Spatial Information Systems (SIS) COMP Formats and Standards

Standards and Formats GIS vendors provide their own proprietary formats GIS vendors provide their own proprietary formats Examples: Examples: – Shapefile (ESRI ArcView format): based on a non-topological representation (as for spaghetti data structure) (as for spaghetti data structure) – Coverages (ESRI ArcInfo format): based on a topological representation – DXF (Drawing Interchange Format: developed within Autodesk’s AutoCAD sw) sw) – etc.

Shapefiles Shapefile (ESRI ArcView format): based on a non-topological representation Shapefile (ESRI ArcView format): based on a non-topological representation (as for spaghetti data structure) (as for spaghetti data structure) Topological/connectivity relations are calculated on-the-fly Topological/connectivity relations are calculated on-the-fly A map is composed of different layers (non-overlayed approach) 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: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: 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):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 system and DB developed by the US Census Bureau: based on topological representation representation – SDTS (Spatial Data Transfer Standard: developed by the US Geological Survey): based on a complex topological representation Survey): based on a complex topological representation – etc.

TIGER files Based on topological representation Based on topological representation All objects in one single layer: overlayed approach All objects in one single layer: overlayed approach All intersections are stored explicitly even if they do not correspond to All intersections are stored explicitly even if they do not correspond to geographic objects geographic objects This intersection point is stored explicitly (i.e., lines are split)

TIGER files (cont.d) Based on topological representation Based on topological representation Entities: Entities: – points – chains (endnodes and shape points) – polygons Relations: 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 Half plane representation: polygons are defined as intersection of a number of half planes (each corresponding to one of their sides) 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 a i x+b i y<0 (corresponding to half planes) The points that belong to the interior of the polygon satisfy the constraints a i x+b i y<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 Realms: planar graphs defined over discrete domains (i.e., grids - not the Euclidean plane) the Euclidean plane) Realm objects: points, lines and regions defined in terms of finite Realm objects: points, lines and regions defined in terms of finite representations representations Lines and regions defined in terms of realm points and segments Lines and regions defined in terms of realm points and segments Intersections of lines occur only at realm points 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) 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 A realm Realms (cont.d)

More complicated sets of data: not just a polygonal subdivision but also isolated points and dangling edges (inside and outside faces) 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 Classical data structures (e.g., DCEL) must be extended to be able to capture these cases Realms: remarks