2. Or : Why doesn’t my GPS elevation agree with other measurements of elevation?

3. The Earth is not a perfect sphere.

4. The Earth is slightly flattened at the poles, and bulges slightly at the equator.

5. In addition, the surface of the Earth, even at sea level, is not flat and smooth.

6. Axis of Earth’s Rotation Earth’s Center The “Geoid” (Exaggerated) The “Geoid” represents the global, mean (average) sea level (The “actual” Earth at sea level)

7. We mathematically represent the Earth in a theoretical model called the “ellipsoid” (a solid ellipse). The “ellipsoid” Center of the ellipsoid

8. This “model” of the Earth is stored in your GPS unit.

9. Axis of Earth’s Rotation Earth’s Center The “Geoid” (Exaggerated) The Ellipsoid Center of the Ellipsoid The Geoid and the Ellipsoid do not necessarily coincide.

10. In current models, the differences between the Geoid and Ellipsoid range from -106 m to +85 m.

11. The “Geoid” The Ellipsoid Surface measurements (made by surveying) give our height above the GEOID, while GPS measurements give our height above the ELLIPSOID.

12. Geoid Ellipsoid Earth’s Surface What Do We Measure? Surveying measures our height above the Geoid. GPS measures our height above the Ellipsoid.

13. The difference you see between your GPS measurement of elevation and that determined by other methods depends, in part, on the local difference between the Geoid and the Ellipsoid.