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Vertical Control Network - History and Background

The original City Bench Level System (the A-Bench system) was run in 1954. From 1954 to 1967 this system was expanded with third order level runs tying to various “A-benches.” In 1967, all City Bench Marks were adjusted to conform to the 1956 USC&GS adjustment.

In 1965, a second order level run between selected USC&GS benches disclosed a material discrepancy between the USC&GS bench U-106 and surrounding USC&GS bench marks. It was decided that U-106 had subsided and to adjust its elevation accordingly.

In 1978, second order bench level runs between USC&GS  bench marks U-106, W-106, V-106 and RV-175 disclosed further movement of U-106. An adjustment upwards was made to the City elevation of U-106.

Between 1973 and 1978 extensive second order bench level runs were undertaken throughout the city. Using this information an adjustment was made of the entire city bench mark system. This involved the adjustment of existing benches and the addition of many new benches. It was decided to accept the elevation of USC&GS bench mark W-106 due to its centrality and the extent of its compatibility with surrounding bench marks.

As there were few USC&GS bench marks that could be used for the 1973 to 1978 expansion this expansion proceeded through the use of closed loops. Errors on the order of 0.004 ft/ sq. root of miles of run were substantiated. It thus seemed reasonable to hold the City’s own work and presume that other USC&GS benches had moved relative to W-106. The nature and character of these benches and the geological activity in this area tended to justify this presumption.

With the expansion of the network into the Oakmont area in 1991 the tie to H-205 Reset 1977 was extremely close, thus the elevation for H-205 Reset 1977 was accepted and held.

Almost every City bench mark has been directly tied to with 2nd order level runs since 1972. The elevation of each bench mark is given in feet and decimal fractions thereof. Each bench mark that has been directly tied to the City’s system with 2nd order level runs since 1972 has the month and year of its most recent tie at the end of its description.

In 1991 the NGS adjusted their benchmarks in the Santa Rosa area an average of 0.26-ft. lower. The City of Santa Rosa did not adjust to these newly adjusted elevations. To convert from the City Bench Mark System to NGVD 29 elevations the 0.26-ft must be subtracted. The NGS VERTCON program can then be used to convert to NAVD 88 elevations.

The relationship of the City’s system to the pre-1991 elevations are as follows:

Bench Mark                    NGS Elev.                City Elev.           Date

RV-175                          144.898                 144.868            1978

 

V-106 Reset 1978           158.805                 158.803            3/81

V-106 Reset 1978           158.805                 158.799            6/82

V-106 Reset 1978           158.805                 158.779           11/84

 

W-106 Reset 1965          165.331                 165.331            1978

W-106 Reset 1965          165.331                 165.331             3/81

W-106 Reset 1965          165.331                 165.331            6/82

 

U-106                            157.608                 157.548           1978

U-106                            157.608                 157.556            3/81

U-106                            157.608                 157.548            6/82

 

E-205 Reset 1951           204.145                 204.064            11/81

E-205 Reset 1989           204.153                 204.088              9/89

 

H-205 Reset 1977           422.833                 422.833           12/91

 

W-1396 Reset 1987         94.7125 (adj. 91)    95.010              2/91

W-1396 Reset 1991          93.833                  94.130              9/91

W-1396 Reset 1991                                      94.014              7/04