Tài liệu Pro Spatial with SQL Server 2012 pptx

www.it-ebooks.info For your convenience Apress has placed some of the front matter material after the index. Please use the Bookmarks and Contents at a Glance links to access them. www.it-ebooks.info v Contents at a Glance Contents vii Foreword xxi About the Author xxii About the Technical Reviewer xxiii Acknowledgments xxiv Introduction xxv ■ Chapter 1: Spatial Reference Systems 1 ■ Chapter 2: Spatial Features 21 ■ Chapter 3: Spatial Datatypes 51 ■ Chapter 4: Creating Spatial Data 77 ■ Chapter 5: Importing Spatial Data 101 ■ Chapter 6: Geocoding 139 ■ Chapter 7: Precision, Validity, and Errors 163 ■ Chapter 8: Transformation and Reprojection 187 ■ Chapter 9: Examining Spatial Properties 211 ■ Chapter 10: Modification and Simplification 253 ■ Chapter 11: Aggregation and Combination 273 ■ Chapter 12: Testing Spatial Relationships 293 ■ Chapter 13: Clustering and Distribution Analysis 327 www.it-ebooks.info vi ■ Chapter 14: Route Finding 353 ■ Chapter 15: Triangulation and Tesselation 387 ■ Chapter 16: Visualization and User Interface 419 ■ Chapter 17: Reporting Services 445 ■ Chapter 18: Indexing 471 ■ Appendix 499 Index 519 www.it-ebooks.info C H A P T E R 1 ■ ■ ■ 1 Spatial Reference Systems Spatial data analysis is a complex subject area, taking elements from a range of academic disciplines, including geophysics, mathematics, astronomy, and cartography. Although you do not need to understand these subjects in great depth to take advantage of the spatial features of SQL Server 2012, it is important to have a basic understanding of the theory involved so that you use spatial data appropriately and effectively in your applications. This chapter describes spatial reference systems—ways of describing positions in space—and shows how these systems can be used to define features on the earth's surface. The theoretical concepts discussed in this chapter are fundamental to the creation of consistent, accurate spatial data, and are used throughout the practical applications discussed in later chapters of this book. What Is a Spatial Reference System? The purpose of a spatial reference system (sometimes called a coordinate reference system) is to identify and unambiguously describe any point in space. You are probably familiar with the terms latitude and longitude, and have seen them used to describe positions on the earth. If this is the case, you may be thinking that these represent a spatial reference system, and that a pair of latitude and longitude coordinates uniquely identifies every point on the earth's surface, but, unfortunately, it's not quite that simple. What many people don't realize is that any particular point on the ground does not have a unique latitude or longitude associated with it. There are, in fact, many systems of latitude and longitude, and the coordinates of a given point on the earth will differ depending on which system was used. Furthermore, latitude and longitude coordinates are not the only way to define locations: many spatial reference systems describe the position of an object without using latitude and longitude at all. For example, consider the following three sets of coordinates: • 51.179024688899524, –1.82747483253479 • SU 1215642213 • 581957, 5670386 These coordinates look very different, yet they all describe exactly the same point on the earth's surface, located in Stonehenge, in Wiltshire, United Kingdom. The coordinates differ because they all relate to different spatial reference systems: the first set is latitude and longitude coordinates from the WGS84 reference system, the second is a grid reference from the National Grid of Great Britain, and the third is a set of easting/northing coordinates from UTM Zone 30U. Defining a spatial reference system involves not only specifying the type of coordinates used, but also stating where those coordinates are measured from, in what units, and the shape of the earth over which those coordinates extend. www.it-ebooks.info CHAPTER 1 ■ SPATIAL REFERENCE SYSTEMS 2 Many different spatial reference systems exist, and each has different benefits and drawbacks: some offer high accuracy but only over a relatively small geographic area; others offer reasonable accuracy across the whole globe. Some spatial reference systems are designed for particular purposes, such as for nautical navigation or for scientific use, whereas others are designed for general global use. One key point to remember is that every set of coordinates is unique to a particular spatial reference system, and only makes sense in the context of that system. Modeling the Earth The earth is a very complex shape. On the surface, we can see by looking around us that there are irregular topographical features such as mountains and valleys. But even if we were to remove these features and consider the mean sea-level around the planet, the earth is still not