Transcript Document
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