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What does height really mean? Part I: Introduction

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This is the first paper in a four-part series considering the fundamental question, "what does the word height really mean?" National Geodetic Survey (NGS) is embarking on a height modernization program in which, in the future, it will not be necessary for NGS to create new or maintain old orthometric height benchmarks. In their stead, NGS will publish measured ellipsoid heights and computed Helmert orthometric heights for survey markers. Consequently, practicing surveyors will soon be confronted with coping with these changes and the differences between these types of height. Indeed, although "height'" is a commonly used word, an exact definition of it can be difficult to find. These articles will explore the various meanings of height as used in surveying and geodesy and present a precise definition that is based on the physics of gravitational potential, along with current best practices for using survey-grade GPS equipment for height measurement. Our goal is to review these basic concepts so that surveyors can avoid potential pitfalls that may be created by the new NGS height control era. The first paper reviews reference ellipsoids and mean sea level datums. The second paper reviews the physics of heights culminating in a simple development of the geoid and explains why mean sea level stations are not all at the same orthometric height. The third paper introduces geopotential numbers and dynamic heights, explains the correction needed to account for the non-parallelism of equipotential surfaces, and discusses how these corrections were used in NAVD 88. The fourth paper presents a review of current best practices for heights measured with GPS.

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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