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UNC-Chapel Hill GEOG 070 - Understanding Spatial Data Models

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Understanding Spatial Data ModelsUnderstanding Spatial Data ModelsPhenomena Organized by LayersSpatial Data ModelsVector Data Model - ObjectsVector Data Model - ObjectsThe Arc-Node Data StructureVector Data Model - ObjectsVector Data Model - ObjectsVector Data Model - ObjectsVector Data Model - ObjectsVector Data Model - ExampleVector Data Model - ExampleVector Data Model - ExampleVector Data Model - ExampleVector Data Model - AttributesVector Data Model - NetworksVector Data Model - NetworksVector Data Model - ScaleVector Data Model - ScaleVector Data Model - ScaleVector Data Model - Sliver PolygonsVector Data Model - TopologyVector Data Model - TopologyVector Data Model - TopologyTopology – Area DefinitionTopology - PointsTopology - ContainmentTopology - ContainmentTopology - LinesTopology - ConnectivityTopology - PolygonsTopology - ContiguityDavid Tenenbaum – GEOG 070 – UNC-CH Spring 2005Understanding Spatial Data Models• Geographic/spatial databases are models of reality representing geographic phenomena– These representations are necessarily incomplete models, including only those aspects of reality which are pertinent to our interests– We need to understand the limitationsinherent in modeling reality using simple data objects (in terms of their ability to represent detail as a function of scale, relationships etc.)David Tenenbaum – GEOG 070 – UNC-CH Spring 2005Understanding Spatial Data ModelsThe next two lectures have the following goals:• Understand the two data models and their pros and cons:– Vector–Raster• Understand the various ways in which data objects can be used to represent real-world entities in both data and modelsDavid Tenenbaum – GEOG 070 – UNC-CH Spring 2005Phenomena Organized by Layers• In a GIS, real-world entities are typically stored in layers, which are referred to in different GIS software using a variety of terms–Themes– Covers– CoveragesDavid Tenenbaum – GEOG 070 – UNC-CH Spring 2005Spatial Data Models• Rasteruses individual cells in a matrix, or grid, format to represent real world entities• Vectoruses coordinates to store the shape of spatial data objectsDavid Tenenbaum – GEOG 070 – UNC-CH Spring 2005Features in the real world are represented by objects that are chosen to approximate their shapeGeographic Primitives•Points–0 dimensional •Lines–1 dimensional•Polygons–2 dimensionalVector Data Model - ObjectsDavid Tenenbaum – GEOG 070 – UNC-CH Spring 2005Vector Data Model - Objects• Point is the primary data object, from which all other higher-order objects are constructed(20,55)(30,80)(25,60)–position denoted by an (x,y) coordinate pair–used for objects too small to be represented as an areaDavid Tenenbaum – GEOG 070 – UNC-CH Spring 2005Benefit:•The arc-node structure allows more efficient data storage for vector dataHow does it work?•It stores data so that nodes construct arcs, and arcs construct polygons•Nodes define the two endpoints of an arc. They may or may not connect two or more arcs.•An arc is the line segment between two nodes. The points between two nodes defining the shape of an arc are called vertices. Nodes and vertices are represented as x, y coordinates.The Arc-Node Data StructureDavid Tenenbaum – GEOG 070 – UNC-CH Spring 2005Vector Data Model - Objects•Lines/Arcs– these are formed by joiningmultiple points– points at the junctions of lines are called nodesNodeVertexDavid Tenenbaum – GEOG 070 – UNC-CH Spring 2005Vector Data Model - Objects• Polygons– these are composed of multiple lines or arcs– They are required to have the property of closure, meaning that the multiple lines/arcs must form a closed shape for it to be a polygonDavid Tenenbaum – GEOG 070 – UNC-CH Spring 2005Vector Data Model - Objects• We can use discrete points, lines and polygons to represent discrete objects such as cities, roads and political boundaries in a spatial databaseBrazilInterstate95CityHospitalDavid Tenenbaum – GEOG 070 – UNC-CH Spring 2005Vector Data Model - Objects• Each spatial data object is assigned a unique object ID (unique identifier)• Spatial coordinates are stored in tables for each data object:– Lines• Coordinates for endpoints• Vertices along line (optional)– Polygons• Multiple (x,y) coords defining polygon• List of lines that define the polygon boundaryDavid Tenenbaum – GEOG 070 – UNC-CH Spring 2005HousesRoadRiverLakeLakeThis example map includes features that need each of the point, arc, and polygon components of vector data model to represent themVector Data Model - ExampleDavid Tenenbaum – GEOG 070 – UNC-CH Spring 2005111213141516171819201234567891021X,Y Coord.AttributeID#36,20nul2135,27nul2032,36nul1921,37nul1814,30nul1714,37nul164,32house156,32house14---------44,50nul632,50nul523,50nul421,50nul314,50nul21,50nul1Vector Data Model - ExamplePointsDavid Tenenbaum – GEOG 070 – UNC-CH Spring 2005X,Y CoordEnd NodeStart NodeAttributeID#32,36…23,50519river2235,27…32,361920river2135,27…36,202120shoreline2036,20…35,272021shoreline1925,1…36,20218river1821,37…21,50318road1714,37…21,371816road1614,37…14,50216road15---------------44,50…44,176map border632,50…44,5065map border523,50…32,5054map border421,50…23,5043map border314,50…21,5032map border21,50…14,5021map border11112131415161718192012345678910212322Vector Data Model - ExampleArcsDavid Tenenbaum – GEOG 070 – UNC-CH Spring 200512345678Bounding LinesAttributeID#4,22,23lake819,20lake75,6,7,18,19,21,22nul63,23,21,20,18,8,13,14,16,17nul52,17,16,15nul412,13,9,10nul31,15,14,12,11nul21,2,3,4,5,6,7,8,9,10,11map exterior1Vector Data Model - ExamplePolygonsDavid Tenenbaum – GEOG 070 – UNC-CH Spring 2005Vector Data Model - Attributes• Every object is assigned a unique ID• That ID is used to link the object with the attribute data that is stored in the attribute tableDavid Tenenbaum – GEOG 070 – UNC-CH Spring 2005Vector Data Model - Networks• There is a special subset of vector data models that are used to address network problems (e.g. routing traffic using mapped streets) - these are known as spatial network modelsDavid Tenenbaum – GEOG 070 – UNC-CH Spring 2005Vector Data Model - Networks• In a spatial network model, directional components are assigned to lines using From and To Nodes to indicate the allowed direction of travelArcABFFromTo001002003005002005David Tenenbaum – GEOG 070 – UNC-CH Spring 2005Vector


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