TEACHING GEOG 385.02/GTECH 785.02  GIS APPLICATIONS IN SOCIAL GEOGRAPHY Back to MP home page

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Vector Data Structures

By the end of this lecture you should be able to:

• Describe three levels of vector data encoding.

• Describe the advantages, in general, of topologically encoded vector data.

• Tell what type of vector encoding IDRISI, ArcView, and ArcInfo use and how that determines the type of vector analysis that they can do.

 Vector Data Structures

Three types of vector structures differ by the type of encoding of spatial vector files

Type of vector encoding corresponds to a level of complexity of vector files

· Spaghetti encoding

· Feature-encoded

· Topologically encoded

1. Spaghetti Encoding

• Simple lines, digitized as a sequence of points with x,y coordinates
• Visual effect of a polygon, but no polygon features are stored
• No attributes (no features to link to…)

• No neighborliness (spatial relationships) among features
• Used by cartographers in early automated cartography
• No analysis possible, display only

2. Feature Encoded

• Recognizes vector features as independent objects (points, lines, and polygons).
• Feature ID, feature type, and points (IDRISI vector files).
• Features exist but are not connected, no neighborliness (lines don't join in a network, polygons are not neighbors).
• Attributes are stored in a relational table (linked to feature Ids)
• Polygons:
• created by making the first and last point the same.
• digitized independently from each other; boundaries of neighboring polygons are digitized and stored twice.
• Problems: doubling of info, slivers, weak analysis.

 3. Topologically Encoded

• Recognizes vector features as spatially interconnected objects
• To capture relationships of connectivity and be able to use them in automated spatial analysis (not just visually), we need to explicitly record topology of vector features.

Topology (from geometry)

• Geometric properties of objects that remain constant when objects are stretched or bended and are independent of any coordinate system and scale of measurement

• In maps - connections between features expressed as relations of adjacency (of areas), containment (islands), and connectivity (lines as roads)

• In vector data structures – a method of coding and storing the data such that the connections between features are known to the GIS program.

Definitions

• Arc - uncrossed line that has a direction
• Nodes - points at which a line begins and ends
• Vertices (points) - points in an arc
• Lines consist of arcs (form a network)
• Polygons - consist of arcs + info about left and right polygon + unique ID attached to a polygon locator.
• Attributes - in a relational attribute table

Advantages:

• No slivers – boundaries are digitized only once, each arc is stored only once.
• Allows for automatic error detection – can check for coherence of all arcs and polygons.
• Can do analysis with neighborliness (complex computations are needed, e.g. for buffering).
• Allows for full vector data analysis including polygon overlay and network analysis.
• Arc/Info (editing + analysis); Cartalinx (editing), ArcView (partially).

ArcView vector structure

• ArcView vector files are called Shape files (based on shapes a.k.a. features - points, lines, polygons).
• It is feature encoded non-topological vector format (e.g. polygon boundaries are stored twice).
• Includes certain spatial information - spatial index (e.g. more than Idrisi vector format).

• .shp - the file that stores the feature geometry.
• .shx - the file that stores the index of the feature geometry.
• .dbf - the dBASE file that stores the attribute information of features. When a shapefile is added as a theme to a view, this file is displayed as a feature table.
• Other files are added that store more indexes incurred during analysis.
• All files must be stored together.

Two types of spatial analysis

• Attribute and spatial querying in ArcView GIS per se

• Topological analysis in special extensions (add-ons), e.g., geoprocessing and network analyst. Involves creating new features and changing topology.