The California Surveyor - CLSA

North American
Vertical Datum
Introduction
What would you do if you discovered
that your local benchmarks were
two and a half feet off and that the
only vertical control suitable for your
project was 20 miles away? This
hypothetical situation is today the
reality which presents a challenging
dilemma for the surveying and
mapping community. The challenge
is the implementation of the North
American Vertical Datum of 1988
(NAVD 88), and the dilemma is the
serious lack of NAVD 88 benchmarks
in many regions of the country (e.g.
most of California).
Surveyors are now well acquainted
with a transition from the
NAD 27 to NAD 83 horizontal
datum. This transition, although
complicated and painful to many,
was greatly simplified by the fact
that precise horizontal control surveying
in NAD 27 was limited
primarily to the geodetic community.
Also, the publication of NAD
83 in 1986 coincided with the advancement
of practical and affordable
GPS surveying - thereby providing
a general upgrading of the
precision and importance of horizontal
control. Precise automatic levels
have been in the surveyor's equipment
locker since the early 1960's,
and the profession has been accustomed
to relatively sophisticated
vertical control surveying as a result.
A datum change of 90 cm will
not go unnoticed by the profession.
If we as plane surveyors are to
effectively deal with a transition from
the National Geodetic Vertical
Datum of 1929 (NGVD 29) to NAVD
of 1988
By Gregory A. Helmer, PLS
88, we need to begin with a basic
understanding of geodetic height
systems. What made NGVD 29 obsolete,
and why is NAVD 88 superior?
With a little knowledge of geodesy
we can at least answer these
questions, and begin to understand
the reasons for a new datum, even
if we resent the intrusiveness of such
a change.
Vertical Control
As one stands at a given point on the
earth, it is intuitively obvious which
direction is up. And it would seem
to be a simple matter to measure
how far up or down one point is
from an other. In fact, this limited
concept was for most of us one of
our first experiences in surveying,
and has served as the basis for a
tremendous amount of good quality
differential leveling. Plus, H.I., minus,
elevation... Within the confines
of most surveying practice we are
not significantly restricted by this
concept; but as we expand our interest
to areas of regional and geographic
extent, and as we introduce
advanced measuring techniques (i.e.
GPS), we must consider the dynamics
of height systems. Differential
leveling is a good place to start. And
if we seriously consider what we
typically take for granted, a fundamental
principal becomes obvious.
As a leveling instrument is set up at
successive locations, its vertical axis
is coincident with the gravity vector
at that point, and its line of site is
perpendicular to that gravity vector.
As the force of gravity changes, the
level line changes. Therefore the
surface that a level describes is not a
plane or a sphere nor any other
defined geometric figure, but is an
irregular surface which is perpendicular
to the force of gravity at every
location. This irregular surface is
called an equipotential surface. Equipotential
means that the potential
gravity is the same at all locations.
In other words, it requires no energy
to overcome the force of gravity as
an object moves from one location to
an other on an equipotential surface.
There are an infinite number of equipotential
surfaces surrounding the
earth, somewhat like the layers of an
onion. Since the earth's gravitational
field is quite complex, each of these
equipotential surfaces has its own
distinct shape (i.e. they are not
parallel).
The Geoid
The equipotential surface which most
closely fits mean sea level is called
the geoid. While the concept of mean
sea level, and hence the geoid, is
easily understood, its realization is
much more problematic. Variations
in wind patterns, and in ocean currents
and salinity are responsible for
sea surface topography of a meter or
more. Sea surface topography presents
a direct ambiguity in the definition
of mean sea level. Variations
in the earth's gravity field impact
the actual shape of the geoid to a far
greater magnitude. Geoidal undulations
of as much as 100 meters are
the result of the uneven distribution
of the earth's mass together with the
effect of centrifugal force from the
earth's rotation. Centrifugal force at
the earth's equator effectively reduces
the acceleration of gravity by
0.35% compared with gravity at the
poles where centrifugal force is zero.
The ellipsoidal shape of the planet,
having a polar diameter which is 42.8
km. (26.6 mi.) shorter than its equatorial
diameter, contributes to a total
increase (including the centrifugal
effect) in the acceleration of gravity
of approximately 0.5% at the poles.
The shape of the geoid becomes even
more uncertain when we observe the
effect of variation in land masses and
the density of material within and
below the earth's crust. The gravity
16 The California Surveyor Fall 1992