In 2015, scientists from the International Association of Geodesy adopted a significant advancement in measuring elevation—the International Height Reference Frame (IHRF). This new framework provides a universal standard for altitude, functioning similarly to how latitude and longitude define geographical locations.

New Developments in Timekeeping

A decade after the IHRF’s implementation, geodesists are set to enhance this standard with the deployment of the most precise clock ever sent to space: the Atomic Clock Ensemble in Space (ACES). Launched from Florida last month, ACES is destined for the International Space Station (ISS).

The Precision of ACES

Developed by the European Space Agency, ACES utilizes two interconnected atomic clocks—one operating on cesium atoms and the other on hydrogen—to achieve unprecedented accuracy. Unlike traditional pendulum clocks, which can be off by a second each day due to environmental factors, or current GPS atomic clocks that might lose a second every 3,000 years, ACES is designed to maintain its precision for an astounding 300 million years without error. According to Luigi Cacciapuoti, an ESA physicist involved in the project, this reliability positions ACES as a crucial tool for both scientific and practical applications.

Supporting Global Synchronized Time

Once operational in space, ACES will connect with some of Earth’s most accurate timekeeping systems to form a synchronized global clock network. This network’s primary aim is to conduct tests of fundamental physics, but it also holds specific significance for the field of geodesy.

Impact on Elevation Measurements

By employing ACES, geodesists can enhance gravitational measurements, leading to a more precise definition of the “zero point” for global elevation. This zero point serves as the baseline from which all height measurements are derived, facilitating international efforts to monitor phenomena such as sea-level changes and infrastructure projects involving water management, like dams and canals.

Historical Context and International Collaboration

The importance of a unified elevation standard is further underscored by past disputes, such as the differing measurements of Mount Everest’s height between China and Nepal. Prior to the adoption of the IHRF, China recorded the mountain’s elevation at 8,844.43 meters, while Nepal measured it at 8,848 meters. Thanks to the new framework, both nations have agreed on a consensus height of 8,848.86 meters.

Understanding the Geoid Model

In establishing this standard, geodesists construct a model of Earth known as the geoid. This model represents a surface where every point experiences uniform gravitational pull. Consequently, if a canal is dug along the height of the geoid, the water level will remain constant, providing a reliable system for determining altitude across the globe.