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1 ' NOVEMBER 1996 CITYWIDE ADJUSTMENT In 1991 Towill Inc., a surveying and photogrametnc engineering company, was hired by Schaaf & '•Wheeler, a consulting civil engineering company, commissioned by FEMA to produce a flood plain study of San Joaquin County Utilizing the official NGVD29 elevations,Towill ran several Second Order Class I level loops from the SJCO BM "FE 17 6" along HWY 26 and the diverting canal into east Stockton,then north along ' HW88 to Lockeford and west along Bear Creek to northwest Stockton The average difference between the current City of Stockton(COS) elevations and the Towill elevations is-0.7 ft. The City venfied this difference utilizing the Global Positioning System(GPS) surveys performed in the fall of 1993 and the winter of 1994 ' Utilizing the Second Order differential leveling data gathered by the COS survey crew and the GPS elevation differences with direct ties to the Towill Elevation Reference Marks(ERM), a least square adjustment was completed Those bench marks that were not surveyed were adjusted based on the existing elevation differences ' and the correction at the nearest occupied bench mark Several new bench marks were also added Bench Marks 340 and 341 were set for future use as ' GPS "Base Stations", and Bench Marks 1009, 1014 thru 1017, 1019, and 1033 thru 1036 correspond to the numbers assigned to the GPS surveyed points shown on the Record of Survey Book 33 Page 20, numbers 1040 thru 1042 shown on Record of Survey Book 33 Page 142 Also those bench marks numbered 342 thru 375 are ' the Towill ERM set in the Stockton area All existing bench marks and survey monuments established by Federal Agengies(USCGS, USGS, USCE, NGS)found and elevated are now listed in a separate section titled "Existing US Survey Monuments" This will include the COS bench marks previously labeled "A", "B", etc ' and will utilize the National Geodetic Survey(NGS)PID number and include some of the official data published for the specific monument Also, the nurnencal listing of bench marks was revised and moved to the end of the bench mark book and labeled Appendix A, titled "City of Stockton Bench Mark Elevation Adjustment and Datum Comparison"(see index). This Appendix shows a comparison of the intial elevations and the newly adjusted ones Also included are the elevations based on the new North American Vertical Datum of 1988(NAVD88) which are provided for information only Although these elevations were published by the ' National Geodetic Survey(NGS) in 1990, we will continue to utilize the NGVD29 newly adjusted elevations provided herein and designated as "COS96 Local Adjustment" The following is the current conversion from ' the old original COS Datum to the NGVD29, COS96 Adjustment Original COS Datum Bench Mark "G"Elevation= 98 41 ' NGVD29 Datum COS96 Elevation= JJL48 Conversion = 86.93 1 1 1 i 1 a