a regular shape. In fact, it is so unique that geophysicists have a specific word solely used to describe the shape of the earth: the geoid. The surface of the geoid is smooth, but it is distorted by variations in gravitational field strength caused by changes in the earth's composition. Figure 1-1 illustrates a depiction of the shape of the geoid. Figure 1-1. The irregular shape of the earth. In order to describe the location of an object on the earth’s surface with maximum accuracy, we would ideally define its position relative to the geoid itself. However, even though scientists have recently developed very accurate models of the geoid (to within a centimeter accuracy of the true shape of the earth), the calculations involved are very complicated. Instead, spatial reference systems normally define positions on the earth's surface based on a simple model that approximates the geoid. This approximation is called a reference ellipsoid. ■ Note Not only is the geoid a complex shape, but it is also not constant. Astronomical and geophysical forces, climatic changes, and volcanic activity all contribute to changes in the earth's structure that continuously alter the shape of the geoid. www.it-ebooks.info CHAPTER 1 ■ SPATIAL REFERENCE SYSTEMS 3 Approximating the Geoid Many early civilizations believed the world to be flat (and a handful of modern day organizations still do, e.g., the “Flat Ear th Society,” http://www.theflatearthsociety.org). Our current understanding of the shape of the earth is largely based on the work of Ancient Greek philosophers and scientists, including Pythagoras and Aristotle, who scientifically proved that the world is, in fact, round. With this fact in mind, the simplest reference ellipsoid that can be used to approximate the shape of the geoid is a perfect sphere. Indeed, there are some spatial reference systems that do use a perfect sphere to model the geoid, such as the system on which many Web- mapping pr oviders, including Google Maps and Bing Maps, are based. Although a sphere would certainly be a simple model to use, it doesn't really match the shape of the earth that closely. A better model of the geoid, and the one most commonly used, is an oblate spheroid. A spheroid is the three-dimensional shape obtained when you rotate an ellipse about its shorter axis. In other words, it's a sphere that has been squashed in one of its dimensions. When used to model the earth, spheroids are always oblate—they are wider than they are high—as if someone sat on a beach ball. This is a fairly good approximation of the shape of the geoid, which bulges around the equator. The most important property of a spheroid is that, unlike the geoid, a spheroid is a regular shape that can be exactly mathematically described by two parameters: the length of the semi-major axis (which represents the radius of the earth at the equator), and the length of the semi-minor axis (the radius of the earth at the poles). The properties of a spheroid are illustrated in Figure 1-2. Figure 1-2. Properties of a spheroid. ■ Note A spheroid is a sphere that has been “flattened” in one axis, and can be described using only two parameters. An ellipsoid is a sphere that has been flattened in two axes; that is, the radii of the shape in the x-, y-, and z-axes are all different. Although referred to as a reference ellipsoid, in practice most models of the earth are actually spheroids, because ellipsoid models of the world are not significantly more accurate at describing the shape of the geoid than simpler spheroid models. www.it-ebooks.info CHAPTER 1 ■ SPATIAL REFERENCE SYSTEMS 4 The flattening ratio of an ellipsoid, f, is used to describe how much the ellipsoid has been “squashed,” and is calculated as f = (a – b ) / a where a = length of the semi-major axis; b = length of the semi-minor axis. In most ellipsoidal models of the earth, the semi-minor axis is only marginally smaller than the semi-major axis, which means that the value of the flattening ratio is also small, typically around 0.003. As a result, it is sometimes more convenient to state the inverse flattening ratio of an ellipsoid instead. This is written as 1/f, and calculated as follows. 