Geog 458: Geog 458: Map Sources and Map Sources and

ErrorsErrorsJanuary 9 2006January 9 2006

Representing GeographyRepresenting Geography

OutlinesOutlines

1.1. Importance of geographic Importance of geographic representationrepresentation

2.2. How do human conceptualize How do human conceptualize surroundings?surroundings?

3.3. How is human conceputalization How is human conceputalization coded in computer databases? coded in computer databases?

4.4. Reading Spatial Data Organization Reading Spatial Data Organization in FGDC Metadata Content Standardin FGDC Metadata Content Standard

1. Importance of 1. Importance of geographic informationgeographic information

What is geographic representation?What is geographic representation? Why is geographic representation Why is geographic representation

important?important?

What is representation?What is representation?

Viewing the real world (or surroundings, Viewing the real world (or surroundings, reality) reality)

Forcing the real world into a manageable Forcing the real world into a manageable conceptconcept

Human conceptualization of the worldHuman conceptualization of the world Putting the real world into a computerPutting the real world into a computer

How would you represent the followings in a How would you represent the followings in a computer? Does the same data model fit all?computer? Does the same data model fit all?

Vegetation, soil, traffic volume, land use, tornado, Vegetation, soil, traffic volume, land use, tornado, clouds…clouds…

Data modeling (building data model)Data modeling (building data model)

Why is representation Why is representation important?important?

Forms the basis of metadata organizationForms the basis of metadata organization Identification, spatial data organization, entity Identification, spatial data organization, entity

and attributeand attribute

Many operations depend on data modelMany operations depend on data model RoutingRouting Spatial interpolationSpatial interpolation

Forms the basis for understanding data Forms the basis for understanding data structure and data format used in a GIS structure and data format used in a GIS Vector, point, line, node, area, topologyVector, point, line, node, area, topology Raster, pixel, tessellation, sampling, planar Raster, pixel, tessellation, sampling, planar

enforcementenforcement

2. How humans 2. How humans conceptualize conceptualize surroundings?surroundings?

1)1) Object vs. fieldObject vs. field

2)2) DimensionalityDimensionality

3)3) Space, time and attributeSpace, time and attribute

4)4) ScaleScale

Do-It-YourselfDo-It-Yourself Exploring different ways to represent spatial Exploring different ways to represent spatial

conceptsconcepts Group A: how land parcel is represented in a GISGroup A: how land parcel is represented in a GIS Group B: how elevation is represented in a GISGroup B: how elevation is represented in a GIS Group C: how tornado is represented in a GISGroup C: how tornado is represented in a GIS Group D: how traffic volume is represented in a GISGroup D: how traffic volume is represented in a GIS

Each group should discuss the representation Each group should discuss the representation of each theme in terms of (1) object vs. field of each theme in terms of (1) object vs. field (2) dimensionality (3) how space, time and (2) dimensionality (3) how space, time and attribute are organized (4) appropriate scale attribute are organized (4) appropriate scale of space and time of space and time

Land parcel & ElevationLand parcel & Elevation We can observe discrete boundary of We can observe discrete boundary of

land parcel but we can’t observe that of land parcel but we can’t observe that of elevationelevation

Land parcel forms polygon in terms of Land parcel forms polygon in terms of geometry, but elevation does not geometry, but elevation does not conform to well-defined geometryconform to well-defined geometry

Identity change over time is gradual in Identity change over time is gradual in the corresponding time scale the corresponding time scale temporal element is largely ignored, the temporal element is largely ignored, the relation between space and attribute relation between space and attribute determines representationdetermines representation

Traffic volume & Traffic volume & TornadoTornado

Movement of things with some Movement of things with some pronounced properties across geographic pronounced properties across geographic pathpath

Can have all dimensionalities by leaving Can have all dimensionalities by leaving out less important detailsout less important details

Temporal element is importantTemporal element is important Finer temporal scale is required to Finer temporal scale is required to

describe the subject properly describe the subject properly attribute attribute is largely ignored, the relation between is largely ignored, the relation between space and time determines space and time determines representationrepresentation

(1) Object vs. Field(1) Object vs. Field Discrete objectDiscrete object

Identifiable boundaries Identifiable boundaries easier for manipulation easier for manipulation like a tabletop object conceived easily by direct like a tabletop object conceived easily by direct human experiencehuman experience

Building, population, county boundaryBuilding, population, county boundary Continuous fieldContinuous field

Variation in a given spatial scale Variation in a given spatial scale requires spatial requires spatial sampling scheme (tessellation)sampling scheme (tessellation)

