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Why Elevation Follows Gravity: The Geoid Beneath Every Height
GNSS measures height above a mathematical ellipsoid, but practical elevation depends on gravity, the geoid, and a physical definition of level.
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Measuring the World
Discover how we determine position, distance, height, gravity, and time, and what those measurements reveal about Earth and the worlds beyond it.
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Recent Articles
Surveying a Moving Seafloor: How GNSS-Acoustic Geodesy Tracks Tectonic Plates
Satellites cannot reach through seawater, so marine geodesists combine GNSS at the surface with acoustic ranging below to measure tectonic motion on the deep ocean floor.
Measuring the Earth–Moon System: Lunar Laser Ranging and Millimeter Geodesy
How Apollo-era retroreflectors let observatories measure the Moon, test gravity, and strengthen lunar reference frames.
The New NSRS Is Coming: A Field-to-Office Guide for Surveyors
An entry-level, field-to-office guide to NATRF2022, NAPGD2022, SPCS2022, epochs, height modernization, and preparing survey workflows for the modernized National Spatial Reference System.
A History of Datums in America, Part 2: NAD 83, HARN, NAVD 88, and the GPS Era
How NAD 83 evolved through the 1986 adjustment, 83/90-era networks, HARN, FBN, CORS96, NSRS2007, and NAD 83(2011)—and how NAVD 88 replaced NGVD 29.
A History of Datums in America, Part 1: From Colonial Surveys to NAD 27 and NGVD 29
From colonial metes-and-bounds surveys and Mason and Dixon to the Coast Survey, U.S. Standard Datum, NAD 27, State Plane coordinates, and NGVD 29.
Finding the Moon: Reference Frames, Optical Sights, and Apollo Navigation
How inertial platforms, optical observations, radio tracking, maps, and reference frames guided Apollo between Earth and Moon.
Surveying the Highest Peak, Part 3: GPS, Geoids, Snow, and the Modern Height of Everest
Why modern Everest measurements depend on GNSS, gravity, snow depth, reference frames, and a clear definition of height.
Surveying the Highest Peak, Part 2: How Peak XV Became Mount Everest
The distant observations, calculations, corrections, and people behind the first accepted height of the world’s highest mountain.
Finding Level: From Water Levels and Spirit Bubbles to Digital Heights
Establishing height has always required a definition of level, a reliable sight line, and a path of observations strong enough to reveal accumulated error.