71

Reviewing the work of the Board of Estimate and Apportionment level survey, it
appears that no plan could have been devised which would have led to the securing
of more satisfactory results. Had a larger appropriation been available at the outset
the work could have been materially expedited by placing several parties in the field.
The services of additional computers would likewise have greatly hastened the com¬
pletion of the task, as the necessity is apparent, in work of this kind, for very thor¬
ough checking of the field notes and office computations.

The examination of all the circuit closures, of the divergence (B-F) between the
two runnings of each section and of the corrections applied, and a consideration of
other evidence bearing upon the accuracy of the work, justify the conclusion that
the leveling done by the Board of Estimate and .Apportionment in New York City
and vicinity compares very favorably with the highest standard heretofore attained
in precise leveling in the United States,
 

HISTORY  OF THE  DATUM   PLANES  NOW  IN   USE  IN
NEW YORK CITY.

When New York City consisted only of Manhattan Island, the city's engineers
referred all their elevations to one plane known as the City Datum or Public Works
Datum. The consolidation of 1898, however, brought into the engineering work of
the greater city, the datum planes of the various departments of the new boroughs,
so that to-day we have ten or more distinct datum planes to which our elevations are
referred.

As the work of some of the city departments overlaps, the engineer who has to
use the elevations of streets arid structures determined by the different departments
is put to considerable trouble to reconcile these elevations and this condition very
frequently results in much confusion and loss of time in making the adjustments
required to secure uniformity.

On account of this multiplicity of datums, many entirely new systems of levels
have been established, and a great amount of computation of use only to the particular
department concerned, has been required. Because of the unsatisfactory and more
or less imaginary character of many of the datum planes, there has been during recent
years a growing desire on the part of the city engineers of all boroughs to secure
the official adoption of one datum plane for the whole city.

Prior to 1870, the municipal topographical work of the City of New York was
done by engineers and surveyors in private practice, each one of whom had his own
system of bench marks, referred to some mythical high water. The history of the
more important departmental datum planes now in use in New York City will be
briefly outlined.

The Public Works Datum: The oldest datum of which we have any record is the
City Datum or Public Works Datum of Manhattan. The original bench mark of this
datum was the water-table at the base of acolumn at the southwest corner of the
old almshouse at Bellevue Hospital, and its elevation was fixed at "20.558 feet above
high water mark."

This elevation is said to have been established about 1790 by a surveyor named
John Randel, Jr. The datum plane based upon this determination is still in use as
a plane of reference.

By reason of the frequent references to the name of Randel in association with
the city plan and the establishment of the first recorded datum plane, it may be of
interest to briefly sketch the origin and character of his connection with this work.