Geoid Monitoring Service (GeMS)

The National Geodetic Survey (NGS) established the Geoid Monitoring Service (GeMS) in 2019 to account for geoid changes in time. This product is actively investigating all potential physical processes which could modify the geoid over time and how to properly incorporate these changes into the National Spatial Reference System (NSRS).


GeMS Overview

  • What is GeMS? The Geoid Monitoring Service is an NGS project to capture and model time-dependent changes in the geoid and geopotential.

  • Why do we need GeMS? NGS needs to monitor geoid change in North America to maintain a centimeter-accurate vertical datum. Geoid change can exceed 1 centimeter per decade in some places.

  • How do these models get made? The models are a combination of satellite gravity models from NASA's GRACE and GRACE Follow-On missions and supplemental local mass change models.

  • How does this impact me? NSRS users should expect to see time-dependent orthometric heights in NAPGD2022 and future NGS products.

  • How can I help? GeMS is always interested in input for observation, reconnaissance, and modeling. GeMS will announce calls to action as its needs evolve. Contact Us

Models

GeMS has a number of interrelated models associated with it. All of these models are derived from the same underlying satellite gravity information provided by the GRACE and GRACE-FO missions. As these models are developed, tools and interfaces (like OPUS) will be adapted to provide model values at a user-requested location. The various models are:

  • Experimental Dynamic Global Geopotential Model (xDGGM)
  • Experimental Dynamic Geoid Model (xDGEOID)
  • Experimental Dynamic Deflection of the Vertical Model (xDDEFLEC) - North-South and East-West components
  • Experimental Dynamic Gravity Model (xDGRAV)
  • Experimental Dynamic Digital Elevation Model (xDDEM)

Validation

GeMS collects terrestrial geodetic data to supplement modeling efforts. Terrestrial data helps model validation, provides continuity between satellite missions, and augments the resolution of satellite models. There are three separate components of GeMS validation:

  • GeMS Validation Surveys (GeMS-VS)
  • Absolute Gravity Network
  • Continuous Gravity