Transcript Representations / Models Longley et al., chs. 1
Representations / Models
Longley et al., chs. 1-3 Zeiler, chs. 2-3
Why Representations or Models?
• •
How do we know what we know?
Human sight
–
Visible spectrum, horizon at ~10km visibility 100 km
•
Human sound
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50Hz to 15KHz up to 100 m
•
Taste, Touch, Smell
Knowing the World
•
Everything else via communication
–
Speech
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Text
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Photographs
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Radio, TV
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Maps
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Internet
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Databases
Surface of the Earth?
•
500,000,000 sq km
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on average 100 sq m is sensed directly p = 100/500,000,000,000,000 m p = 0.0000000000002 or 2 x 10 -13
spatially
•
5 billion years
–
we live through ~70 p = 70/5,000,000,000 p = 0.000000014 or 1.4 x 10 -8
temporally
\
we know almost nothing of the surface of the Earth via our senses!
Communicated Information
• • • • • •
decide where to go as tourists, shoppers choose study areas for research manage parks, reserves choose where to live address urban congestion All such information must use a representation (space and time)
Jonathan Raper’s Week in 2-D
1km Each color= 1 day Courtesy Jonathan Raper of City University London, GISci 2002 Keynote Darker= later in the day
Jonathan Raper’s Month in 3-D
X & y axes are spatial and z is seconds from midnight. Points are from GPS carried on all journeys with static time auto completed. Model produced by Earthvision (http://www.dgi.com/) Courtesy Jonathan Raper of City University London, GISci 2002 Keynote
More Representations in Space/Time
Representation in Space/Time
•
What would more detail show?
•
Infinite complexity Simplification
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must reduce to manageable volume
Where do Representations occur?
• • • • • •
the human mind, in memory and reasoning speech written text photographs digital databases GIS !
Representations are crucial...
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for communication (metaphors, expressions)
•
going beyond the space-time limits of our senses
•
dealing with an infinitely complex world
DIGITAL Representation
• • •
how to express knowledge exclusively in 0s and 1s?
how to describe complexity of world in 0s and 1s?
the fundamental question of data modeling for GIS
Communication via a channel
Communication via a channel
• • • • •
How efficient is the channel?
Is there information that can't be expressed?
Text omits gesture, pronunciation, voice inflection GIS as a communication channel?
What information about a place can't be expressed in GIS?
Communication via a channel
•
What if the sender and receiver can't understand each other?
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different language
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different alphabet
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different GIS
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interoperability
Geographic Representation
• • •
“Location, location, location!”
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to map, to link based on the same place,
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to measure distances and areas Time
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height above sea level (slow?)
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Sea surface temperature (fast) Attributes
– –
physical or environmental soci-economic (e.g., population or income)
Geographic Representation
The “atom” of geographic information < location, time, attribute > “It’s chilly today in Corvallis” < Corvallis, today, chilly > “at 44
°
N, 123
°
E at 12 noon PST the temperature was 60
°
F”
Describing LOCATION
Time ok, Attributes Not Always
• •
“chilly” is subjective and relative 60
°
F generally understood
•
did Hugh Grant climb a hill or a mountain?
Ontology
• • • •
Ontology: the study of the basic elements of description "what we tell about" semantics, “semantic interoperability” discrete objects ontologies and fields are two different www.ucgis.org
Research Challenge in Ontology
A Coastal “Geo-Ontology”
Courtesy Jonathan Raper of City University London, GISci 2002 Keynote
A Complete Representation
• • • •
DIGITAL and GEOGRAPHIC complete representation of the planet past, present, and future
“ Digital Earth ”
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a “camera” pointed at a sunlit Earth
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a virtual, immersive world
Simulated image from NASA’s SRTM, Carrizo Plain, S. Calif.
The Fading of Digital Earth, v. 1.0
Longley et al., p. 463
• • • •
No effective interoperability between datasets No common vision of cooperation Low awareness by the public, inaccessibility Many stand in the gap now but challenges remain
– – – – –
Google Earth http://topex.ucsd.edu/marine_topo/globe.html http://www.virtualocean.org
NASA World Wind - http://worldwind.arc.nasa.gov/ ArcGIS Explorer powered by ArcGIS Server
“Atoms” of Geographic Information
• • • • •
an infinite number two-word description of every sq km on the planet, 10 Gb store one number for every sq m, 1 Pb (trillion bytes) www.ccsf.caltech.edu/~roy/dataquan/ Too much for any system How to limit?
Limiting Detail
• •
aggregate, generalize, approximate ignore the water?!
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2/3 of planet!
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one temperature for all of Corvallis
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one number for an entire polygon
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sample the space
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only measure at weather stations, temp. varies slowly all geographic data miss detail
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all are uncertain to some degree
The Problem of Infinite Complexity
• • •
many ways of limiting detail a GIS user must make choices GIS developers must allow for many options
•
Most important option is how we choose to think about the world
Objects and Fields
How many students at OSU?
Clouds in sky?
Fish in the sea?
Atmospheric highs in N. hemisphere today?
• • • • •
Objects Well-defined boundaries in empty space “Desktop littered w/ objects” World littered w/ cars, houses, etc.
Counts 49 houses in a subdivision
Fields:
care to count every peak, valley, ridge, slope???
What constitutes a “mountain?”
•
1000 ft was magic number but how?
Fields
what varies continuously and how smoothly measurable at every point on a surface
• Radiation captured by satellite • Elevation • Temperature • Soil type • Soil pH • Rainfall • Land cover type • Ownership An image of part of the lower Colorado River in the southwestern USA. The lightness of the image at any point measures the amount of radiation captured by the satellite's imaging system. Image derived from a public domain SPOT image, courtesy of Alexandria Digital Library, University of California, Santa Barbara.
Fields
• • • •
each variable has one value everywhere variable is a function of location field = a way of conceiving of geography as a set of variables, each having one value at every location on the planet z f = f (x,y,z, t )
Fields and Objects
• • •
Objects are intuitive, part of everyday life
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May overlap Fields worth measuring at every point
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Often associated with scientific measurements
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surfaces, fronts, highs, lows
Both objects and fields can be represented either in raster or in vector form
One Variable as Pt (grid or sample), TIN Raster, Poly, Contours What changes? Representation or phenomenon?
Gateway to the Literature
• • • • • Goodchild, M. F., M. Yuan, Cova, T. Towards a general theory of geographic representation in GIS.
Int. J. Geog. Inf. Sci.
21(3-4): 239-260, 2007.
Comber, A., P.R. Fisher, J., and R. Wadsworth, Integrating land-cover data with different ontologies: Identifying change from inconsistency,
Int. J. Geog. Inf. Sci.
,
18
(7), 691-708, 2004.
Golledge, R., The Nature of Geographic Knowledge,
Annals of the AAG
, 92(1): 1-14, 2002.
Kavouras, M., M. Kokla, and E. Tomai, Comparing categories among geographic ontologies,
Comp. Geosci
,
31
(2), 145-154, 2005.
Kuhn, W., Semantic reference systems,
Int. J. Geog. Inf. Sci.
,
17
(5), 405-409, 2003.