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FORWARD The following is a chronological description of the City of Stockton's bench mark elevation and datum history: ORIGINAL CITY OF STOCKTON DATUM The original City of Stockton(COS) datum was an assumed elevation of 100 feet at an unknown bench mark in the general vicinity of McCleod Lake This was probably done to avoid negative elevations in those Iareas below sea level 1982 CITYWIDE CONVERSION TO NGVD29 In 1982 the City hired R W SIEGFRIED (RWS) & ASSOC to determine the elevations for the control monuments, Bench Marks 1 thru 34, established in PHASE I of the COS horizontal control survey, as shown on Record of Survey Book 28 Page 58 The elevations were established by a Second Order level circuit and utilized the National Geodetic Vertical Datum of 1929 (NGVD29) The official adjustment of 1967 listed the elevation of 12 966 for U S Coast & Geodetic Survey (USC&GS) Bench Mark"C 1041"(COS "D", see U S I survey monument descriptions on page 23, Permanent Identification(PID) JS 1385) and was used as the initial bench mark to determine all subsequent elevations A conversion from the original COS datum to the NGVD29 was then determined by comparing the elevations of U S PID HS0393 (originally COS bench mark"G") at City Hall near McCleod Lake as follows Original COS Datum Bench Mark Elevation= 98 41 NGVD29 Datum COS 1982 Elevation= 12 0 Conversion= 86 32 NOTE: See Appendix A for additional elevation and datum comparisons. FEBRUARY 1984 ADDITIONS In January and February 1984, elevations were determined by RWS for Bench Marks 35 through 46, as shown on Record of Survey Book 28 page 119 A second order level circuit was run from COS Bench Mark I "D" to Bench Marks 16 and "F", utilizing the NGVD29 Datum elevations as established above In January of 1984,a report of the Vertical Control Survey for the East Stockton Sanitary Sewer Project (ESSSP) was published This report was prepared by George S Nolte and Associates The report lists those bench marks used to establish their Datum as well as those new bench marks which were established In February of 1984 ties were made between the NGVD29 Datum as established above and the ESSSP Datum The results are as follows IRC I NGVD29 D ESSSP DATUM DIFFERENCE C O S -B M "F" to ESSSP T B M 170(BM No 86) 20 860 20 574 +286 C O S -B M "26" to U S C &G S C-833 (BM No L) 14 693 14 327 +360 C O S -B M "28"to ESSSP T B M 203 (BM No 99) 12 402 12 087 +315 Average difference between the 1982 Stockton/NGVD29 Datum and the ESSSP Datum is + 0 322 032 feet was added to all of the bench marks listed in the ESSSP report In turn, these adjusted bench marks are included herein as Bench Marks 47 through 115 and "J" through "M" JUNE 1984 ADDITIONS In April and May 1984, elevations were determined by RWS for Bench Marks 116 through 134, as shown ' on Record of Survey Book 28 Page 138 A second order level circuit was run from and tied back to Bench Mark 31 ' 1