City of Madison, Wisconsin - Old GPS Community Base Station

Base station information
(Revised August 26, 2016)

This base station is a Trimble Alloy GNSS reference 672 channel receiver with a Zephyr Geodetic antenna with ground plane, and is located at the City of Madison Engineering Division Operations Facility located at 1600 Emil St in Madison, WI. The base station uses an elevation mask of 13 degrees and a PDOP mask of 6.0.

GPS correction file data is internally processed within the Alloy receiver. The correction data is logged at 5 second intervals and Rinex data is logged at 10 second intervals. Data is logged 24 hours a day, 7 days a week. Any anticipated outages in data logging will be announced in advance, if possible, on the base station status announcements page

The reference position of this base station is:

GeNeral Information regarding the establishment of the reference position:
The latitude and longitude are in the NAD83(1997) datum, determined by Ayres and Associates, Inc. in August 2005 from National Geodetic Survey (NGS) stations Burr Jones GPS (NGS PID# DG4237), Emil GPS (DG4231), Edina-Taylor GPS (DG4236) and Owen Park GPS (DG4235) using the First-Order coordinates adjusted by the NGS in January 2004 and published in the NGS database in March 2004.

The ellipsoid height of 232.645 m is computed from a leveled NAVD88 elevation of 266.911 m for the antenna reference point (bottom of antenna housing) plus the geoid separation of –34.266 m computed for this location from the NGS GEOID03 geoid model. The NAVD88 elevation was determined by Ayres and Associates, Inc. and the City of Madison using differential leveling from NGS benchmark 2V02 (DF9800), holding to the Second-Order NAVD88 elevation of 260.406 m published for 2V02 by the NGS in February 2004. 

HORIZONTAL NOTES:
The current NAD83(1997) horizontal position was determined from nearby NGS stations in the City of Madison in order to maximize accuracy relative to those stations. For comparison, the current position is within 0.013 m (horizontal distance) of the NAD83(1997) position determined in August 2004 by a University of Wisconsin-Madison Civil and Environmental Engineering Department study, relative to NGS stations Cottage Grove GPS (OM1258), Kollath (NH1578) and Rock (OM0651) using the A-Order coordinates adjusted by the NGS in April 1999. It is also within 0.019 m of the previous base station position determined from the NGS Online Positioning User Service (OPUS) in the NAD83(CORS96)(EPOCH:2002.0000) datum.

Note that in the Madison area, the previous NAD83(1991) datum adjustment is shifted about 0.04 m, mostly east-west, relative to the NAD83(1997) datum. Subsequent adjustment of NAD83 by the NGS is scheduled for the near future, but any shifts should be minor.

VERTICAL NOTES: 
Ellipsoid heights derived from a geoid model (like GEOID03) do not necessarily match NAD83 ellipsoid heights published by the NGS. A GPS network completed in August 2004 by the University of Wisconsin-Madison Civil and Environmental Engineering Department determined the NAD83 ellipsoid height of the Madison base station as 232.679 m (+/- 0.01 m, 95% confidence), relative to the Third-Order Class I NAD83 ellipsoid heights adjusted by the NGS on April 28, 1999 for stations Cottage Grove GPS (OM1258), Kollath (NH1578) and Rock (OM0651). The ellipsoid height of 232.679 m is 0.034 m larger than the 232.645 m ellipsoid height computed from the NAVD88 elevation and GEOID03. The sign and magnitude of this discrepancy between (NAD83-NAVD88) geoid separations and GEOID03 geoid separations is fairly consistent in the Madison area for Wisconsin Height Modernization Program stations, indicating that GEOID03 creates an ellipsoid approximately parallel to and about 0.02 to 0.04 m above the NAD83 ellipsoid in this area.

By establishing the ellipsoid height of the base station from a leveled NAVD88 elevation and the GEOID03 geoid model, users should be able to produce reliable NAVD88 elevations at other points by using the same GEOID03 geoid model in the roving GPS receiver.

Note that GPS-derived orthometric (NAVD88) elevations are generally not as accurate as elevations produced from differential leveling, due in part to inherent accuracy limitations in GPS-measured ellipsoid heights, and to imperfections in the geoid models which convert ellipsoid heights to NAVD88 elevations.

RECEIVER CONFIGURATION NOTES:
Users are responsible for following good GPS techniques, including proper receiver calibration and independent checks. Below are some particular technical issues to consider:

ANTENNA MODELS:
Precise vertical GPS measurements depend on correct modeling of the antenna vertical phase center offset and variation. The offset of the phase center from the antenna reference point (i.e. bottom of antenna housing) is not a constant value, but rather varies with satellite elevation angle. Different antennas have different offsets and variations; if the base and rover receivers use different antennas, the offsets do not cancel out by relative positioning alone. Also, a local calibration/transformation of the rover to a benchmark will not necessarily correct this, because the offsets vary over time. Precise GPS receivers, such as Real-Time Kinematic (RTK) receivers, must ‘know’ the antenna model for both the base and the rover to correctly account for these offsets. Contact your receiver vendor to accomplish this.

GEOID MODEL:
RTK GPS receivers must have the GEOID03 geoid model loaded in order to produce reliable NAVD88 elevations from the current base station coordinates. Older geoid models (GEOID99, GEOID96) are shifted vertically and even slightly tilted relative to GEOID03. See the “Vertical Notes” section. While local calibration/transformation to one or more NAVD88 benchmarks may account for this discrepancy, this technique carries its own risks.