Is the Earth a perfect sphere?

No. The Earth bulges at the equator due to its rotation, making it an oblate spheroid (an ellipsoid). Furthermore, the actual distribution of mass (mountains, oceans, dense rocks) creates an uneven gravity field, making its true physical shape highly irregular—this shape is called the Geoid.

Why is my GPS elevation wrong?

Your GPS receiver calculates your height based on the mathematical Ellipsoid. However, most maps display height based on the Geoid (Mean Sea Level). Without a local Geoid model loaded into the receiver, the GPS elevation can be off by tens of meters.

Geoid vs. Ellipsoid (The 3 Shapes of Earth)

For Beginners: The Lumpy Potato

When you ask "What is my elevation above sea level?", you are actually asking a very complicated physics question. To understand elevation, surveyors use three different "surfaces" of the Earth:

Topography (The Ground): The actual physical mountains, valleys, and oceans you can touch.
The Ellipsoid (The Math): A perfectly smooth mathematical shape used by GPS satellites to calculate coordinates.
The Geoid (The Physics): A lumpy, potato-like surface defined by Earth's gravity. This is what we actually mean by "Mean Sea Level."

The magic formula: H (True Elevation) = h (GPS Height) - N (Geoid Separation)

If you take a raw GPS reading on a beach at the ocean's edge, it might tell you that your elevation is -30 meters. The GPS isn't broken; it's just doing math against an invisible ellipsoid instead of physical sea level.

The Ellipsoid: Perfect Math for Satellites

GPS satellites orbit the Earth incredibly fast. To calculate where you are on the surface, the computers inside the satellites need the Earth to be a perfect, smooth shape. The Earth bulges slightly at the equator due to centrifugal force, so scientists created the Ellipsoid (specifically the WGS84 Ellipsoid).

The Ellipsoid is a perfectly smooth mathematical balloon. It has no mountains, no valleys, and no gravity anomalies. When your raw GPS tells you your height, it is giving you the Ellipsoid Height (h).

The Geoid: The True "Sea Level"

Gravity on Earth is not uniform. The Himalayan mountains have a lot of mass, which pulls water toward them. Deep ocean trenches have less mass. Because of this, if you let the oceans completely settle without tides or wind, the water level would be lumpy and uneven across the globe.

This lumpy, gravity-defined surface is called the Geoid. It represents Mean Sea Level (MSL). When civil engineers build a canal or a pipeline, they must ensure water flows downhill. Water obeys gravity (the Geoid), not math (the Ellipsoid). Therefore, engineers require Orthometric Height (H), which is your true elevation above the Geoid.

The Geoid Separation (N)

The distance between the smooth Ellipsoid and the lumpy Geoid at any given location is called the Geoid Separation or Geoid Undulation (N). Depending on where you are on Earth, the Geoid can be up to 100 meters above or below the Ellipsoid.

How Surveyors Fix the Problem

A surveyor uses a high-end RTK GPS receiver to get a highly accurate Ellipsoid Height (h). They then load a massive data file onto their data collector called a Geoid Model (such as GEOID18 in the USA). The software looks up the Geoid Separation (N) for their specific latitude and longitude, and subtracts it to give the client the true Orthometric Height (H).