测绘工程专业英语课文翻译(14-17).docVIP

Unit14 Datums, Coordinates and Conversions （已知坐标转换） A datum is the mathematical model of the Earth we use to calculate the coordinates and elevations on any map , chart, or survey system . Geodetic datum s define the size and shape of the Earth and the origin and orientation of the coordinate systems used to map the Earth . Hundreds of different datum s have been used to frame position descriptions since the first estimates of the Earth’s size were made by Aristotle . But all coordinates reference some particular set of numbers for the size and shape of the Earth . For example, the Global Positioning System (GPS) is based on the World Geodetic System 1984 (WGS-84) . Many countries use their own datums when they make their maps and surveys—what we call local datums .（基准是我们用来计算任何地图、海图或测量系统的坐标和海拔的地球的数学模型。大地测量数据定义了地球的大小和形状，以及用于映射地球的坐标系的起源和方向。数以百计的不同的数据已被用于帧位置描述，因为第一次估计地球的大小是由亚里士多德。但是所有坐标都引用了地球的大小和形状的一些特定的数字集。例如，全球定位系统（GPS）是基于世界大地测量系统1984（WGS-84）。许多国家使用自己的数据时，他们使他们的地图和调查，我们称之为局部基准。） Horizontal and Vertical Datums（水平和垂直数据） In geodesy two types of datums must be considered: a“ Horizontal Datum ”for location ,which for m s the basis for the computations of horizontal control surveys in which the curvature of the Earth is considered and a“ Vertical Datum ”for elevation or to which elevations are referred . Vertical control networks provide elevations with reference to a surface of constant gravity force called the geoid . Almost all maps and charts use mean sea level (geoid) for elevation . But they might use any of over hundred different horizontal position datums . One example of horizontal datum is a mathematical surface called a reference ellipsoid with which positional information (latitude and longitude) is referenced to . T he coordinates for points in specific geodetic surveys and triangulation networks are computed from certain initial quantities (datums) . Sometimes, a map will have more than one grid on it .Normally,each grid is for a