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| SCIGN GPS Site Monument Information |
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This page contains information on antenna, geodetic and eccentric reference points found at SCIGN GPS sites.
Most SCIGN sites are constructed with a drilled-braced monument, designed at the Scripps Institution of Oceanography by Frank Wyatt and Duncan Agnew. A SCIGN D3 antenna adaptor is then welded to the top of the monument, upon which an antenna is mounted (see picture at left). To determine the monument type for a SCIGN site, check the site log, under the "Site Identification of the GPS Monument" section. There are three important reference marks at drilled-braced monument sites. The first is the antenna reference point (ARP). This is the point the site's antenna height is measured to. Most SCIGN sites use an Ashtech Dorne Margolin with chokerings model (e.g., antenna model type ASH701945B_M; see the IGS Official Antenna and Receiver Model Code List for model descriptions). The IGS Antenna Dimensions page shows that the antenna reference point for this model is the bottom of the pre-amplifier, or BPA. This is the code that appears in the Antenna Reference Point field in the site's log. Next is the geodetic reference point (GRP). This is the point from which the antenna height is measured. This is also the point to which site coordinates are published. For SCIGN sites fitted with a SCIGN D3 adaptor (again, available in the Monument section of the site log), this is always the top of the center support bolt in this adaptor. See below for a diagram displaying this and other features of SCIGN monuments. The eccentric reference point (ERP) is last. This is a drilled mark on the north-facing monument leg, and is used for two purposes: to provide a secondary measurement if the adaptor is removed from the monument, and to allow surveyors a point to which to easily level to on the monument. It is not possible for surveyors to remove the SCIGN dome which encloses the adaptor (and geodetic reference point).
Therefore, a final important measurement is from the ERP to the geodetic reference point. This value is determined by adding the distance from the ERP to the bottom of the antenna adaptor (typically 0.1016m; listed as the "antenna adaptor divot distance" in the Monument section of the site log) PLUS the distance from the bottom of the antenna adaptor to the top of the center support screw (0.02286 m): Surveyors leveling to a SCIGN eccentric reference point should keep in mind that this height is x meters (0.12446 m in the example above) below the published antenna height, which is measured at the geodetic reference point. Finally, some software uses the L1 phase center reference point. Differences in height from this point to the antenna reference point can be found on the IGS Antenna Dimensions page. For example, the distance from the L1 phase center to the antenna reference point for an Ashtech Dorne Margolin antenna is 0.110m. For more information, please contact Paul Jamason. Diagrams (courtesy of Frank Wyatt [IGPP] and his research group): |
