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All geographic information systems (GIS) are built using formal models that describe how things are located in space. A formal model is an abstract and well-defined system of concepts. It defines the vocabulary that we can use to describe and reason about things. A geographic data model defines the vocabulary for describing and reasoning about the things that are located on the earth. Geographic data models serve as the foundation on which all geographic information systems are built.   We are all familiar with one model for geographic information—the map. A map is a scale model of reality that we build, using a set of conventions and rules (for example, map projections, line symbols, text). Once we construct a map, we can use it to answer questions about the reality it represents. For example, how far is it from Los Angeles to San Diego? Or, what cities lie along the Mississippi River? The map model also serves as a tool for communicating facts about geography visually: Is the terrain rough? Which way is north? In fact, when we see a map, we often understand things that might not even occur to us as specific questions.   Maps work because we know the “rules” of conventional map reading: blue lines are rivers, North is toward the top of the page, and so on. In a similar way, geographic data models define their own set of concepts and relationships, which must be understood before you can expect to create or interpret your own data model. These concepts relate to how you can represent geographic information in a computer system, rather than, as in the map example, on paper.   In Modeling Our World, Michael Zeiler has written an excellent primer for understanding the various models used to represent geographic information in ArcInfo™ 8 software. He presents, using straightforward text and excellent illustrations, the concepts and vocabulary employed in the design, implementation, and use of the ArcInfo 8 geographic database. In addition to explaining the ArcInfo data model (objects, features, surfaces, networks, images, and so forth) in detail, Michael also provides good insight into how to use this framework to design useful information models that fit your particular needs.   This book serves a variety of different purposes. For the geographer or scientist, it defines a conceptual context for representing geographic information. For the GIS specialist, it serves as a guidebook in designing and using geographic databases. Finally, it introduces database concepts to a geographic audience, and geographic concepts to the database specialist.    ArcInfo 8 defines a unified framework for representing geographic information in a database. Several different generic data models are supported within this framework: • cell-based or raster representation • object-based or feature-based representation • network or graph-element representation • finite-element or TIN representation   Each of these generic models has its own vocabulary used to define and reason about geographic information. When we decide to represent roads, rivers, terrain, or any sort of phenomena in a GIS, we need to decide exactly how we define information in terms of these generic models. As chapter 1 points out, there are many ways that information can be modeled in a GIS. The representation you choose for the data model will affect how you sample and measure geographic information, how you display it visually, and which relationships between elements can be represented, as well as query and analysis operations that can be applied to the information.   Some have asserted that we should hide representational models for geographic information (features, geometry, rasters, surfaces, and so on)from the users of geographic information systems. Somehow, these representational concepts are considered “implementation details.” In this view, a single real-world thing, such as the Mississippi River, should be modeled as a single thing within the GIS. Perhaps, behind the scenes, the system could automatically use multiple representations for these real-world things. If you ask “What is upstream?” it could use a network representation of the river. If you ask “What is the surface area of the water?” it could use a polygon feature representation. If you ask “What area does it drain?” it could use a surface or terrain representation, and so on. While it may be desirable to hide these concepts from some consumers of geographic information, I believe that a strong understanding of geographic data models and representations is crucial to the correct design and use of geographic information systems. Geographic data models act as the lens or filter through which we perceive and interpret the infinite complexity of the real world. It is only in the context of representations of the Mississippi River, for example, that we can define specific properties, behavior, or even its identity as a “thing of interest.” Understanding geographic data model concepts is central to knowing how to define and collect geographic information. It is also crucial for correctly interpreting the results derived from the analysis of geographic information. This is similar to the role that statistics and sampling theory play in the natural sciences.   For the GIS specialist, this book serves as an introduction to a new object-relational model for representing features, spatial relationships between features, and other thematic relationships. This new model is significantly richer in its ability to represent features with associated behavior, relationships, and properties than the current coverage or shapefile model. If you are already familiar with coverages, shapefiles, and database tables, the new model is a dramatic extension of concepts and capabilities with which you are already familiar. Our goal in building the new feature data model has been to move as much specialized application logic (for example, maintaining connectivity or relational integrity between objects) as possible into the scope of the data model itself. This allows more of the GIS application to be defined using rules in the data model, rather than custom application logic written for each application. For other aspects of the data model, which may already be familiar to the reader, the specific jargon and concepts used in ArcInfo 8 (for topics like image data, as an example) are clearly introduced and defined.   This book also connects the specialized world of geographic information systems and the broader world of object-relational databases. ArcInfo now supports the direct use of standard relational database technology as an integral part of the GIS. This introduces some new concepts to the GIS community. Topics such as transaction models for simultaneous editing of a shared, seamless database are described in detail. For the GIS specialist, this provides a good introduction to standard database concepts. For the database specialist, this book serves as a good answer to the question “what is so special about spatial?”   Working with geographic information systems is fun for me because it serves to integrate concepts and ideas from a variety of different disciplines— geometry and networks from applied mathematics, sampling and measurement theory from remote sensing and physics, information modeling and multiuser database issues from information technology. In working with GIS, we get to integrate all of this in a single, useful framework for building real systems. This book presents that synthesis, based on our work with ArcInfo 8. I hope you find this book useful and stimulating as a basis for your own work in geographic information systems.   Contents PREFACE ........................................................................................................................... vii ACKNOWLEDGMENTS ............................................................................................. ix CHAPTER 1: OBJECT MODELING AND GEODATABASES ........................ 1 Modeling objects with GIS ...................................................................................... 2 The progress of geographic data models .............................................................. 