Elevation

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Elevation and GPS
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Or Why doesnt my GPS elevation agree with other
measurements of elevation?
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The Earth is not a perfect sphere.
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The Earth is slightly flattened at the poles, and
bulges slightly at the equator.
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In addition, the surface of the Earth, even at
sea level, is not flat and smooth.
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Axis of Earths Rotation
The Geoid represents the global, mean (average)
sea level (The actual Earth at sea level)
The Geoid (Exaggerated)
Earths Center
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The ellipsoid
We mathematically represent the Earth in a
theoretical model called the ellipsoid (a solid
ellipse).
Center of the ellipsoid
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This model of the Earth is stored in your GPS
unit.
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Axis of Earths Rotation
The Geoid (Exaggerated)
The Geoid and the Ellipsoid do not necessarily
coincide.
The Ellipsoid
Center of the Ellipsoid
Earths Center
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In current models, the differences between the
Geoid and Ellipsoid range from -106 m to 85 m.
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The Geoid
Surface measurements (made by surveying) give our
height above the GEOID, while GPS measurements
give our height above the ELLIPSOID.
The Ellipsoid
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What Do We Measure?
GPS measures our height above the Ellipsoid.
Surveying measures our height above the Geoid.
Earths Surface
Geoid
Ellipsoid
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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.