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1. Home
2. Education
3. Geodesy
4. Gravity

**Gravity** Global Positioning Tutorial  Geoid Earth with inset of North Americ*Imagine if all of the Earth topography, mountains and valleys were scoured off leaving a continuous world's ocean completely at rest, without the effects of currents, weather and tides. The effect of the Earth's gravity on this hypothetical world mean sea level is represented by the geoid. However, because the Earth's gravity is not equal in all places, this hypothetical ocean is not perfectly smooth. The strength of the Earths' gravity, and consequent effect on the shape of the geoid is represented by color variations in this image.*  Gravity is the force that pulls all objects in the universe toward each  other. On Earth, gravity pulls all objects "downward" toward the center  of the planet. According to Sir Isaac Newton's Universal Law of  Gravitation, the gravitational attraction between two bodies is  stronger when the masses of the objects are greater and closer  together. This rule applies to the Earth's gravitational field as well.  Because the Earth rotates and its mass and density vary at different  locations on the planet, gravity also varies.  Geoid Earth with inset of Indian Ocean*In this global map of the geoid (Geoid99), the area in blue near India indicates a region where the Earth's gravity is weaker. In this area, the mean sea level is lower because the hypothetical ocean has been pulled away from the area of lower gravitational force towards other areas of the globe where the gravitational forces are stronger.*  One  reason that geodesists measure variations in the Earth's gravity is  because gravity plays a major role in determining mean sea level.  Geodesists calculate the elevation of locations on the Earth's surface  based on the mean sea level. So knowing how gravity changes sea level  helps geodesists make more accurate measurements. In general, in areas  of the planet where gravitational forces are stronger, the mean sea  level will be higher. In areas where the Earth's gravitational forces  are weaker, the mean sea level will be lower. To measure the Earth's gravity field, geodesists use instruments in  space and on land. In space, satellites gather data on gravitational  changes as they pass over points on the Earth's surface. On land,  devices called gravimeters measure the Earth's gravitational pull on a  suspended mass. With this data, geodesists can create detailed maps of  gravitational fields and adjust elevations on existing maps. Gravity measurements accurately reflect elevation changes on the Earth's surface.
  Geodesy **Lessons** Welcome What is Geodesy? The History of Geodesy The Geoid What is a Datum? The Horizontal Datum The Vertical Datum Gravity The National Spatial Reference System The Global Positioning System CORS and GIS References
  More **Information** Roadmap to Resources Subject Review Is the Earth round? (Ocean Facts)  Geodesy Education Heard of geodesy? These videos from NOAA's National Geodetic Survey offer a deep dive into the science of knowing where you – and everything else is in the world!
  The National Science Teaching Association (NSTA) includes this resource in its database. NSTA provides educators and students access to Web-based, educationally appropriate science content that has been formally evaluated by master teachers.
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  **Last updated:** 02/26/21 **Author:** NOAA How to cite this article Contact Us


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