大地水準面、地形水準面、準大地水準面.

「地球形狀」，除了是一個科學的議題，亦是測量的根本依據，更是多種應用實踐的起點。在這個議題中，「大地水準面」、「地形水準面」、「準大地水準面」，是在「地球形狀」的物理描述中相互關聯、糾結，既相似又不同的概念。對應地，產生不同的「高程系統」，由「大地水準面」起算的是「正高」，由「準大地水準面」起算的是「正常高」。本文謹就這三個「地球形狀」模型，以盡量最少化數學性推演與描述的方式，探討其間的相似與不同。

“Figure of the Earth”is not only a science subject, but also providing the fundamental datum for surveying and mapping. Consequently, it is the corner stone of a number of important applications.“Geoid”,“Telluroid”, and“Quasi-Geoid”are three related concepts in this subject. They bear both similarity and differences. Different height systems are correspondingly established. Orthometric height is referenced to geoid, and normal height is referenced to quasi-geoid. This note introduces these three concepts in a descriptive manner for reducing the amount of complex mathematical derivations.