2-12 Vertical Datums
a. A Vertical Datum is a level surface or arbitrary
level to which elevations are referred. Usually, the
geoid (Mean Sea Level) is that surface; however, other
vertical datums may include: Mean Low Level, the
level at which the atmospheric pressure is 29.92 inches
of mercury (1013.2 mbs), or an arbitrary starting
elevation. Vertical datums are usually defined as a
surface of "0" elevation and can also be called Altitude
Datums.
b. Since it is impossible to determine exactly where
the geoid intersects a land mass, it is impossible to use
the geoid itself as the actual vertical datum.
Historically, tide gauge measurements were averaged
over nineteen years to establish a local mean sea level.
These Mean Sea Level datums are very close to the
geoid but not exactly. For this reason, level lines ran
from tide gauge marks in different regions do not
connect exactly at the same elevation. In the United
States, the National Geodetic Vertical Datum (NGVD)
of 1929 replaced the Mean Sea Level 1929 and has
since been updated to the North American Vertical
Datum (NAVD) 1988. This new vertical datum, which
is based on tide gauge measurements and precise
geodetic leveling, has extended a "common" vertical
network to most of the continental U.S. The North
American Vertical Datum is considered to be within a
few meters of the geoid; there is greater uncertainty in
the relationship between other local vertical datums
and the geoid throughout the world.
c. Because of the uncertainty between local Mean Sea
Level Datums and the geoid, and because the exact
relationships between those datums is unknown; all
elevations should be considered to be referenced to the
Mean Sea Level Datum for the purpose of shifting
between vertical datums.
2-13 Horizontal Datums (Geodetic Datums)
a. General. A Horizontal Datum is a set of
quantities which fix an ellipsoid to a specific position
and orientation. The point at which the ellipsoid is
fixed is called the Datum Point. There are two types of
datums to be considered: Surface and Geocentric.
b. Surface Fixed Horizontal Datum.
1. A surface fixed horizontal datum is a set of
quantities relating to a specific point on the surface of
the earth which fixes an ellipsoid to a specific location
and orientation with respect to the geoid in that region.
The center of the ellipsoid and the center of mass of
the earth do not coincide. See Figure 2-10. Examples
of surface datums are: NAD 27, Tokyo, and ARC
1950.
2. A surface datum is generally defined by five
quantities: Latitude ( ), Longitude ( ), and geoid
f
l
height (N) at the Datum Point; semi-major axis (a),
and either semi-minor axis (b) or flattening (f) of the
reference ellipsoid. A geodetic azimuth is sometimes
listed as a defining parameter for a horizontal datum.
3. This type of datum can cover from very small
areas to very large regions of the earth. The geoid
separation at the datum point is generally zero.
However, as you move away from the datum point, the
geoid separation increases, creating the need for a new
datum. In many cases, the same ellipsoid, fixed to a
different location and orientation, is used.
ELLIPSOID
EARTHS
SURFACE
Datum Point
Figure 2-10 Surface Fixed Ellipsoid
c. Geocentric Horizontal Datum
1. A geocentric horizontal datum is one which
specifies that the center of the reference ellipsoid is
placed at the center of mass of the earth. This point at
the center mass of the earth is also the datum point.
See Figure 2-11. Examples of geocentric datums are
the World Geodetic Systems.
2. At least eight constants are required to define a
geocentric datum. Three to specify the location of the
origin of the coordinate system, three to specify the
orientation of the coordinate system, and two to specify
the dimensions of the reference ellipsoid.
3. Geocentric datums generally cover a large area of
the world and in some cases are global in extent. The
geoid separation remains relatively small for the entire
region covered by the datum. The World Geodetic
DRAFT
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