The California Surveyor - CLSA

vector is drawn toward areas of
greater mass and therefore the geoid
bends upward in these areas. Conversely,
in areas with a deficiency of
mass, the geoid bends downward.
In California, geoidal undulations
vary by several meters:
Location Geoid Height (from OSU 91A)
Near Yuma, AZ
Santa Barbara
Lone Pine
Sacramento
Crescent City
-33.2 m
-36.8 m
-26.0 m
-29.8 m
-25.2 m
-109 ft.
-121 ft.
-85 ft.
-98 ft.
-83 ft.
Undulations of 50 ppm are not
uncommon in the mountainous
areas of the state. Globally, the geoid
height varies +/-100 meters. One of
the greatest challenges remaining for
geodetic science is the precise mapping
of the detailed undulations of
the geoid.
Vertical Datum
Several possibilities exist, and have
been used, to define a vertical datum.
A tide gauge can locate mean
sea level at a particular location by
observations of sea level variations
over an extended period of time
(typically a 19 year cycle). As already
discussed, mean sea level at one
location is not necessarily on the
same equipotential surface as mean
sea level at an other location. The
NGVD 29 resolved this conflict by
constraining the general adjustment
at 26 tide gauges. Five sites in
Canada, and 21 sites in the U.S.A.
were held fixed in the general adjustment
of 106,724 km. (66,315 mi.)
of leveling. This means that NGVD
29 is neither an equipotential surface
nor an accurate approximation of the
geoid.
In the definition of NAVD 88, the
limitation of a datum surface distorted
to fit sea surface topography
was considered incompatible with
modern surveying technology. Space
based geodetic surveying techniques
(e.g. GPS, SLR, VLBI) are capable of
providing extremely accurate geodetic
height differences in terms of
height above a reference ellipsoid.
The prospect of these methods in
conjunction with a precise geoid is
too valuable to sacrifice to a distorted
datum.
The NAVD 88 was defined by a
single least squares adjustment of
1,300,000 km. (808,000 mi.) of leveling
data in the U.S.A., Canada, and
Mexico. This adjustment was completed
in June of 1991 and included
80,000 km. (49,700 mi.) of new leveling
in the U.S.A., VLBI height differences,
and a total of 709,000 marks.
The Coast and Geodetic Survey (formerly
the National Geodetic Survey)
selected the new International Great
Lakes Datum of 1985 (IGLD 85) local
mean sea level height at station
POINT-AU-PERE (Father Point) /
RIMOUSKI as the minimum-constraint
datum point for NAVD 88.
POINT-AU-PERE/RIMOUSKI is located
at the mouth of the St.
Lawrence River in Quebec, Canada.
It was selected to minimize the impact
of NAVD 88 on the national
mapping programs of the U.S.A. and
Canada, and upon international
treaties regarding shipping in the
Great Lakes.
Orthometiric Heights
Examination of the elevation change
from NGVD 29 to NAVD 88 in
California reveals differences of 60
cm. (1.97 ft.) to 120 cm. (3.94 ft.) (see
Figure V. But the change is not of a
uniform slope or curve. Why is it
that the elevation change is so
irregular? Well, some of the irregularity
is attributable to the distortion
in NGVD 29 from sea surface topography,
and some is from the errors
which exist in the NGVD 29 leveling
and adjustments, but the majority of
the contouring is the result of refinement
of a geodetic term known as
the orthometric height correction.
As was previously stated, no two
equipotential surfaces are parallel.
Any line of differential levels naturally
passes through an infinite number
of equipotential surfaces since it
is run on the ground at varying distances
from the geoid. The
orthometric height correction attempts
to compensate for the
non-parallism of the equipotential
surfaces on which the leveling
observations were made. This correction
requires precise gravity
measurements to be processed simultaneously
with the leveling data.
The NAVD 88 adjustment utilized
improved gravity measurements to
accurately model the orthometric
height correction. The greatest magnitude
for the correction occurs in
the mountainous regions where
deflection of the vertical is
greatest and the NGVD 29 models
were most deficient.
California NGVD 29 to NAVD 88 Figure 1
OREGON
NAVD 88 Leveling
NGVD 29 to NAVD 88 (CM)
CONTINUED ON PACE 18
Fall 1992 The California Surveyor 17