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## GIS Data Models III

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**GIS Data Models III**GEOG 370 Instructor: Christine Erlien**Representing Geographic Space: Vector Data Structures**• Represent spatial locations explicitly • Relationships between entities implicit • Space between geographic entities not stored**Vector Data Models**• Multiple data models • Examination of relationships • Between variables in 1 map • Among variables in multiple maps • Data models • Spaghetti models • Topological models • Vector chain codes**Vector Data Model: Spaghetti**• Simplest data structure • One-to-one translation of graphical image • Doesn’t record topology relationships implied rather than encoded • Each entity is a single piece of spaghetti Point Line Area very short longer collection of line segments • Each entity is a single record, coded as variable-length strings of (X,Y) coordinate pairs • Boundaries shared by two polygons stored twice**Vector Data Model: Spaghetti**From Fundamentals of Geographic Information Systems, Demers (2005)**Vector Data Model: Spaghetti**• Measurement & analysis difficult • All relationships among objects must be calculated independently • Relatively efficient for cartographic display • CAC • Plotting: fast www.gis.niu.edu/Cart_Lab_03.htm**Vector Data Model: Topological**• Topology: Spatial relationships between points, lines & polygons • Topological models record adjacency information into data structure • Line segments have beginning & ending • Link: Line segment • Node: Point that links two or more lines • Identifies that point as the beginning or ending of line • Left & right polygons stored explicitly**Vector Data Model: Topological**From An Introduction to Geographic Information Systems, Heywood et al. (2002)**Topological Data Models**• Multiple models • GBF/DIME (geographic base file/dual independent map encoding) • TIGER (topologically integrated geographic encoding and reference system) • POLYVRT (POLYgon conVERTer)**Topological Data Models: GBF/DIME**• Created by U.S. Census Bureau • Both street addresses & UTM coordinates defined for each link • Topology + direction • Left/right • From/to From Fundamentals of Geographic Information Systems, Demers (2005)**Topological Data Models: TIGER**Point Area • Designed for use with the 1990 U.S. Census • Block-level maps • Points, lines & areas explicitly addressed • Census blocks can be retrieved directly by block number Coordinates Line From Fundamentals of Geographic Information Systems, Demers (2005)**Topological Data Models: POLYVRT**Entities stored separately but linked to one another through pointers Chains: Collections of line segments with directional information (from-to, left/right) From Fundamentals of Geographic Information Systems, Demers (2005)**ArcGIS/Arcview’s shapefile**Shapefile Comprised of 3 file types *.shp contains coordinates *.shx is an index file *.dbf is an attribute file in dbase format Where is topology? Not explicitly stored Created on-the-fly Shapefile in-between spaghetti & topological data structures**Compacting Vector Data Models**• Compact data to reduce storage • Freeman-Hoffman chain codes • Each line segment • Directional vector • Length • Non-topological • Analytically limited limits usefulness to storage, retrieval, output functions • Good for distance & shape calculations, plotting**Vector Model to Represent Surfaces: TIN**TIN allows us to record topographic data as points in a regular or irregular grid.**Vector Model to Represent Surfaces: TIN**From Geographic Information Systems & Science, Longley et al. (2005)**Vector GIS: Hybrid & Integrated Systems**• Hybrid system • Links graphic data structures with DBMS • Efficiently manage both graphics & attribute data • Allows raster & vector data types From Fundamentals of Geographic Information Systems, Demers (2005)**Vector GIS: Hybrid & Integrated Systems**• Integrated system • Entities’ coordinate data stored as relational table • Topological data stored as separate table in same database • Attributes can be • Stored in same tables as graphic entities • Stored as separate tables & linked relationally • GIS more closely integrated with DBMS than in hybrid system**Vector GIS: Object-Oriented Database Management Systems**• Emerging as an alternative to hybrid or integrated models • Extends the integrated model by incorporating a spatial query language • Objects inherit properties from the class of objects that they belong to • Variable types & operations particular to that class • Example: ArcGIS’ geodatabase • Shift from previous hybrid orientation