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