Unit 13 Geoid and Reference Ellipsoid（大地水准面和参考椭球） The Earth‟s physical surface is a reality upon which the surveying observations are made and points are located.（地球物理表面【或者说，地球自然表面】是一个实体，测量工作【observation 观测】在其上进行，点位在其上进行定位。） However, due to its variable topographic surface and overall shape, it cannot be defined mathematically and so position cannot be computed on its surface.（然而，由于【due to】它的起伏不定的【variable 可变的、不定的，这里按中文习惯译为起伏不定的】地形表面和总的【overall】形状，它不能被数学的定义，因此点位也不能在其上进行计算。） It is for this reason that in surveys of limited extent, the Earth is treated as flat and plane trigonometry used to define position.（正是因为这个原因，在有限范围内的测量中，地球被当成平的，并用平面三角学【trigonometry三角学】来确定位置。） If the area under consideration is of limited extent, orthogonal projection of this area onto a plane surface may result in negligible distortion.（如果在考虑中的【under consideration在考虑中的】区域是有限范围的，该区域在一个平面【plane surface】上的正交投影【orthogonal projection】导致的【直译为导致，可以意译为：其结果】是可以忽略的【negligible 可以忽略的】变形【distortion】） Plane surveying techniques could be used to capture field data and plane trigonometry used to compute position.（平面测量技术可以被用来获取外业数据，平面三角学用来计算位置坐标） However, if the area extended to a large area beyond limitation and treated as a flat surface the effect of the Earth‟s curvature will produce unacceptable distortion.（然而，如果该区域延伸【extend】为一个大的区域超过了限度，把它当成一个平面，地球的曲率影响【effect】将产生不可被接收的【unacceptable】变形） It can also be clearly seen that the use of a plane surface as a reference datum for the elevations of...
