data.