SaLIS Vol. 66, No. 3
September 2006
What Does Height Really Mean? Part IV: GPS Heighting
Thomas H. Meyer, Daniel R. Roman, and David B. Zilkoski
This is the final paper in a
four-part series examining the fundamental question, “What does the word height
really mean?” The creation of this series was motivated by the National
Geodetic Survey’s (NGS) embarking on a height modernization program as a result
of which NGS will publish measured ellipsoid heights and computed Helmert orthometric heights for
vertical bench marks. Practicing surveyors will therefore encounter Helmert orthometric heights computed
from Global Positioning System (GPS) ellipsoid heights and geoid
heights determined from geoid models as their
published vertical control coordinate, rather than adjusted orthometric
heights determined by spirit leveling. It is our goal to explain the meanings
of these terms in hopes of eliminating confusion and preventing mistakes that
may arise over this change. The first paper in the series reviewed reference
ellipsoids and mean sea level datums. The second
paper reviewed the physics of heights culminating in a simple development of
the geoid in order to explain why mean sea level
stations are not all at the same orthometric height.
The third paper introduced orthometric heights, geopotential numbers, dynamic heights, normal heights, and
height systems. This fourth paper is composed of two sections. The first
considers the stability of the geoid as a datum. The
second is a review of current best practices for heights measured with the
Global Positioning System (GPS), essentially taking the form of a commentary on
NGS’ guidelines for high-accuracy ellipsoid and orthometric
height determination using GPS.
OPUS Observations
Peter Lazio
Although primarily a
positioning service, the coordinate values provided by the National Geodetic
Survey’s On Line Positioning User Service (OPUS) may also be used as
observations in a least squares adjustment.
This is made possible by the inclusion of the coordinate covariance
matrix in the OPUS extended data report.
A covariance matrix is one of the necessary components in a least
squares adjustment. We will develop the
other necessary component for a least squares adjustment, namely, the
mathematical model, and demonstrate how commercial least squares adjustment
software can be used to incorporate OPUS in a network adjustment. OPUS vectors
and coordinates can be used as observations to rigorously combine multiple OPUS
solutions for the same station and or they can be combined with other
measurement methods in a network adjustment.
Location of Boundaries Defined by Sequential
Conveyances—A Question of Timing
Andrew C. Kellie,
and Joseph B. Curd, Jr.
Whether a land boundary has
been created by sequential or simultaneous conveyance materially affects the
conduct of the boundary retracement. This is because
the courts have used different rules to apportion excess and deficiency
occurring between parcels created by simultaneous and sequential conveyances.
Sequential parcels are those created by conveyances written at different times
and made without reference to a common scheme of subdivision. This paper
examines the general rules that have been developed for the retracement
of parcels created by sequential conveyances. In addition, the authors examine
specific cases where the courts have ruled on the location of boundaries for
sequentially created parcels.
Solar and Celestial Observations for Direction and
Position Determination
Jacob Dunham, Nick Battjes,
Elizabeth Chesla, and Matthew Gotham
Solar and celestial
observations have always been an important aspect of surveying; they have
played a crucial role in society ever since people traveled beyond their
traditional communities and started wondering where they were. Solar and
celestial observations have been used to determine the Mason-Dixon Line, the
true North for Public Land Surveys, the direction of Mecca, and many other
boundaries and places on Earth. It is precisely because solar and celestial
observations play such an important role in determining position and direction
that surveyors should know the history behind them and how to perform them,
should the need arise. This paper takes an in-depth look at the history of
solar and celestial observations and then goes on to discuss the instruments
and procedures used to perform such observations. Finally the paper discusses
methods of calculating direction using observed data.
Solar and Celestial Observations for Position and
Azimuth Determination: Artillery Surveying
in Vietnam
Daniel P. Engle, Joel Metzger, Joe Schott, Justin Hinchcliff, Charlie Kuckenbecker,
and Cody Schanfish
When man wanted to know
where he was on this Earth, or what direction he was going, he looked up to the
heavens. The stars are the key. Some appear to be fixed, while others revolve
around them. By knowing the apparent places of these fundamental stars and
making the appropriate measurements, position and azimuth can be determined.
These ancient techniques are still used by surveyors today. In
the mid-1960s, artillery surveyors stationed in Vietnam used solar and Polaris
observations to establish a common grid for orienting the guns. The
University of Akron student surveying team, who participated in the 2006 NSPS
Student Surveying Competition, paid tribute to these brave soldiers by using
the same equipment and procedures to determine an assigned azimuth. They did so
wearing the uniforms of U.S. Army artillery surveyors of the Vietnam War.