42 U. S. COAST AND GEODETIC SURVEY 



Unlike mean sea level, mean low water as determined in different 

 localities is not even approximately in the same level surface but 

 varies with the range of tide, and for this reason it is not adapted to 

 leveling operations over large areas. Even in a small area there will 

 be slight differences in mean low water from point to point; for 

 example, mean low water as determined at the navy yard is 0.07 foot 

 lower than the mean low water at Commonwealth Pier No. 5 in 

 South Boston. For practical purposes, however, a low-water datum 

 may be adopted which will approximately represent the mean low 

 water over a limited area. 



For Boston Harbor such a datum has been selected and designated 

 as the "Boston low-water datum." This datum is defined as being 

 14.69 feet below bench mark 1 on the old dry dock at the navy yard 

 in Charlestown. It was established by the Coast Survey many years 

 ago from observations at the navy yard and has since been used by 

 the United States Army Engineers and others. This datum is 

 0.04 foot above the best-determined mean low water at the navy 

 yard and 0.03 foot below the best-determined mean low water at 

 Commonwealth Pier No. 5 for all observations available at this time. 

 Because of long usage and close agreement with the later results 

 from many years of observations, the continued use of this Boston 

 low-water datum is very desirable. 



Boston city base. — The Boston city base is a datum which has 

 been used by the city of Boston for many years. According to the 

 report of the city engineer of Boston for the year 1904, it is 15.11 

 feet below the "new" bench mark on the coping of the old dry dock, 

 which is bench mark 2 of the Coast and Geodetic Survey. The 

 elevation relative to bench mark 1 on the coping at the entrance to 

 the dry dock is also given in the same report, but bench mark 2 is 

 the basic bench mark for the Boston city levels. 



Navy-yard datum. — The datum used by the Navy Department for 

 work in the vicinity of the navy yard at Charlestown has been 

 defined as a plane 114.54 feet below "Navy-Yard Bench Mark 1," 

 which is bench mark 2 of the Coast and Geodetic Survey. 



Summary of datums. — The relations of these datums to the sea- 

 level datum as determined by the leveling of 1923 are as follows: 



Feet 



Sea-level datum (13.304 feet below B. M. 7) 0. 00 



Boston low-water datum (14.69 feet below B. M. 1). —4. 87 



Boston city base (15.11 feet below B. M. 2) —5. 65 



Navy-yard datum (114.54 feet below B. M. 2) —105. 08 



BENCH MARKS 



A bench mark is a definite point on a more or less permanent 

 object used as a reference for elevations. The standard bench mark 

 used by the Coast and Geodetic Survey is a brass disk 33^ inches in 

 diameter containing the inscription "U. S. COAST & GEODETIC 

 SURVEY BENCH MARK," together with other information. 

 Other well-defined points are also used as bench marks. Through 

 bench marks tidal datums determined from observations are preserved 

 for the future use of the engineer. 



The following is a list of the several groups of bench marks around 

 Boston Harbor which are described in this volume: 



