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In 1993 NAVD 88 was affirmed as the
official vertical datum in the National Spatial Reference System (NSRS) for the
Conterminous United States and Alaska (see Federal Register Notice (FRN))
Synopsis
A vertical datum is a surface of
zero elevation to which heights of various points are referred in order that
those heights be in a consistent system. More broadly, a vertical
datum is the entire system of the zero elevation surface and methods of
determining heights relative to that surface. Over the years, many different
types of vertical datum have been used. The most predominant types today are
tidal datum's and geodetic datum. Tidal datum are determined by averaging the
level of water at a tide gage over time. Some simple examples of these are Mean
Sea Level (MSL), Mean Low Water (MLW) and Mean Higher High Water (MHHW). Mean
Sea Level (MSL) is a tidal datum which is computed by the Center for
Operational Oceanographic Products and Services (CO-OPS), as part of the
National Tidal Datum Epoch (NTDE) based on data collected over a 19-year tide
cycle. It pertains to Local Mean Sea Level (LMSL) at the tide station at which
it was observed and should not be confused with any other vertical datum,
including LMSL at other tide stations. The current NTDE for the United States
is 1983-2001.
Geodetic datums are predominantly
determined through a process of surveying known as geodetic leveling,
determining the height differences between points in the ground known as bench
marks. These height differences can only yield actual heights at the benchmarks
if at least one datum origin point is chosen to serve as the absolute level of
the vertical datum. It is frequently the practice of those responsible for
defining a geodetic datum, to choose a datum origin point that is also at a
tide gage so a relationship between the tidal and geodetic datum exists, though
this is by no means a requirement.
In the United States and its
territories, NOAA's National Geodetic Survey (NGS) responsibilities include
defining and providing access to and the maintenance of geodetic vertical
datum's. These datum's are part of the overall National Spatial Reference
System (NSRS). Currently five vertical datums exist: North American Vertical
Datum of 1988 (NAVD 88), American Samoa Vertical Datum of 2002 (ASVD02),
Guam Vertical Datum of 2004 (GUVD04), Northern Marianas Vertical Datum of
2003 (NMVD03) and Puerto Rico Vertical Datum of 2002 (PRVD02). Additionally,
Virgin Islands Vertical Datum of 2009 (VIVD09) will soon be published and
a new Hawaii vertical datum is in the planning stage.
Each of these datum's has either a
single datum origin point, otherwise known as a primary bench mark, or
one such point per island in its coverage area. Every datum origin point
in these five datum's is at a tide gage. To determine the heights of the
respective origin points, first local mean sea level relative to the respective
NTDE was determined. Next, a nearby bench mark was chosen as the datum
origin point for a particular continuous land area (which could be an
island or an entire continent). The height above LMSL for those origin bench
marks was determined by leveling between the tide station reference mark
and the bench mark that is serving as the datum origin point. This height
above LMSL was held fixed as the single initial constraint for that particular
land area.
The Vertical Mark Number (VM) and
Permanent IDentifier (PID) shown on the bench mark sheet are unique identifiers
for bench marks in the CO-OPS and NGS databases, respectively. For bench marks
where both VM and PID are indicated, both idal and geodetic vertical
datum elevations are available. Contemporary geodetic vertical datum's
established by NGS are realized through the publication of heights derived from
a simultaneous, least squares minimally constrained adjustment of leveling
observations between passive geodetic control spanning the region for which the
datum is defined. North American Vertical Datum of 1988 (NAVD 88)
North American Vertical Datum of
1988 (NAVD 88) consists of a leveling network on the North American Continent,
ranging from Alaska, through Canada, across the United States, affixed to a
single origin point on the continent:
Tide Station & Location = Pointe-au-Pere,Rimouski, Quebec, Canada
PID = TY5255
GSD* Designation = 54L071
Bench Mark = 1250 G
Ht above LMSL(Meters) = 6.271
* Geodetic Survey of Canada = GSD
In 1993 NAVD 88 was affirmed as the
official vertical datum in the National Spatial Reference System (NSRS) for the
Conterminous United States and Alaska (see Federal Register Notice (FRN)).
Although many papers on NAVD 88 exist, no single document serves as the
official defining document for that datum. Detailed information about NAVD 88
is available here.
Abbreviations
(MLW)..........................................................................................................................Mean Low Water (MHHW)..........................................................................................................Mean Higher High Water
(CO-OPS)................................................Center for Operational Oceanographic Products and Services
(NTDE)......................................................................................................National Tidal Datum Epoch
(NGS)...............................................................................................................National Geodetic Survey
(NSRS)..............................................................................................National Spatial Reference System
(NAVD 88)..............................................................................North American Vertical Datum of 1988
(ASVD02).............................................................................American Samoa Vertical Datum of 2002
(GUVD04)................................................................................................Guam Vertical Datum of 2004
(NMVD03).........................................................................Northern Marianas Vertical Datum of 2003
(PRVD02).......................................................................................Puerto Rico Vertical Datum of 2002
(VIVD09)...................................................................................Virgin Islands Vertical Datum of 2009
(VM).....................................................................................................................Vertical Mark Number
(PID)........................................................................................................................Permanent IDentifier
(NSRS)..............................................................................................National Spatial Reference System
(FRN).................................................................................................................Federal Register Notice
(MSL).............................................................................................................................Mean Sea Level
Reference:
http://www.ngs.noaa.gov/datums/vertical/
Stephen F. Moran PLS, MBA has been
surveying since 1986. He obtained his professional registration in 2000. He
holds a BS Degree in Mathematics and an MBA from the University of Maryland
University College.
