Methods we utilized to establish these elevations on

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methods we utilized to establish these elevations on the BPA and GRM at the CGPS stations. Preliminary Project Planning (before issuance of contract) Selection of CGPS Stations This project was to cover the ten southern counties of California – from the 36 th parallel down (San Luis Obispo, Kern, San Bernardino, Santa Barbara, Ventura, Los Angeles, Orange, Riverside, San Diego and Imperial Counties). The first step of this project was to select a group of stations that would provide a good geometry over these counties in the final adjustment. With a finite budget in mind, we had to determine the number of stations to level to, the specification to use, the equipment to use, the proximity of the station to existing NAVD88 first order benchmarks, the vertical stability of the station (proximity to seismic or subsidence areas, etc.) and the type of CGPS monument. The CSRC works in conjunction with the NGS and it was important that upon completion these elevations would be accepted into the national database, so we decided to complete the leveling at the Second Order, Class II criteria of the FGCS Specification and Procedures for Electronic Digital/Bar Code Leveling Systems (FGCS, 1994). This Version 1.10 (October 28, 2004) Page 2 of 36 6/14/2018 2:20 AM
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specification would be accepted by NGS and would also provide an elevation ≤ 5mm accuracy. This accuracy for our vertical control would allow the CSRC to achieve the end goal of orthometric heights at ± 2cm accuracy as stated in the CSRC Master Plan for the Geodetic Control (CSRC, 2003). One mandate of the CSRC is to provide contracting opportunities for the private sector since we receive funding from the federal government. This leveling project was to be contracted out so when deciding what equipment to use, it was decided to use the newer electronic digital leveling equipment since it is commonly in use by the private sector and would provide a broader opportunity for respondents. Both of these first two decisions had details that later had to be worked out with NGS and will be detailed below. A major determining factor in selecting the stations to level was proximity to existing NAVD88 first order benchmarks. Searches of the NGS database were done to determine the number of first-order adjusted benchmarks within a two mile radius of the station. Stations were selected that had at least 4 or more benchmarks within the two mile radius, knowing that in some cases the elevations would have changed over the years and that leveling would have to be extended to check into additional benchmark(s) to meet the specifications. Along with proximity of the benchmarks, site accessibility/condition and elevation difference were also reviewed. Several sites were originally accepted based on proximity of benchmarks and network geometry, but later had to be rejected based on the severe elevation difference, and therefore, expense, to level to the site. Another seemingly suitable site was rejected due to frequent vandalism, which put into question the long- term viability of the site as part of the network. Stations were also reviewed for known
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