HOLMES & NARVER, Inc.
ENGINEERS-CONSTRUCTORS
TO:
S$. P. Howell
__JOB:
942
FROM:
L. S. Hammond
__re:____Surveys
Page 2
(2)
Vertical Control
DATE:__July 31, 1957
The vertical inter-relaticn of
stations on HOW and NAN will require an extensive
survey, but can be accomplished to well within the
tolerance of 1:25,000 and approaching first order
leveling tolerances of 0.017 times the square root
of the distance in miles.
The following recommended
procedures include reciprocal leveling over the
water between islands and sand spits along the east
reel.
(a)
The atoll chart indicates the possibility of
observing between HOW and NAN by occupying eleven
instrument stations.
See attached sketch.
A
terminal point on HOW and on NAN, seven on existing
sand spits or islands, and two on observing tri-
pods which will be placed midway between land areas
where the over water distance is excessive.
These
tripods will be similar to the observing tripods
used on DOG and GEORGE during the CASTLE operation.
They will be required in pairs approximately twenty
feet apart.
(b)
The required equipment will include a first order
level equal to the Wild NW 111, two Invar leveling
rods and large targets of special design, which can
be observed at long distances. This equipment is
not available at the jobsite at this time.
(ec)
The leveling will be performed to procedures adopted
by the U.S. Coast and Geoditic Survey and other
organizations for leveling across stretches of water,
with some modification to meet the problem of this
operation.
A brief description of the method is as follows and more detailed
procedures will be set up if a jobsite reconnaissance determines
that the method is feasible.
.
Permanent bench marks are established at the seven land locations.
Pairs of tripods at the two water locations. These tripods must be
very stable. The observing is then accomplished between pairs of
bench marks.e The level is set up about twenty feet from the near
bench mark and a backsight is taken on the near rod.
Next, the
telescope is focused on the distant rod with the bubble centered and
the target is made to coincide with the middle wire. Radio communications will be required for this operation. Upon completion of the