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What Does Height Really Mean? Part II: Physics and Gravity

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This is the second paper in a four-part series considering the fundamental question, "what does the word height really mean?" The first paper in this series explained that a change in National Geodetic Survey's policy, coupled with the modern realities of GPS surveying, have essentially forced practicing surveyors to come to grips with the myriad of height definitions that previously were the sole concern of geodesists. The distinctions between local and equipotential ellipsoids were considered, along with an introduction to mean sea level. This paper brings these ideas forward by explaining mean sea level and, more importantly, the geoid. The discussion is grounded in physics from which gravitational force and potential energy will be considered, leading to a simple derivation of the shape of the Earth's gravity field. This lays the foundation for a simplistic model of the geoid near Mt. Everest, which will be used to explain the undulations in the geoid across the entire Earth. The terms geoid, plumb line, potential, equipotential surface, geopotential number, and mean sea level will be explained, including a discussion of why mean sea level is not everywhere the same height; why it is not a level surface.

Document Type: Research Article

Publication date: June 1, 2006

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  • Surveying and Land Information Science (SaLIS) is the official publication of the American Association of Geodetic Surveying (AAGS) and the Geographic and Land Information Society (GLIS).

    SaLIS is a scientific journal devoted to reporting research and new work conducted to advance geodetic surveying, land surveying, large-scale mapping, and geographic information systems designed to advance the development and management of the cadastral parcel data layer and other land information applications. SaLIS publishes research articles, technical papers, technical notes, papers on the current state of surveying education, surveying history, book reviews, and current literature reviews. Every four years, the journal publishes the U.S. Report to the International Federation of Surveyors (FIG). The Proceedings of the Surveying Teachers Conference are published bi-annually.

    For information about AAGS visit
    For information about GLIS visit

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