Achieving Vertical Accuracy: The Role of EGM96 and EGM2008

When someone checks their altitude on a standard smartphone GPS app, the reading is often wildly inaccurate—sometimes off by tens of meters. In civil engineering and surveying, where water drainage and foundation depths are calculated in millimeters, this level of error is catastrophic. The root of this problem lies in the difference between Ellipsoidal Height and Orthometric Height.

The GPS Elevation Problem

GPS satellites calculate your position relative to a perfectly smooth, mathematical model of the Earth called the WGS84 ellipsoid. Therefore, the altitude given by raw GPS is "Ellipsoidal Height." However, the actual Earth is lumpy and irregular due to varying gravitational pulls. Engineers design projects based on "Orthometric Height," commonly known as Mean Sea Level (MSL).

The Geoid Solution

To convert Ellipsoidal Height to MSL, surveyors must subtract the "Geoid Undulation (N)" value from the raw GPS altitude. The Geoid is an equipotential surface of the Earth's gravity field that closely approximates global mean sea level. To find this N value, surveyors rely on global geopotential models.

EGM96 vs. EGM2008

For many years, the Earth Gravitational Model 1996 (EGM96) was the industry standard. It provided a reliable, albeit coarse, grid for calculating geoid undulation worldwide. However, as precision requirements increased, a better model was needed.

Enter EGM2008. Released by the National Geospatial-Intelligence Agency (NGA), EGM2008 represents a monumental leap in geodetic science. It offers a much higher resolution grid (up to 1-minute resolution in specific calculations), accounting for fine-scale gravitational anomalies caused by mountains and deep ocean trenches. By integrating EGM2008 directly into field surveying software, engineers can instantly achieve highly accurate MSL readings without requiring complex desktop post-processing.