The California Surveyor - CLSA

CONTINUED FROM PACE 1 7 N AVD 88
Since the vast majority of users of NGVD 29 have not been seriously restricted
by its deficiencies, there would seem to be little reason to accept such
a difficult task as this transition represents. This is especially true since the
availability of NAVD 88 marks is so limited. Future plans call for the readjustment
of additional data sets into NAVD 88 (i.e. COE, USGS, and from
NGVD 29). This is however; questionable for California due to the highly
active crustal motion in the State.
So, why would you opt to tie your project into the new datum? How about
to conform to FEMA flood plain studies, or what if you're
doing federally funded Caltrans work, or Corp of Engineers mapping? All
of these agencies have acknowledged NAVD 88 and have either established,
or are working on, policies to address its implementation. It is only a matter
of time before this trickles down to the local level as did NAD 83. NAVD
88 is already being used and will become a requirement for an increasing
number of projects, necessitating extensive vertical control surveys to density
the network. The good news is that a tremendous amount of effort is going
into the development of precise geoid models to facilitate GPS leveling.
GEOID 90 is a three minute grid of geoid heights developed by C&GS, with
an accuracy of 2 - 10 cm. (0.06 - 0.36 ft.). This level of accuracy meets the
requirements for many vertical projects where it is being used to great
advantage. C&GS has programs underway to study the geoid in much
greater detail. One such study is being conducted in conjunction with
Caltrans in the San Diego County area with hopes of obtaining accuracy at
the 1 - 5 cm. (0.03 - 0.18 ft.) level.
There is no question that NAVD 88 will be accepted as the principal
vertical datum. Much work however is needed to facilitate the transition. The
work effort required to accomplish this will become the responsibility of
those prepared to meet the challenge. •
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1 8 The California Surveyor Fall 1992