14基准、坐标和变换.doc

Unit 14 Datums, Coordinates and Conversions（基准、坐标和变换）A datum is the mathematical model of the Earth we use to calculate the coordinates and elevations on any map, chart, or survey system.（基准是我们在任何地图、海图【chart海图】或测量系统中用来计算坐标和高程的地球数学模型）Geodetic datums define the size and shape of the Earth and the origin and orientation of the coordinate systems used to map the Earth.（大地基准能定义地球的大小和形状，以及用来给地球绘图的坐标系统的原点和坐标定向【orientation】）Hundreds of different datums have been used to frame position descriptions since the first estimates of the Earths size were made by Aristotle.（自亚里士多德【Aristotle】第一次对地球的大小作出估计【estimate估计】以来，上百种不同的基准被用来构架【frame构造v.】位置的表述【description】）But all coordinates reference some particular set of numbers for the size and shape of the Earth.（但是所有的坐标参考对于地球大小和形状有一些特殊）For example, the Global Positioning System (GPS) is based on the World Geodetic System 1984 (WGS-84).（例如，GPS就是基于1984世界大地系统（WGS-84）【美国军方采用 IUGG国际大地测量与地球物理联合会第 17届大会大地测量常数的推荐值GRS80建立的大地坐标系】建立的）Many countries use their own datums when they make their maps and surveys-what we call local datums.（许多国家进行他们的测量和绘图时用他们自己的基准我们称之为区域基准）Horizontal and Vertical Datums（水平和竖直基准）In geodesy two types of datums must be considered: a “Horizontal Datum” for location, which forms 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.（大地测量中两个基准的类型应该考虑：一个是位置的“水平基准”，形成考虑地球曲率的水平控制测量计算的基础；一个是高程的“竖直基准”或者说高程所参考的【refer涉及】）Vertical control networks provide elevations with reference to a surface of constant gravity force called the geoid.（竖直控制网提供关于【with reference to】一个被称为大地水准面的恒【constant不变的】重力表面【重力等位面】的高程）Almost all maps and charts use mean sea level (geoid) for elevation. （几乎所有地图和海图都使用平均海平面（大地水准面）作为高程基准）But they might use any of over hundred different horizontal position datums.（但是它们可能使用任一上百种不同的水平位置的基准）One example of horizontal datum is a mathematical surface called a reference ellipsoid with which positional information (latitude and longitude) is referenced to.（）The coordinates for points in specific geodetic surveys and triangulation networks are computed from certain initial quantities (datums).（精密的【specific详细而精确的；特殊的】大地测量和三角测量【triangulation三角测量的】网中，点位坐标由一些【cerrtain某些、一些】初始量【quantity】（基准）计算得到）Sometimes, a map will have more than one grid on it. Normally, each grid is for a different datum.（有时，一张地图有超过一个格网存在。通常，每个格网有一个不同的基准）Horizontal datum and coordinates: A horizontal datum is a surface of constant values that forms the basis for the computations of horizontal control surveys.（水平基准和坐标：水平基准是一个恒值的曲面，形成水平控制测量的基础）In a horizontal datum a reference ellipsoid is used as a mathematical approximation of the shape of the Earth.（水平基准中，一个参考椭球被用来当作地球形状的数学近似【approximation近似值】）Five parameters are required to define a horizontal datum: two to specify the dimensions of the ellipsoid, two to specify the location of an initial point (origin), and one to specify the orientation (i.e., north) of the coordinate system.（定义一个水平基准需要五个参数【parameter】：两个确定椭球的尺度，两个确定一个原点【initial初始的，initial point即大地原点】的位置，一个确定坐标系统的方向（也就是，北方向）For example, the radius and flattening of the ellipsoid selected for the computations to specify the dimensions of the ellipsoid, the longitude and latitude of an initial point (origin) to specify the location and an azimuth of a line (direction) to some other (triangulation) station to specify the orientation.（例如，被选择来计算的【用来计算的】椭球的半径和扁率确定椭球的尺度，原点的经纬度确定位置【指椭球定位】，一条线（方向）到其它某个（三角测量）站点的方位角来确定方位【椭球定向】）A change in any of these quantities affects every point on the datum.（这些量中任一个的改变都会影响到基准面上每个点）For this reason, while positions within a system are directly and accurately reliable, data such as distance and azimuth derived from computations involving geodetic positions on different datums will be in error in proportion to the difference in the initial quantities.（因为这个原因，当在一个系统内的位置是直接和精确可靠的，如距离和方位等得自【derived from得自】计算的数据包括在不同基准上的大地测量位置将出现错误，错误与初始值之间的差异成比例）The two main horizontal datums used in the U.S. are the North American Datum of 1927 (NAD27) and the North American Datum of 1983 (NAD83).