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									Spatial Information Systems (SIS)
           COMP 30110

       Formats and Standards
                  Standards and Formats
• GIS vendors provide their own proprietary formats
  Examples:
    – Shapefile (ESRI ArcView format): based on a non-topological representation
      (as for spaghetti data structure)
    – Coverages (ESRI ArcInfo format): based on a topological representation

    – DXF (Drawing Interchange Format: developed within Autodesk’s AutoCAD
      sw)

    – etc.
                             Shapefiles
• Shapefile (ESRI ArcView format): based on a non-topological representation
  (as for spaghetti data structure)


• Topological/connectivity relations are calculated on-the-fly

• A map is composed of different layers (non-overlayed approach)




         Idrography layer                     Road network layer
                                                                   Entities from the
                                                                      two layers
                                                                       intersect
                                    Note

•ArcView is a desktop GIS:
    – Used for visualisation more than for map making
    – Does not provide all full functionality provided by ArcInfo
    – Easily integrates with other applications, DBMSs etc.


• ArcInfo is a dedicated GIS system:
    – Used more for map making
    – Provides complete GIS functionality
    – It was not designed to be integrated with other applications
    – No high-level (SQL-like) query language
             Standards and Formats (cont.d)
•Formats defined by official organisations (standards):

    – TIGER (Topologically Integrated Geographic Encoding and Referencing):
      system and DB developed by the US Census Bureau: based on topological
      representation

    – SDTS (Spatial Data Transfer Standard: developed by the US Geological
      Survey): based on a complex topological representation

    – etc.
                              TIGER files
• Based on topological representation

• All objects in one single layer: overlayed approach

• All intersections are stored explicitly even if they do not correspond to
  geographic objects

                                                               This intersection
                                                                point is stored
                                                                explicitly (i.e.,
                                                                lines are split)
                 TIGER files (cont.d)

• Based on topological representation
• Entities:
    – points
    – chains (endnodes and shape points)
    – polygons
• Relations:
    – VE
    – FE
    – EV
    – EF
    Additional information: polygon centroids, attribute information,
    isolated points, dangling edges, etc.
     Other ways of representing vector data:
                 Half-plane representation
• Half plane representation: polygons are defined as intersection of
  a number of half planes (each corresponding to one of their sides)

• The points that belong to the interior of the polygon satisfy the constraints
                 aix+biy<0 (corresponding to half planes)
Example:

                                                                 x-y>0
                                                                       ^
                                                                  x<7
                                                                      ^
                                                                  y>1
                                                                      ^
                                                                  y<3
                                                                      ^
    Other ways of representing vector data:
       Realms (Güting and Schneider 1993)

• Realms: planar graphs defined over discrete domains (i.e., grids - not
          the Euclidean plane)

• Realm objects: points, lines and regions defined in terms of finite
         representations

• Lines and regions defined in terms of realm points and segments

• Intersections of lines occur only at realm points
                       Realms (cont.d)

• Intersections of lines occur only at realm points




• Therefore a realm is represented by means of a set of points and a set
of non-intersecting segments (they only “touch” at their endpoints)
            Realms (cont.d)


• A realm
                     Realms: remarks
• More complicated sets of data: not just a polygonal subdivision but
also isolated points and dangling edges (inside and outside faces)




• Classical data structures (e.g., DCEL) must be extended to be able to
capture these cases

								
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