Point to Ponder “I think there is a world market for maybe five computers.” »Thomas Watson,...
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Point to Ponder
• “I think there is a world market for maybe five computers.”
» Thomas Watson, chairman of IBM, 1943
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Spatial Data Representation
• Maps and GIS are models of reality
– They emphasize some aspects of reality in a cartographic (and database) representation while ignoring or greatly simplifying other aspects of reality.
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Spatial Data Representation
• Abstraction
– the process of interpreting what can be sensed from the real world into representational symbols (e.g. the category forest)
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Spatial Data Representation
• Data Modeling
– the process of abstraction from the real world for the purpose of representation in a GIS (or other information system)
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Spatial Data Representation
Conceptual Model
Data Model
Data Structure
File Structure
Data
Modeling
Levels
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Spatial Data Representation• What geographic entities can be identified in this
landscape?
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Spatial Data Representation• The geographic entities (things) that are
represented in a GIS depend on:
– worldview/perspective– knowledge– purpose
of the GIS designer and the GIS user
– and the GIS representational structure
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Spatial Data Representation
• Geographic entities can be conceptualized as:
– points – lines – areas – surfaces
[volumes]
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Spatial Data Representation
• How geographic entities are conceptualized depends on:
– your worldview/perspective– your knowledge– your purpose– your scale of conceptualization
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Spatial Data Representation
– your scale of conceptualization
State College in the context of its location in the state (point feature)
State College in the context of its size and shape (area feature)
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Spatial Data Representation
• Geographic entities can be spatially (graphically) represented as spatial elements:
– points 0 dimensional– lines 1 dimensional– areas 2 dimensional– surfaces 2.5 dimensional– volumes 3 dimensional
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Spatial Data Representation
• Most geographic entities exist in three dimensions through time
• The conceptualization of the geographic entity to be represented governs the choice of graphic representation as a (set of) spatial element(s)
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Spatial Data Representation
• Some spatial representation choices are easier than others:– roads are represented as lines on road maps of
Pennsylvania
• Some spatial representation choices are more difficult:– especially the representation of events and
processes (e.g. soils)
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Spatial Data Representation
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Spatial Data Representation• Cartographic generalization - reducing information
so it can be more clearly cartographically represented
Feature elimination - eliminating portions of features
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Spatial Data Representation
Feature smoothing - graphically simplifying features
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Spatial Data Representation
• Can distinguish between two different conceptualizations of geographic phenomena:
– object view: empty space ‘littered’ with objects (e.g. lakes)
– field view: a geographic entity that ‘varies’ across a space (e.g. elevation)
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Areas within 1 km of hazardous facility in the Philadelphia area
Elevation of the State College area
Object View Field View
Spatial Data Representation
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Spatial Data RepresentationObject or field conceptualizations can be spatially represented in different ways, e.g. the elevation field represented as a surface of shading values (left) or as a set of discrete contour lines (right).
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Spatial Data Representation
Conceptual Model
Data Model
Data Structure
File Structure
Data
Modeling
Levels
Elevation varies over the surface of the earth
Set of lines that represent elevation (contours)
Vector-Relational
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Spatial Data Representation
• Geographic entities that are conceptualized as surfaces can be thought of as:
– discrete: spatial extent of an entity defined by sharp boundaries (e.g. population density per census block group)
– continuous: varies ‘smoothly’ over space (e.g. temperature)
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Spatial Data RepresentationPopulation density by county in Pennsylvania changes in discrete steps
Elevation changes smoothly and continuously over space
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Spatial Data Representation• Is land cover (forested, urban, agricultural, etc.)
conceptualized with an object view or a field view?
– Is land cover an assemblage of individual areas, each a certain type of cover (object view)? Or is land cover a spatially varying property that can be measured at any point in space (field view)?
• Is land cover spatially continuous or discrete?
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Spatial Data Representation
• GIS is a model of the real world
• The representation of the real world in a GIS is determined by: – the conceptualization of the real world– the structure of the GIS database representation
• spatial elements GIS supports
• data structures underlying the conceptual level