CONTENTS.
 

PAGE

General statement..........................................................        o

Instruments used...........................................................       lo

Recovery of Coast Survey bench marks........................................       lo

Reconnaissance.............................................................       lo

Inspection of instruments....................................................       lo

Organization of field party...................................................       lo

The precise level...................................... .■.....................       11

The Icvelin? rods...........................................................       22

The rod lengths.............................................................       23

Foot plates and foot pins....................................................       25

General instructions for precise leveling........................................       26

Examples of record and computation..........................................       28

Corrections in precise spirit-leveling...........................................       32

Computation of precise levels....................................................       2^

Correction tables...........................................................       38

River crossings.............................................................       ^^2

The precise level net........................................................       49

Changed elevations of old bench marks........................................       51

Board of Estimate and Apportionment datum plane chart.......................       54
 

Bench marks..
 

54
 

Supports for rods and instrument.............................................       61

Settling of rods and instrument...............................................       62

Winter work...............................................................       63

So-called precise leveling.....................................................       63

Computation and adjustment................................................       63

Precision of the results......................................................       67

Condition of atmosphere and length of sight...................................       68

Rate of progress....................................................■........       69

Amount of field work........................................................       6g

Conclusions..........................................................., , . , .       70

History of the datum planes now in use in New York City.......................       71

The question of coastal subsidence in New York City and vicinity................       78

Description of bench marks..................................................       89

Types of bench marks used..................................................       89

Standard elevations of bench marks in the Borough of Richmond and in New Jersey. .       93

Descriptions of bench marks in the Borough of Richmond and in New Jersey........       97