Temperature, population density, tax rate per Temperature, population density, tax rate per countycounty

Any limitations?Any limitations? Soil boundary, mental map of localitiesSoil boundary, mental map of localities Planar enforcementPlanar enforcement (i.e. polygon doesn’t overlap) (i.e. polygon doesn’t overlap)

(2) Dimensionality(2) Dimensionality By identifying dimensionality of object, you can By identifying dimensionality of object, you can

place them in Euclidean geometryplace them in Euclidean geometry Zero-dimension (point)Zero-dimension (point) One-dimension (line)One-dimension (line) Two-dimension (area)Two-dimension (area) Three-dimension (volume)Three-dimension (volume)

Any limitations? Any limitations? Even though most of data format (e.g. shapefile) Even though most of data format (e.g. shapefile)

available in GIS is only allowed to have one of possible available in GIS is only allowed to have one of possible dimensions, reality is that some object (e.g. lake) can dimensions, reality is that some object (e.g. lake) can have multiple dimensions depending on scale and have multiple dimensions depending on scale and applicationsapplications

Multiple representationMultiple representation (i.e. representing geographic (i.e. representing geographic entities across multiple scales)entities across multiple scales)

(3) Space, Time, and (3) Space, Time, and AttributeAttribute

Geographic information has three Geographic information has three components: space, time and attribute components: space, time and attribute

Data model can be understood Data model can be understood through measurement frameworkthrough measurement framework

Measurement framework: all of three Measurement framework: all of three components are not fully measured, components are not fully measured, but rather one of them is measured, but rather one of them is measured, controlled, and fixed respectively controlled, and fixed respectively (Sinton 1978)(Sinton 1978)

(4) Scale(4) Scale Scale determines the way in which phenomenon is Scale determines the way in which phenomenon is

describeddescribed Astronomy, geography, human biologyAstronomy, geography, human biology Universe time, geological time, geographic timeUniverse time, geological time, geographic time

Scale influences the way humans cognize surroundingsScale influences the way humans cognize surroundings Our experience of surroundings is quite different depending on Our experience of surroundings is quite different depending on

scale: e.g. tabletop object (direct experience) versus citiesscale: e.g. tabletop object (direct experience) versus cities Scale Scale human conceptualization human conceptualization data model data model GIS GIS

implementationimplementation Representing reality in some geometry (e.g. city as Representing reality in some geometry (e.g. city as

point) is a reasonable approximation only at a point) is a reasonable approximation only at a particular scaleparticular scale

Scale determines accuracy and thus fitness of use of Scale determines accuracy and thus fitness of use of data to particular purposesdata to particular purposes

Is GIS a container of digital Is GIS a container of digital maps?maps?

Partially yes (mainly because we are used to it)Partially yes (mainly because we are used to it) DRG, DLG, DOQQ, Satellite imageDRG, DLG, DOQQ, Satellite image

But pitfall of limiting your thought on geographic But pitfall of limiting your thought on geographic representation to this doesn’t help us explore representation to this doesn’t help us explore other potential geographic representationsother potential geographic representations The way of describing geographic information is not The way of describing geographic information is not

necessarily limited to point, line, polygon, and surface, necessarily limited to point, line, polygon, and surface, but rather but rather relationalrelational or propositional (e.g. I live in or propositional (e.g. I live in Seattle, you should turn left at the intersection) Seattle, you should turn left at the intersection) first- first-order logic (deductive database)order logic (deductive database)

City does not have geometry and attributes, but also has City does not have geometry and attributes, but also has different different functionsfunctions (as adminstrative unit, as economic (as adminstrative unit, as economic unit, as ecological unit) unit, as ecological unit) object-oriented database object-oriented database

Geography is not the same as geometry indeed!Geography is not the same as geometry indeed!

3. How is human 3. How is human conceptualization coded in conceptualization coded in

computer databases?computer databases? Putting discrete objects into a computer Putting discrete objects into a computer

(vector GIS)(vector GIS)1)1) Spatial primitivesSpatial primitives

2)2) TopologyTopology

3)3) Generalization algorithmGeneralization algorithm Putting continuous fields into a computer Putting continuous fields into a computer

(raster GIS)(raster GIS)4)4) Gridded rasterGridded raster

5)5) TINTIN Georelational model as a special case of Georelational model as a special case of

vector topological model (ArcInfo coverage)vector topological model (ArcInfo coverage)

(1) Spatial primitives(1) Spatial primitives

Three spatial primitives are used to Three spatial primitives are used to represent discrete objectsrepresent discrete objects Point: as a point (x, y)Point: as a point (x, y) Line: as a set of verticesLine: as a set of vertices Polygon: as a set of vertices that form Polygon: as a set of vertices that form

to closeto close

(2) Topology(2) Topology What is topology?What is topology?

