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 obrvations 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【obrvation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左边英语】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 ud 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 ud to capture field data and plane trigonometry ud 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 en that the u of a plane surface as a reference datum for the elevations of points is totally unacceptable.（同样可以明显的【clearly】看到，用平面作点的高程的参考基准是完全【totally完全地】不可被接受的【不可以的】。）
Therefore, to reprent horizontal positions and elevations on maps and charts, we need a mathematical model of the Earth which includes a t of numbers for the size and shape of the Earth.（因此，为了在地图和海图【chart海图、图表】上表示【reprent】出水平和高程位置，我们需要一个地球的数学模型，它包括一系列地球大小和形状的参数【number这里译为参数parameter参数】）
We will define a mathematical surface that approximates to the shape of the area under consideration and then fit and orientate it to the Earth’s surface.（我们将定义一个数学表面接近【approximates接近v.】于被考虑的【under consideration】区域的形状，然后使之与地球表面相符合【fit and orientate：fit是适合的意思；orientate是定向的意思】）
Such a surface is referred in surveying as a ‘reference ellipsoid’.（这样一个表面在测量里被称为【refer提到、涉及】“参考椭球”）
The Geoid （大地水准面）
Since the physical surface of the Earth can’t be ud as a computational surface, a mean
a level surface is instinctively taken into consideration.（由于【既然】地球的自然表面不能被当作一个计算面来用，平均海平面自然的【instinctively本能地、自然地】被考虑【被考虑作计算面】）
剩菜能吃吗Mean a level (MSL) is defined as the average level of the ocean surface for all stages of the tide after long periods of obrvations.（平均海平面被定义为在经过长时间的海潮的观测后得到的整个时期【stage阶段、时期】的海洋表面的平均水平面）
We u MSL as a plane upon which we can reference or describe the heights of features on, above or below the ground.（我们用平均海平面当作一个平面，在上面我们可以定位【reference原意为名词，这里词性改变——动词：定位、参考】或描述高于、低于或在地面上特征的高度）
By extending the earth’s MSL through the land areas, an equipotential surface approximately at狗晚上能看见吗 MSL would be formed.（将地球的平均海平面延伸穿过陆地，一个近似【approximately近似地】于平均海平面的等位面【大学生如何学习equipotential surface等位面】形成了）
Such a surface is called the ‘geoid’. Thus by definition, the geoid is an equipotential surface of the Earth gravity field that most cloly approximates the mean a surface.（这样一个面被称为“打的水准面”。这样，依照定义，大地水准面是一个地球重力场的等位面【equipotential surface】，非常近似于平均海平面）
The geoid is only a theoretical surface, which is perpendicular at every point to the direction of gravity.张爱玲散文《迟暮》（大地水准面仅仅是一个理论的曲面，在每个点上都垂直【perpendicular耶稣受难记垂直的】指向重力方向）
You can’t e it, touch it or even dig down to find it. The shape of geoid can be actually measured which is bad on gravity data collected worldwide.（你看不到它，摸不到它，更不能下挖来找到它。大地水准面的形状事实上可以被测出——基于世界范围的重力数据采集）
Although the gravity potential is everywhere the same and the surface is smoother than physical surface of the Earth, it still contains many irregularities which render it unsuitable mathematical location of planimetric position.（尽管重力位【gravity potential，potential
位、电压、潜能】是处处相等的【指的是大地水准面的】，并且该曲面必地球自然表面药平滑的多，它依然包含了许多不规则之处【irregularity】，使得【render三国志翻译】它不适于平面位置【planimetric position】的精确定位【mathematical除有‘数学的’意思，也有‘精确的’意思】
The irregularities are thought to be due to mass anomalies throughout the Earth.给员工开会的简短讲话（这种不规则被认为是由于【due to由于 due应当的】遍及地球【throughout介词：遍及】的质量【mass这里是质量的意思】分布不规则【anomaly异常、不规则】）
The geoid remains important to the surveyor, as it is the surface to which all terrestrial measurements are related.（由于它是对于所有有关的陆地测量的参考面【surface这里译为参考面】，对测量者来说大地水准面依然【remain】重要）
As the direction of the gravity vector (termed the vertical) is everywhere normal to the geoid, it defines the direction of the surveyor’s plumb-bob line.（由于重力矢量（称之为【term称为】垂线【vertical垂线n.、垂直的adj.】）的方向在各处都垂直【normal垂直的、正交的、垂线、法线】于大地水准面，由测量者的铅垂线【plumb-bob line】方向就可
以表示【define明确表示】）