1 / f = a / (a – b) The inverse-flattening ratio of an ellipsoid model typically has a value of approximately 300.Given the length of the semi-major axis a and any one other parameter, f, 1/f, or b, we have all the information necessary to describe a reference ellipsoid used to model the shape of the earth. Regional Variations in Ellipsoids There is not a single reference ellipsoid that best represents every part of the whole geoid. Some ellipsoids, such as the World Geodetic System 1984 (WGS84) ellipsoid, provide a reasonable approximation of the overall shape of the geoid. Other ellipsoids approximate the shape of the geoid very accurately over certain regions of the world, but are much less accurate in other areas. These local ellipsoids are normally only used in spatial reference systems designed for use in specific countries, such as the Airy 1830 ellipsoid commonly used in Britain, or the Everest ellipsoid used in India. Figure 1-3 provides an (exaggerated) illustration of how different ellipsoid models vary in accuracy over different parts of the geoid. The dotted line represents an ellipsoid that provides the best accuracy over the region of interest, whereas the dash-dotted line represents the ellipsoid of best global accuracy. Figure 1-3. Comparison of cross-sections of different ellipsoid models of the geoid. www.it-ebooks.info CHAPTER 1 ■ SPATIAL REFERENCE SYSTEMS 5 It is important to realize that specifying a different reference ellipsoid to approximate the geoid will affect the accuracy with which a spatial reference system based on that ellipsoid can describe the position of features on the earth. When choosing a spatial reference system, we must therefore be careful to consider one that is based on an ellipsoid suitable for the data in question. SQL Server 2012 recognizes spatial reference systems based on a number of different reference ellipsoids, which best approximate the geoid at different parts of the earth. Table 1-1 lists the properties of some commonly used ellipsoids that can be used. Table 1-1. Properties of Some Commonly Used Ellipsoids Ellipsoid Name Semi-Major Axis (m) Semi-Minor Axis (m) Inverse Flattening Area of Use Airy (1830) 6,377,563.396 6,356,256.909 299.3249646 Great Britain Bessel (1841) 6,377,397.155 6,356,078.963 299.1528128 Czechoslovakia, Japan, South Korea Clarke (1880) 6,378,249.145 6,356,514.87 293.465 Africa NAD 27 6,378,206.4 6,356,583.8 294.9786982 North America NAD 83 6,378,137 6,356,752.3 298.2570249 North America WGS 84 6,378,137 6,356,752.314 298.2572236 Global Realizing a Reference Ellipsoid Model with a Reference Frame Having established the size and shape of an ellipsoid model, we need some way to position that model to make it line up with the correct points on the earth's surface. An ellipsoid is just an abstract mathematical shape; in order to use it as the basis of a spatial reference system, we need to correlate coordinate positions on the ellipsoid with real-life locations on the earth. We do this by creating a frame of reference points. Reference points are places (normally on the earth's surface) that are assigned known coordinates relative to the ellipsoid being used. By establishing a set of points of known coordinates, we can use these to "realize" the reference ellipsoid in the correct position relative to the earth. Once the ellipsoid is set in place based on a set of known points, we can then obtain the coordinates of any other points on the earth, based on the ellipsoid model. Reference points are sometimes assigned to places on the earth itself; The North American Datum of 1927 (NAD 27) uses the Clarke (1866) reference ellipsoid, primarily fixed in place at Meades Ranch in Kansas. Reference points may also be assigned to the positions of satellites orbiting the earth, which is how the WGS 84 datum used by GPS-positioning systems is realized. When packaged together, the properties of the reference ellipsoid and the frame of reference points form a geodetic datum. The most common datum in global use is the World Geodetic System of 1984, commonly referred to as WGS 84. This is the datum used in handheld GPS systems, Google Earth, and many other applications. A datum, consisting of a reference ellipsoid model combined with a frame of reference points, creates a usable model of the earth used as the basis for a spatial reference system. We now need to consider what sort of coordinates to use to describe positions relative to the chosen datum. www.it-ebooks.info CHAPTER 1 ■ SPATIAL REFERENCE SYSTEMS 6 There are many different sorts of coordinate systems, but when you use geospatial data in SQL Server 2012, you are most likely to use a spatial reference system based on either geographic or projected coordinates. Geographic Coordinate Systems In a geographic coordinate system, any location on the surface of the earth can be defined using two coordinates: a latitude coordinate and a longitude coordinate. The latitude coordinate of a point measures the angle between the plane of the equator and a line drawn perpendicular to the surface of the earth to that point. The longitude coordinate measures the angle (in the equatorial plane) between a line drawn from the center of the earth to the point and a line drawn from the center of the earth to the prime meridian. The prime meridian is an imaginary line drawn on the earth's surface between the North Pole and the South Pole (so technically it is an arc, rather than a line) that defines the axis from which angles of longitude are measured. The definitions of the geographic coordinates of latitude and longitude are illustrated in Figure 1-4. Figure 1-4. Describing positions using a geographic coordinate system. ■ Caution Because a point of greater longitude lies farther east, and a point of greater latitude lies farther north, it is a common mistake for people to think of latitude and longitude as measured on the earth's surface itself, but this is not the case: latitude and longitude are angles measured from the plane of the equator and prime meridian at the center of the earth. www.it-ebooks.info [...]