SaLIS Vol. 66, No. 3, September 2006

What Does Height Really Mean? Part IV: GPS Heighting, by 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, by 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, by 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, by 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, by 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.