Conceptual models of real world geographical phenomena

● From real world to abstraction● Conceptual models● Data models and representation

Is the geographic world a jig-saw puzzle of polygons, or a club-sandwich of data layers? (Couclelis, 1992)

Maps and GIS are models of realityThey emphasize some aspects of reality in a cartographic (and database) representation while ignoring or greatly simplifying other aspects of reality.

Abstraction the process of interpreting what can be sensed from the real world into symbols

Data Modeling the process of abstraction from the real world for the purpose of representation in a GIS (or other information system)

Entities or Fields

● Entities – space is peopled with 'objects'– discontinuous objects, that are spatially delimited by

precise edges and, in case, characterized by specific attributes

– recognize the entity then define boundaries and location

– “discrete data”– administrative boundaries, phonelines,

transportation networks

Entities or Fields

● Fields– space in terms of continuous Cartesian coordinates

in two or three dimensions– phenomena that are continuous almost everywhere

in their definition domain– attribute vary smoothly and continuously over space– understand spatial variation then recognize 'things"– “continuous data”– elevation, temperature, satellite data

Layers● a logical separation of

mapped information according to theme

● each layer is thematic and reflects either a particular use or a characteristics

● series of overlays

Data Models & Representation

● Spatial/Geometric– location or spatial disribution of a phenomenon

● Attributes– descriptive information associated with geometry

Basic Data Models

Vector Data Model (entities)

Raster Data Model (fields)

Vector Data Model

The directional linear connection between two pointsThe root of vector data model lies in cartographyBasic elements are points with an x,y coordinatesSeries of points that when joined with straight lines, form the graphic representation of that feature unlimited precision

Vector Data Model

A series of points that are connected or joined form a line. A type of line feature that does not intersect with any other line is referred to as arcs. A series of arcs defined a polygon. A series of polygon is a surface within the vector system.

Object representation

Point (oil wells, fire hydrant)

Lines (railway, roads, rivers)

Area/Polygon (land parcel, forest stand, lakes)

Topology

Connections & relationships between geographic features based on location.Spatial relationships are implicit on map sheets.Standing on a street corner looking at a map is a pretty easy way to identify intersecting streets and properties that are adjacent.The computer ‘sees’ these relationships by means of explicitly encoded topology.

Raster Data ModelsEstablishes a pattern of similar grids or cells over a geographic areaThe location of each cell is defined by its row and column indicesThe value assigned to a cell either reflects the dominant feature (attribute) at that grid space or indicates the presence of a preferred feature in case where two or more features are present within a single grid spaceBasic unit in a raster is a cell (or grid or pixel)Analytically more powerful than the vector based GIS

Raster Data Model

Direct calculations using raster layers

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5

3

20

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Raster or Vector?

Comparison between raster and vector

ComplexSimpleGeneralization

AverageGoodArea analysis

GoodPoorAnalysis in network

HighLowGeometrical accuracy

ComplexSimpleData structure

GoodAverageGraphic treatment

SmallLargeData volume

SlowRapidData collection

VectorRaster

Attribute Data

Is the geographic world a jig-saw puzzle of polygons, or a club-sandwich of data layers? (Couclelis, 1992)

References

Burrough, P. A. & McDonnel R.A. 1998. Principles of Geographical Information System. New York, USA: Oxford University Press

Neteler, M. & Mitasova, H. 2004. Open source GIS: a GRASS GIS approach, 2nd edition. The Netherlands: Kluwer Academic Publishers

Heywood, I. et. al. 1998. An Introduction to Geographical Information System. New York, USA: Addison Wesley Longman

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