有別於一般人的印象，地球並非完美的球體，甚至也不是簡單的卵形物體。一篇題為 Mean Sea Level, GPS, and the Geoid (2003) 的文章指出，地球表面的不平滑，比我們想像之中誇張得多。地面上不均勻的重力分佈，令海平面亦隨之而有高低起伏：
That the earth does not have a geometrically perfect shape is well established, and the geoid is used to describe the unique and irregular shape of the earth. However, only recently have the more substantial irregularities in the surface created by the global mean sea level (MSL) been observed. These irregularities are an order of magnitude greater than experts had predicted. Controlled by the gravitational potential of the earth, these irregularities form very gentle but massive 「hills」 and 「valleys.」
文章還給出各種情況下，地球海平面分佈的電腦模擬，有學趣者可以讀讀原文：Mean Sea Level, GPS, and the Geoid (2003)。
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