基准（Datum）
坐标系统，如经纬度，需要能够数学定义的具备子午线（meridian）、纬线的形状，和已确定的原点、定向参数等。全球、洲际和地区地图上的经纬度坐标通常是以格林尼治为本初子午线，以南北极为90度。然而，精确地大地测量需要更多的准确定义，基准就是一个选定的椭球和一系列地理坐标系（geographic coordinate system）的定义参数。
区域基准（regional），有时称作当地基准（local），具有自己的原点，一些物理点和参考方位。例如，NAD-27的原点就是在堪萨斯州meades Ranch的一系列参考桩。其他的定义参数包括原点处的大地水准面差距（geoid/ellipsoid separation）值，大地水准面和参考椭球面的角度差（此处存疑）。
世界基准（World），顾名思义，可以在全球范围内使用，其坐标是通过多种方法确定的，包括长时间卫星数据的观测。由于卫星绕地球质心（center of earth's mass）飞行，那么质心可认为是该基准的原点。像区域基准一样，每个世界基准也都有各自选定的椭球。
区域基准和世界基准有一个很重要的区别：区域基准具有内在局限性，即本身存在大地测量形变，这些在世界基准中是不存在的。这是由于区域基准的大多数测站是建立在“旧”基准上，尽管那些使用经纬仪（transit）和链尺（chain）的测量员异乎寻常的小心仔细，世界基准还是具有更高的相对精度（relative precision），毕竟其坐标的获取是以卫星测距和其他高科技手段为基础。
要想真正说明白基准的话题很难，但从根本上讲：使用多种基准使同一个位置具有多种地理坐标，反之，相同的坐标值但参考基准不同，那么代表的也是地球上不同的位置。因此就会有不同基准下坐标的互相表示，这将靠基准转换（datum shift）完成。
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原文：
Datums
While a coordinate system such as latitude and longitude requires a mathematically defined shape on which the parallels and meridians will exist, some defining origin and orientation parameters are also required. The latitude and longitude coordinates on world, continental, and regional maps usually reference Greenwich as the zero meridian and the poles as 90 degrees north and south. However, accurate geodesy requires far more precise definitions. A datumis an adopted ellipsoid and a precise set of defining parameters for a geographic coordinate system.
Regional datums, sometimes called local datums, have as their origin, some physical point and reference azimuth (a second point). For example, the origin of the NAD-27 is a set of reference monuments at Meades Ranch, Kansas. Additional defining parameters include a value of for the geoid/ellipsoid separation at the origin, and the angular orientation of these two surfaces with respect to each other.
World datums, as their name suggests, are utilized worldwide, and their coordinates are determined through various means including protracted observations of satellite data. Since satellites will circle the center of earth's mass, this center could be considered to be the datum's origin. As with regional datums, each world datum has its own adopted ellipsoid.
It is important to point out one particular difference between local and world datums. Local datums are typically inferior in makeup, that is, they inherently contain geodetic warps and tensions that are not present in world datums. This is due to the manner in which the majority of stations were established in these 'older' datums. Despite the incredible care with which traverses were conducted by surveyors with transits and chains, world datums, whose coordinates are established using satellite ranging and other sophisticated techniques, offer far superior relative precision.
It is impossible to cover the topic of datums with the detail it deserves, but fundamentally, the consequence of having different datums is that a single physical location can be said to have as many sets of geographic coordinates as there are datums for the region. Conversely, the same set of coordinates, but referenced to different datums, have different physical locations on the earth. Therefore, there arises a need to compute coordinates expressed in one datum given coordinates expressed in another. This is accomplished using one of several well published mathematical transformations called datum shifts.
源文档 <http://blog.sina.com.cn/s/blog_5654be540100o09j.html>