... REFERENCE SYSTEMS • Details of all the geographic spatial reference systems supported by SQL Server 2012 are contained within a system catalogue view called sys .spatial_ reference_systems SQL Server also supports data defined using any projected spatial reference system • The Well-Known Text format is a standard format used to express the properties of a spatial reference system If you are interested in... a 2D plane, SQL Server supports projected coordinate data defined from any projected coordinate reference system, and the details of such systems are not listed in sys .spatial_ reference_systems To illustrate how spatial references are represented in WKT format, let’s examine the properties of the EPSG:4326 spatial reference by executing the following query SELECT well_known_text FROM sys .spatial_ reference_systems... The Hammer–Aitoff map projection is illustrated in Figure 1-6 9 www.it-ebooks.info CHAPTER 1 ■ SPATIAL REFERENCE SYSTEMS Figure 1-6 The Hammer–Aitoff map projection Mercator Projection The Mercator map projection is an example of a conformal map projection A conformal map projection is any projection that preserves the local shape of objects on the resulting map The Mercator projection was first developed... system, embedded within brackets following the GEOGCS keyword The reason is that a projected system must first define the three-dimensional, geodetic model of the earth, and then specify several additional parameters that are required to project that model onto a plane ■ Note The Well-Known Text format in which SQL Server stores the properties of spatial reference systems in the sys .spatial_ reference_systems... represent all of the necessary parameters of a spatial reference system These reference numbers are called spatial reference identifiers (SRIDs) One authority that allocates SRIDs is the European Petroleum Survey Group (EPSG), and its reference identification system is implemented in SQL Server 2012 Whenever you use any of the spatial functions in SQL Server that involve stating the coordinates of... shape and position These shapes are called geometries The spatial functionality in SQL Server is based on the Open Geospatial Consortium’s “Simple Features for SQL Specification”, which you can view online at http://www.opensgeospatial.org/standards/sfs This standard defines a number of different types of geometries, each with different associated properties In this chapter, each of the different types... item of spatial data in SQL Server is an example of one of the ten instantiable classes shown with a solid border Figure 2-1 The inheritance hierarchy of geometry types Instantiable types (those types from which an instance of data can be created in SQL Server 2012) are shown with a solid border SQL Server 2008 provided only a single instantiable type of Curve (the LineString), and only a single type... ■ SPATIAL FEATURES ■ Note Some GIS systems make a distinction between a LineString and a Line According to the Open Geospatial Consortium specification (a standard on which the spatial features of SQL Server 2012 are largely based), a Line connects exactly two Points, whereas a LineString may connect any number of Points Because all Lines can be represented as LineStrings, of these two types SQL Server. .. defined using projected coordinates, because these take place on a simple 2D plane Therefore SQL Server can support data defined using any projected coordinate system The parameters of each geographic spatial reference system in sys .spatial_ reference_systems are stored in the well_known_text column using the Well-Known Text (WKT) format, which is an industrystandard format for expressing spatial information... criticism, the Mercator projection is still used by many applications, including Bing Maps and Google Maps, and it is probably one of the most instantly recognizable of all geographical images of the world 10 www.it-ebooks.info CHAPTER 1 ■ SPATIAL REFERENCE SYSTEMS Figure 1-7 The Mercator map projection Equirectangular Projection The equirectangular projection is one of the first map projections ever to . of the spatial features of SQL Server 2012, it is important to have a basic understanding of the theory involved so that you use spatial data appropriately. identification system is implemented in SQL Server 2012. Whenever you use any of the spatial functions in SQL Server that involve stating the coordinates