CH2-1空间信息基础.ppt

靖娟利 土木工程系,第二章 空间数据结构,主要内容,空间信息基础,1,空间数据的表达,2,空间数据模型与数据结构,3,矢量数据模型,3.1,3.2,不规则镶嵌数据模型,3.3,栅格数据模型,2.1 空间信息基础,一、地球模型,地球自然表面,水准面,大地水准面,地球椭球体,地球形状的概括,一、地球模型,一、地球模型椭球体 Spheroids and spheres,The shape and size of a geographic coordinate systems surface is defined by a sphere or spheroid. Although the earth is best represented by a spheroid, the earth is sometimes treated as a sphere to make mathematical calculations easier. The assumption that the earth is a sphere is possible for small-scale maps (smaller than 1:5,000,000). At this scale, the difference between a sphere and a spheroid is not detectable on a map. However, to maintain accuracy for larger-scale maps (scales of 1:1,000,000 or larger), a spheroid is necessary to represent the shape of the earth. Between those scales, choosing to use a sphere or spheroid will depend on the maps purpose and the accuracy of the data.,一、地球模型椭球体 Spheroids and spheres,A sphere is based on a circle, while a spheroid (or ellipsoid) is based on an ellipse. The shape of an ellipse is defined by two radii. The longer radius is called the semimajor axis, and the shorter radius is called the semiminor axis. Rotating the ellipse around the semiminor axis creates a spheroid. A spheroid is also known as an oblate ellipsoid of revolution. The following graphic shows the semimajor and semiminor axes of a spheroid.,各种地球椭球体模型,一、地球模型基准面 Datum,a datum defines the position of the spheroid relative to the center of the earth. A datum provides a frame of reference for measuring locations on the surface of the earth. It defines the origin and orientation of latitude and longitude lines.,Geocentric datums which relates coordinates to the earths center of mass. An earth-centered, or geocentric, datum uses the earths center of mass as the origin. The most recently developed and widely used datum is WGS 1984. It serves as the framework for locational measurement worldwide.,Local datums A local datum aligns its spheroid to closely fit the earths surface in a particular area. A point on the surface of the spheroid is matched to a particular position on the surface of the earth. This point is known as the origin point of the datum. The coordinates of the origin point are fixed, and all other points are calculated from it.,datam,一、地球模型高程系统 Elevation referencing,二、地理参考与坐标系地理参考,绝对参考（direct georeferencing） 地理坐标系（Geographical coordinates） 平面直角坐标系（Rectangular coordinates） 相对参考（relative georeferencing ） 极坐标系（Polar coordinates） 位置便移（Offset distance）,Address and street codes Postal codes and area name Administrative zones and statistical units Grids and map sheets,连续参考系统（Continuous georeferencing systems ）,离散参考系统（Discrete Geoferencing Systems ）,二、地理参考与坐标系坐标系,把大地水准面上的测量成果化算到椭球体面上的计算工作中，所采用的椭球的大小,椭球体,椭球体与大地水准面的相关位置不同，对同一点的地理坐标所计算的结果将有不同的值,基准面,地理坐标系 (geographic coordinate systems ),A geographic coordinate system (GCS) uses a three-dimensional spherical surface to define locations on the earth. A GCS is often incorrectly called a datum, but a datum is only one part of a GCS. A GCS includes an angular unit of measure, a prime meridian, and a datum (based on a spheroid).,投影坐标系 projected coordinate systems,projected coordinate system,projected coordinate system,defined on a flat, two-dimensional surface， has constant lengths, angles, and areas across the two dimensions，and always based on a geographic coordinate system,locations are identified by x,y coordinates on a grid, with the origin at the center of the grid. Each position has two values that reference it to that central location. One specifies its horizontal position and the other its vertical position. The two values are called the x-coordinate and y-coordinate. Using this notation, the coordinates at the origin are x = 0 and y = 0.,三、地图投影 map projections,1,transform its three-dimensional surface to create a flat map sheet.,2,uses mathematical formulas to relate spherical coordinates on the globe to flat, planar coordinates.,3,Different projections cause different types of distortions.,地图投影,直接建立在球体上的地理坐标，用经度和纬度表达地理对象位置,建立在平面上的直角坐标系统，用（x，y）表达地理对象位置,投影,我国常用的地图投影,1：100万：兰勃特投影（正轴等积割圆锥投影）,大部分分省图、大多数同级比例尺也采用兰勃投影,1：50万、1：25万、1：10万、1：5万、1：2.5万、1:1万、 1：5000采用高斯克吕格投影,高斯-克吕格投影 Gauss-Krger,高斯吕格投影在英美国家称为横轴墨卡托投影。美国编制世界各地军用地图和地球资源卫星象片所采用的全球横轴墨卡托投影（UTM）是横轴墨卡托投影的一种变型。高斯克吕格投影的中央经线长度比等于1，UTM投影规定中央经线长度比为0.9996。在6度带内最大长度变形不超过0.04%。,GK,特点归纳,中央经线和赤道为互相垂直的直线，其他经线均为凹向并对称于中央经线的曲线，其他纬线均为以赤道为对称轴的向两极弯曲的曲线，经纬线成直角相交。,特点1,为了保证地图的精度，采用分带投影方法，即将投影范围的东西界加以限制，使其变形不超过一定的限度，这样把许多带结合起来，可成为整个区域的投影,特点3,在这个投影上，角度没有变形。中央经线长度比等于1，没有长度变形，其余经线长度比均大于1，长度变形为正，距中央经线愈远变形愈大，最大变形在边缘经线与赤道的交点上；面积变形也是距中央经线愈远，变形愈大 。,特点2,GK投影分带,1：2.5万1：50万比例尺地形图，采用6度分带；1：1万比例尺地形图采用经差3度分带。,1,6度带是从0度子午线起，自西向东每隔经差6为一投影带，全球分为60带，各带的带号用自然序数1，2，3，60表示。即以东经0-6为第1带，其中央经线为3E，其余类推.（我国11个，13带23带）,2,3度带，是从东经1度30分的经线开始，每隔3度为一带，全球划分为120个投影带。（我国23个，24带46带）,3,我国领土范围为东经73135 ，北纬455 ,GK投影分带,GK坐标值的规定,GK投影每一个投影带的坐标值都是相对于本带坐标原点的相对坐标值，因而各带的坐标完全相同。,1,2,3,我国位于北半球（ 4N55 N ），全部的x坐标值都是正值，在每个投影带中有一半的y坐标值为负