Geodesy 101: The Basics of Earth’s Shape, Gravity, and Positioning
- Anvita Shrivastava
- Apr 30
- 4 min read
Updated: May 7
Geodesy, or 'Earth measurement', is the science that measures and defines the geometry, orientation in space, and gravity field of the planet. Geodesy may seem very niche and complex when many of us have to travel from one location to another daily; however, geodesy is responsible for nearly every technological advancement that affects our daily lives, including GPS navigation, satellite mapping, climate monitoring, and numerous engineering projects.
What Is Geodesy?
Geodesy is a subfield of earth science that specializes in providing accurate measurements and determination of the size, shape, and gravimetric (gravity) characteristics of the planet Earth. Traditionally, we have always represented the Earth as being perfect and/or spherical; however, geodesy identifies that the Earth is not perfect and that it is constantly changing, dynamic, and has an irregular exterior.
Modern-day geodesy uses mathematics, physics, and high-level satellite technology to produce very precise types of positioning and/or spatial data. Spatial data/position determination by means of geodetic data or measurements is used for most professional fields, such as surveyors, geospatial analysis, geophysics, and navigation systems.
Understanding Earth’s Shape
The Sphere Approximation
When it comes to our planet, Earth is basically looked at as a ball because it is easier to understand it this way. If you use the "sphere" model to calculate things about Earth, it will give you a basic answer, but it won't account for the fact that a ball shape does not come close enough to matching the fact that Earth actually bulges at the equator and flattens at the poles.
The Ellipsoid Model
A more accurate depiction of Earth’s actual shape is called a reference or mathematically defined ellipsoid. This is an ellipsoid shape that fits closely to Earth's overall mass distribution and surface elevation. The most common reference or mathematically defined ellipsoid is known as the WGS84 (the World Geodetic System 1984), which is used as a reference for satellite systems as well as worldwide mapping applications using GPS technology.
Key Characteristics of an Ellipsoid:
Semi-major axis/equatorial radius
Semi-minor axis/polar radius
Flattening factor
The Geoid
The geoid describes Earth’s actual shape based on gravitational forces. It is an irregularly shaped surface that represents the mean sea level of the world, but it also extends under the land masses.
Unlike the smooth and regular ellipsoid, the geoid is an irregular surface, which represents where gravity acts upon the Earth's surface. This is why the geoid is essential for any geophysical study or precise measurement of elevation.
Earth’s Gravity Field
The gravity on Earth does not occur evenly over the planet. And, there are some different factors that contribute to the variation in the gravity, for example, the distribution of mass on the Earth is different, such as mountains, ocean trenches, and underground and below the surface of the Earth.
Why Gravity Matters in Geodesy
The shape of the Geoid
The way that satellites orbit the Earth in space
To accurately calculate where something is located
To understand how tectonic plates are moving and how the sea level is changing.
Using satellite missions and gravity measurements taken directly from the Earth, geodesy provides a very accurate measurement of the Earth's gravitational field.
Positioning and Coordinate Systems
Geographic Coordinates
People often express where something is located based on the:
Latitude (north to south)
Longitude (east to west)
Height (how high above sea level something is)
Geodetic Datums
The geodetic datum provides the reference for measuring locations of objects on the Earth's surface. This reference includes:
The surface of the Earth is referred to as an ellipsoid.
The coordinate system
A set of fixed, known locations
The WGS84 is an internationally recognized datum that will be used for GPS (Global Position System) and most other geospatial-related products.
Global Navigation Satellite Systems (GNSS)
Global navigation satellite systems (GNSS): GPS, GLONASS, Galileo, and BeiDou rely on geodetic principles to provide accurate location coordinates regardless of where you are on the Earth.
Using a satellite system, the GNSS provides accurate location coordinates anywhere on the Earth within a specific accuracy, ranging from as little as one meter of accuracy to as little as one centimeter of accuracy, depending on the method of GNSS being utilized.
Applications of Geodesy
Geodesy is important to many different industries and fields of study, including:
Surveying and mapping: Provides the ability to physically measure land and create maps.
Navigation: Provides the GPS capability to guide vehicles and goods.
Earthquake monitoring: Uses geodesy to measure how tectonic plates move.
Climate change studies: Measures ocean level rise and ice sheet movement;
Engineering projects: Involves determining the exact location for building new roads and bridges.
Why Geodesy Matters Today
The world is a fast-paced, rapidly changing environment. As technology evolves, so will the need for accurate, precise, and reliable geospatial (geographic) information.
Supporting emerging technologies such as autonomous vehicles, smart cities, or space exploration requires highly accurate geodetic measurements; it is geodesy that provides the spatial foundation for ensuring that all spatial data is accurate, consistent, and aligned globally.
Geodesy works under the radar, but geodesy plays a role in many aspects of our environment. From smartphone maps through global environmental monitoring, geodesy gives our understanding of the earth, as well as our ability to navigate the earth, a level of precision that someone 100 years ago could not have imagined.
As a workspace professional or someone just getting into geospatial, you must learn the foundation of the Earth’s size, shape, gravity, and position in our data-oriented society today.
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