Spatial Information Systems (SIS)Spatial Information Systems (SIS)

COMP 30110COMP 30110

Formats and StandardsFormats and Standards

Standards and FormatsStandards and Formats

• GIS vendors provide their own proprietary formatsGIS vendors provide their own proprietary formats

ExamplesExamples::– Shapefile (ESRI ArcView format): based on a Shapefile (ESRI ArcView format): based on a non-topological representation non-topological representation

(as for spaghetti data structure)(as for spaghetti data structure)

– Coverages (ESRI ArcInfo format): based on a Coverages (ESRI ArcInfo format): based on a topological representationtopological representation

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

sw)sw)

– etc.etc.

ShapefilesShapefiles

• Shapefile (ESRI ArcView format): based on a Shapefile (ESRI ArcView format): based on a non-topological representation non-topological representation

(as for spaghetti data structure)(as for spaghetti data structure)

• Topological/connectivity relations are calculated on-the-flyTopological/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 layerIdrography layer Road network layerRoad network layer

Entities from the Entities from the two layers two layers intersectintersect

NoteNote

•ArcView is a desktop GIS: ArcView is a desktop GIS: – Used for visualisation more than for map makingUsed for visualisation more than for map making

– Does not provide all full functionality provided by ArcInfoDoes not provide all full functionality provided by ArcInfo

– Easily integrates with other applications, DBMSs etc.Easily integrates with other applications, DBMSs etc.

• ArcInfo is a dedicated GIS system: ArcInfo is a dedicated GIS system: – Used more for map makingUsed more for map making

– Provides complete GIS functionalityProvides complete GIS functionality

– It was not designed to be integrated with other applicationsIt was not designed to be integrated with other applications

– No high-level (SQL-like) query languageNo high-level (SQL-like) query language

Standards and Formats (cont.d)Standards and Formats (cont.d)

•Formats defined by official organisations (standards):Formats defined by official organisations (standards):

– TIGER (Topologically Integrated Geographic Encoding and Referencing):TIGER (Topologically Integrated Geographic Encoding and Referencing):

system and DB developed by the US Census Bureau: based on system and DB developed by the US Census Bureau: based on topological topological

representationrepresentation

– SDTS (Spatial Data Transfer Standard: developed by the US Geological SDTS (Spatial Data Transfer Standard: developed by the US Geological

Survey): based on a complex topological representationSurvey): based on a complex topological representation

– etc. etc.

TIGER filesTIGER files

• Based on Based on topological representationtopological representation

• All objects in one single layer: overlayed approachAll 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 objectsgeographic objects

This intersection This intersection point is stored point is stored explicitly (i.e., explicitly (i.e., lines are split)lines are split)

TIGER files (cont.d)TIGER files (cont.d)

• Based on Based on topological representationtopological representation

• Entities:Entities:

– pointspoints

– chains (endnodes and shape points)chains (endnodes and shape points)

– polygonspolygons

• Relations:Relations:

– VEVE

– FEFE

– EVEV

– EFEF

Additional information: polygon centroids, attribute information, Additional information: polygon centroids, attribute information, isolated points, dangling edges, etc.isolated points, dangling edges, etc.

Other ways of representing vector data:Other ways of representing vector data:

Half-plane representationHalf-plane representation• Half plane representationHalf plane representation: polygons are defined as intersection of : 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 The points that belong to the interior of the polygon satisfy the constraintsconstraints aaiix+bx+biiy<0 (corresponding to half planes)y<0 (corresponding to half planes)

Example:Example:

x-y>0x-y>0 ^ ^

x<7x<7 ^̂

y>1y>1 ^̂

y<3y<3^̂

Other ways of representing vector data:Other ways of representing vector data:

Realms (Güting and Schneider 1993)Realms (Güting and Schneider 1993)

• RealmsRealms: planar graphs defined over discrete domains (i.e., grids - not : planar graphs defined over discrete domains (i.e., grids - not

the Euclidean plane)the Euclidean plane)

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

representations representations

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

• Intersections of lines occur only at realm pointsIntersections of lines occur only at realm points

• Intersections of lines occur only at realm pointsIntersections of lines occur only at realm points

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

Realms (cont.d)Realms (cont.d)

• A realmA realm

Realms (cont.d)Realms (cont.d)

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

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

Realms: remarksRealms: remarks