4 The geodatabase, store of geographic data........................................................... 8 Features in an object-oriented data model ...........................................................10 Serving geographic data .........................................................................................12 Accessing geographic data.....................................................................................14 Building data models..............................................................................................16 Guide to reading UML object diagrams ................................................................19 Technology trends ..................................................................................................21 CHAPTER 2: HOW MAPS INFORM...................................................................... 23 The utility of maps ..................................................................................................24 How maps present information .............................................................................25 The parts of a map .................................................................................................27 Presenting geography with layers ......................................................................... 28 Drawing features with symbols............................................................................. 30 Drawing feature layers............................................................................................32 Classifying attribute values .....................................................................................36 Displaying thematic, spectral, and picture data....................................................38 Visualizing surfaces with TIN layers .....................................................................41 CHAPTER 3: GIS DATA REPRESENTATIONS ................................................... 45 The fundamentals of a GIS....................................................................................46 The diverse applications of GIS ............................................................................48 Three representations of the world .......................................................................51 Modeling surfaces.................................................................................................. 52 Modeling imaged or sampled data ........................................................................54 Modeling discrete features .....................................................................................56 Comparing spatial data representations ................................................................58 CHAPTER 4: THE STRUCTURE OF GEOGRAPHIC DATA........................ 61 The catalog and connections to data....................................................................62 The geodatabase, datasets, and feature classes ...................................................64 ArcInfo workspaces and coverages ..................................................................... 66 Shapefiles and CAD files ........................................................................................68 Maps and layers ......................................................................................................70 Comparing the structure of vector datasets ..........................................................72 Comparing feature geometry in vector datasets ...................................................73 CHAPTER 5: SMART FEATURES ............................................................................. 75 The qualities of features .........................................................................................76 Steps to making features smart ............................................................................. 78 Designing the geodatabase ....................................................................................80 Storing data in tables ..............................................................................................82 The shape and extent of features......................................................................... 84 Attributes: qualities of an object ............................................................................86 Adding simple behavior with subtypes.................................................................88 Validating attributes .................................................................................................90 Relationships among objects .................................................................................92 Extending object classes ........................................................................................96 The geodatabase object model............................................................................. 98 CHAPTER 6: THE SHAPE OF FEATURES ........................................................ 101 Geometry and features .........................................................................................102 Constructing geometry ..........................................................................................105 Testing spatial relationships .................................................................................110 Applying topological operators............................................................................112 Geometry object model........................................................................................114 CHAPTER 7: MANAGING WORK FLOW WITH VERSIONS ................. 115 Using versions .......................................................................................................116 Long transactions and the geodatabase ..............................................................118 The fundamentals of versions .............................................................................120 Editing versioned geodatabases...........................................................................122 Types of work flows .............................................................................................124 CHAPTER 8: LINEAR MODELING WITH NETWORKS ........................... 127 Modeling infrastructure ........................................................................................128 The network model ..............................................................................................130 How features connect ..........................................................................................132 Network features ...................................................................................................134 Network flow .........................................................................................................139 Analysis on a network ..........................................................................................142 Network object model ..........................................................................................145 CHAPTER 9: CELL-BASED MODELING WITH RASTERS ........................ 147 Representing geography with rasters ...................................................................148 Using raster data....................................................................................................150 Raster data model .................................................................................................152 Raster display and analysis ..................................................................................154 The spatial context of rasters ...............................................................................156 Raster formats ........................................................................................................158 Raster object model ..............................................................................................160 CHAPTER 10: SURFACE MODELING WITH TINS ...................................... 161 Representing surfaces ...........................................................................................162 Structure of a TIN .................................................................................................164 Modeling surface features ....................................................................................166