Non-metric properties of geographic objects that Non-metric properties of geographic objects that remain constant when the geographic space of remain constant when the geographic space of objects is distorted (e.g. projection change, objects is distorted (e.g. projection change, transformation) transformation)

Why is topology important? Why is topology important? From data perspectives, it validates geometric From data perspectives, it validates geometric

accuracy (e.g. network connectivity, line accuracy (e.g. network connectivity, line intersection, overlap, duplicate lines) intersection, overlap, duplicate lines) link to link to logical consistency in data qualitylogical consistency in data quality

From analytics perspectives, it optimizes queries by From analytics perspectives, it optimizes queries by storing spatial relation information in a tablestoring spatial relation information in a table

How are they stored in a table? (see next slide)How are they stored in a table? (see next slide)

ConnectivityConnectivity Arc 2 connects Arc 2 connects

from 11 to 12from 11 to 12

ContainmentContainment Polygon C is Polygon C is

surrounded by surrounded by arc 2,4, 9, and 6arc 2,4, 9, and 6

ContiguityContiguity Polygon B/C in Polygon B/C in

the left/right of the left/right of arc 6arc 6

(3) Generalization(3) Generalization

Douglas-Poiker algorithmDouglas-Poiker algorithm Process for simplifying line by reducing the Process for simplifying line by reducing the

number of vertices in its representationnumber of vertices in its representation E.g. Cape Cod in 1:1,000,000 map E.g. Cape Cod in 1:1,000,000 map Cape Cod in Cape Cod in

1:25K1:25K How it works?How it works?

For illustration, see Figure 3.17 at Longley et al (p. 82)For illustration, see Figure 3.17 at Longley et al (p. 82) What does it achieve?What does it achieve?

It attempts to preserve pronounced changes in angle It attempts to preserve pronounced changes in angle within a given tolerancewithin a given tolerance

It reduces the size of data, and speeds up the process It reduces the size of data, and speeds up the process for display and further analysisfor display and further analysis

(4) Raster grid(4) Raster grid Variation in values are stored in each cellVariation in values are stored in each cell

Many geographic data (e.g. satellite image, Many geographic data (e.g. satellite image, air photo) are derived from this data air photo) are derived from this data structurestructure

It takes up large space; it leads to It takes up large space; it leads to development of many different raster development of many different raster compression methodscompression methods

(5) TIN(5) TIN

What is TIN?What is TIN? Triangulated Irregular NetworkTriangulated Irregular Network Represents a surface as contiguous non-Represents a surface as contiguous non-

overlapping triangular plane where three overlapping triangular plane where three points have different z-values (See Figure 8.12 points have different z-values (See Figure 8.12 in Longley et al 2005 (p. 189)in Longley et al 2005 (p. 189)

Why is TIN popular? Why is TIN popular? Allows for varying density in sampling pointsAllows for varying density in sampling points Less storage because it stores only critical Less storage because it stores only critical

points (cf. raster grid) points (cf. raster grid)

Georelational modelGeorelational model

Geometry of spatial object is stored in Geometry of spatial object is stored in a file and attribute is stored in a tablea file and attribute is stored in a table

File is linked to attribute through a File is linked to attribute through a common identifiercommon identifier

Separation between spatial and non-Separation between spatial and non-spatial attribute spatial attribute

See Figure 8.10 (e.g. Arc/Info See Figure 8.10 (e.g. Arc/Info coverage)coverage)

4. Reading Spatial Data 4. Reading Spatial Data Organization in CSDGMOrganization in CSDGM

Do-It-YourselfDo-It-Yourself

What is Indirect Spatial Reference? Take any What is Indirect Spatial Reference? Take any example.example.

What is Direct Spatial Reference Method?What is Direct Spatial Reference Method? What is STDS point and vector type?What is STDS point and vector type? What is “Entity Point”?What is “Entity Point”? What is the difference between point and node?What is the difference between point and node? ResourcesResources

http://fgdc.gov/metadata/csdgm/03.html http://mcmcweb.er.usgs.gov/sdts/SDTS_standard_nov

97/part1b10.html

Metadata from Geospatial One-Stop

Air Quality – Lake Monitoring Sites