（美国使用的两个主要的水平基准是1927北美基准（NAD27）【1927北美坐标系】和1983北美基准（NAD83）【1983北美坐标系】）In 1986, NAD83 replaced NAD27 because the latter was found to be not accurate enough to support modern positioning activities that occur in highly accurate electronic measurement systems and satellite-based positioning systems.（1986年，NAD83代替了NAD27，因为后者被发现不足够准确以支持出现在高精确的电子测量系统和卫基定位系统的现代定位活动）NAD83 is an earth-centered datum and relies on an ellipsoid (and other constants) of the Geodetic Reference System of 1980 (GRS80).（NAD83是一个地心坐标系，以1980大地参考系统（GRS80）椭球（和其它常量）为基础【rely on依靠、依赖】）It is important to note that GPS position calculations are based on the WGS84 datum (World Geodetic System of 1984), which for all practical purposes is identical to GRS80.（需要注意的是，GPS定位计算是基于WGS84基准【坐标系】（1984世界大地坐标系），实际上【for all practical purposes实际上】同GRS80是相同的）In China, Xian Geodetic Coordinate System 1980 is used as a horizontal datum in which the initial point (origin) is in Shanxi Province.（在中国，1980西安大地坐标系【1980西安坐标系】作为水平基准，其原点在陕西省）Vertical datum and heights: The zero surface, to which elevations or heights are referred, is called a vertical datum.（竖直基准和高程：0基准面，高程或海拔所参照的，被称为竖直基准【高程基准】）From previous text we know the geoid is an equipotential surface of the Earth gravity field that most closely approximates the mean sea surface.（在以前的【previous】文章里我们知道大地水准面是一个地球重力场的等位面【equipotential surface】，非常接近于平均海平面。）At every point the geoid surface is perpendicular to the local plumb line.（在每个点上大地水准面都与本地铅垂线正交）It is therefore a natural reference for heights-measured along the plumb line.（因此【therefore因此】这是一个自然的【natural天生的、自然的】高程参考沿铅垂线测得）Heights referred to the geoid are called orthometric heights, which stand in contrast to geodetic (ellipsoidal) heights, which refer to the ellipsoid.（到大地水准面的高程叫正高【orthometric height】，与之相比的是大地高【in contrast to和相比】，其参考面为椭球面）Because we cannot directly see the geoid surface, we cannot actually measure the heights above or below the geoid surface.（因为我们不能直接看到大地水准面，我们就不能实际测量到【above or below高于或低于】大地水准面的高程。）We must infer where this surface is by making gravity measurements and by modeling it mathematically.（我们必须利用重力测量来推断【infer推断】该曲面在哪儿，并将其数学地建模【并建立其数学模型】）For practical purposes, we assume that at the coastline the geoid and the MSL surfaces are essentially the same.（为了实际目的【以应用为目的】，我们假定大地水准面和平均海平面在海岸线处基本上【essentially基本上、本质的】一样）Nevertheless, as we move inland we measure heights relative to the zero height at the coast, which in effect means relative to MSL.（然而，当我们向内陆移动，我们测量高程相对于岸边的0高程，实际上指的是【means意味着】相对于平均海平面）Therefore we use mean sea level as a plane upon which we can reference or describe the heights of features on, above or below the ground.（因此我们用平均海平面当作一个平面，在其上我们可以参考或描述高于或低于地表或地表上的特征的高程）Elevations are not required for most parcel mapping applications.（大多数地块【parcel小包、地块，即小范围的】绘图应用并不要求高程）However, since GPS is a 3D (actually 4D) measuring device, elevations are available for every point.（然而，由于GPS是一个3D（实际上是4D）测量仪器，每个点的高程都是可以用到的【available可用的】）As mentioned earlier, the GPS-derived elevation refers to the ellipsoid (ellipsoidal height), not the mean sea level (orthometric height).（正如早些时候提到的，GPS高程【GPS-derived得自GPS的】是到椭球（大地高）而不是平均海平面（正高）的高）Conversions （转换）A coordinate conversion or transformation is the process of bringing a coordinate from one defined coordinate system (or zone) into another through a series of algorithms based on the latitude/longitude position of the point.（坐标转换【conversion or transformation都是转换】是在点的经纬度位置基础上，通过一连串的运算法则，将坐标从一个定义坐标系统（或坐标带）转换到另一个坐标系统的过程）Coordinate systems based on the same datum retain a perfect mathematical relationship, allowing coordinate values to be precisely transformed between them.（基于相同的基准的坐标系统保留着完美的【perfect】数学关系式，使得坐标值可以在两者之间精确转换）But the coordinates for a point on the Earths surface in one datum will not match the coordinates from another datum for that same point.（但是地球表面点的一个坐标系中的坐标与另一坐标系中相同该点的坐标将不再相吻合【match匹配、符合】）The differences occur because of the different ellipsoids used and the probability that the centers of each datums ellipsoid is oriented differently with respect to the Earths center.（差异的发生是因为使用的椭球不同和每个基准的椭球中心关于地球中心定位不同的可能性【probability】）【used和probabiolity并列，其它都是分别修饰两个词的】A grid shift exists between datums beca