Lecture02 Representing Geography

Digital vs Analog
analog
– – – – – information expressed by scaling quantities good for quantitative information a paper map is analog world is scaled to a miniature representation representative fraction is key, e.g. 1:24,000
How efficient is the channel of communication?
• • • • • Is there information that can't be expressed, e.g. in text What are the limits of a GIS as a communication channel? What information about a place can't be expressed in GIS? Is the message optimally expressed in the coding system? What if the sender and receiver can't understand each other?
• • • • • • decide where to go as tourists, shoppers run large corporations manage agriculture, forestry choose where to live Travel from A to B Understand geography!
Features are symbolized
Why do we use representations? How do we gather information?
• Limits on human senses • Sight
– Visible spectrum (400-800 nm) – LOS, Horizon, Visibility
Digital representation
Much (most?) human communication is now digital
• sent through a "pipe" or transmission channel that can transmit only 0s and 1s • stored on devices that can store only 0s and 1s • processed as 0s and 1s • text in email, word processors uses ASCII (1 & 0) • voice in telephone • music on CD • DVD, digital TV • FAX
Digital equivalents
• • • • Images: JPEG, TIFF, GIF, BMP, ... Sound: MIDI, MP3, WAV FAX: CCITT Maps, geographic information: GIS data models and structures
Digital
• • • • • • From "digit" meaning finger A character in a counting system How many symbols? 0 thru 9, A-Z, a-z, etc. All can reduce to 0 and 1 To all intents and purposes "digital"=“binary”
different language different alphabet different GIS different data model
Geographic Representation
Geographic information • information about some place on the surface of the Earth
• • Odds are trillions to one Lotto odds are 25.8 million to one
• Can extend that through migration, travel
Mike Goodchild’s Counties Visited
5 billion years
• Sound
– Audible spectrum (50-15K Hz) – Range to 100m
• Taste, touch, smell
– Rather limited spatial range – Limited sensory distinction
Everything else we know about the world we know through communication • • • • • • • text speech maps photographs radio, TV Internet databases
The Message on Voyager
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Communication of information via a channel
• Claude Shannon’s Model
• C. E. Shannon: A mathematical theory of communication. Bell System Technical Journal, vol. 27, pp. 379–423 and 623–656, July and October, 1948.
Digits, distance, and communication
• When two humans communicate at a distance, chances are the content is expressed at some point in digital form • The further the distance the more likely the communication is digital • The longer/denser the communication, the more likely it is digital
digital
– – – – information expressed by symbols requires a coding scheme of representation in symbols sender and receiver must agree on the scheme what does digital scale mean?
All such information must use a representation
• What is communicated is a representation of the real thing • Locations in time and space are reduced to a few symbols • Communication requires simplification • The real world is infinitely complex • So representations reduce information to a manageable volume
Digital Coding of Text
• • • • • ASCII code one code per character A = 65, B = 66, etc. 26 letters plus common symbols originally 128, extended to 256 8 binary digits (one byte) per character
• If we live through 70 • p=70/5,000,000,000=0.000 000 014 • So, we can know almost nothing about the surface of the Earth via our senses alone
We rely on communicated information to:
Representations occur:
• In the human mind, in memory and reasoning (e = m c2) • In speech (e.g. acromyns) • In written text (abbreviations, etc.) • In photographs • In digital databases • and in GIS
Knowledge of the surface of the Earth
• 500,000,000 sq km • On average 100 sq m is sensed directly at any point in time • Odds of being in the right place at the right time • p=100/500,000,000,000,000= 0.000 000 000 000 2
Digital coding schemes important in GIS
• ASCII
– eight bits per character, names, text annotation
• integer
– 3 bits per decimal digit, n bits give 2^n options, or 32 bits per whole number (short, long integer)
The digital translation challenge
• How can we express knowledge exclusively in 0s and 1s? • How can we describe what we know about the world in 0s and 1s? • How do we catures as representations?
• float (single precision)
–
• double precision
–
• BLOB binary large object
What if you received this message:
0100 1000 0110 0101 0110 1100 0110 1100 0110 1111 0010 0000 0111 0111 0110 1111 0111 0010 0110 1100 0110 0100
Geog 176B Lecture 2: Representing Geography [Text: Ch. 3]
http://www.mondodisotto.it/imageiraq/
A map is a representation
Geometry is orthographic and scaled
1 sign bit, 7 exponent bits (-63 to +63), 24 mantissa bits (8 significant digits) 1 sign bit, 7 exponent bits, 56 mantissa bits (18 significant digits)