**Spheroid / Ellipsoid**While we are familiar with perfectly spherical representations of the Earth, it may prove surprising to learn that the Earth is more of an

**oblate spheroid, in**simple terms a squashed sphere bulging at the equator, due to the centripetal force of rotating on its axis:

** **

More accurately still, Earth can also be depicted as a rather lumpy rock:

**Geoid**Earth's geoid is a calculated

*surface*of equal gravitational potential energy, representing the shape the

*sea surface*would be if the

*ocean*were not in motion. The geoid is the shape that the ocean surface would take under the influence of the gravity (not a consistent 9.81 m/s²) and rotation of Earth alone, if other influences such as winds and tides (gravitational effects of the Moon and Sun) were absent.

**Undulation**Undulation or geoidal separation is the distance in between the Spheroid and Geoid as illustrated below (undulation is the value -

**N**geoid). Undulation is the result of several phenomena but is primarily caused by variations in earth's gravitational field. Denser areas of the earth have stronger gravitational pull and vice-versa.

In the example NMEA GGA string below, 39.65 Metres is the height of the GPS antenna above the Ellipsoid and 8.54 Metres is the Geoid - Ellipsoid separation (undulation).

**Figure 3 - Undulation **

In the example NMEA GGA string below 39.65 Metres is the height of the GPS antenna above the Ellipsoid & 8.54 Metres is the Geoid - Ellipsoid separation (undulation):

**Geoidal Models **It is not practical to represent undulation as a mathematical formulae (unlike spheroids), undulation is measured and and tabulated as a model. Geoidal models of the earth are often created represented at equally spaced intervals. Current Veripos products and services use either EGM96 or EGM08 models. EGM96 has a spatial resolution of 15 minutes ( approx. 37 million values) and EGM08 2.5 minutes (approx 1.5 million values). The table below details which EGM model is used for each Veripos product:

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