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UNC-Chapel Hill GEOG 070 - Representing the Real World

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Representing the Real WorldRepresenting the Real WorldRepresenting the Real WorldRepresenting the Real WorldDigital RepresentationsDigital RepresentationsBinary NotationBinary NotationElectronic Representation of BitsBits and BytesASCII EncodingRepresenting the Real WorldRepresenting the Real WorldTwo Fundamental Sorts of RepresentationsThe Discrete Object ViewEntities  Data ObjectsCollection of Data Objects  Data ModelThe Discrete Object ViewData Objects and their AttributesData ModelsTopologyConceptualizing TopologyTopology - AdjacencyTopology - ConnectivityTopology - ContainmentTopology - DirectionTopology - ProximityWhy We Need TopologyThe Field ViewTopography Lends Itself to the Field ViewOther Field-like PhenomenaDavid Tenenbaum – GEOG 070 – UNC-CH Spring 2005•The theory of representing the real world in a GIS using digital data•The nature of digital data and binary notation•The discrete object view of the world•Entities, data objects, and data models•Topology and spatial relationships•The field view of the worldRepresenting the Real WorldDavid Tenenbaum – GEOG 070 – UNC-CH Spring 2005To map is to transform information from one form to another --- MathematicsmapRepresenting the Real WorldEarth surface Paper--- GeographyDavid Tenenbaum – GEOG 070 – UNC-CH Spring 2005•Maps are a means by which to convey a lot of spatial information in a succinct package, because they allow us to gain knowledge of places we may never have seen ourselves•Because we get to observe a very small part of the world with our own eyes, we rely on a variety of methods of representation to convey information:•Photographs, spoken descriptions and written text (along with maps) are all familiar kinds of representation we use in everyday life to help us assemble more knowledge about the EarthRepresenting the Real WorldDavid Tenenbaum – GEOG 070 – UNC-CH Spring 2005representationRepresenting the Real WorldEarth surface •In a GIS, the representation of real world phenomena makes use of digital data formatsDigital dataDavid Tenenbaum – GEOG 070 – UNC-CH Spring 2005Digital Representations•You probably have a fairly broad experience of digital representations through the use of computers and the Internet for other (non-geographic) purposes•It is easy to be completely unaware of how digital representations function because technology has advanced sufficiently that their functioning is quite transparent to the consumer of information•e.g. when you browse a web page, you see the formatted hypertext, not the html source code, nor the fundamental binary storage of the dataDavid Tenenbaum – GEOG 070 – UNC-CH Spring 2005Digital Representations•The term digital derives from digits (which in turn is a reference to our fingers) and our system of counting based on the 10 digits of our hands•While our usual counting system makes use of the 10 symbols in the decimal counting system (0 – 9), digital computers use of system of counting that only uses 2 symbols that we represent with 0 and 1.•Computers represent phenomena as binary digits. Every item of useful information about the Earth’s surface is ultimately reduced by a GIS to some combination of 0s and 1s (p. 61 of text)David Tenenbaum – GEOG 070 – UNC-CH Spring 2005Binary Decimal1 digit 0, 1 1 bit 0,1,2,…92 digits 00, 01 2 bits 00, 01,…10, 11 97, 993 digits 000, 001 3 bits 000, 001,010, 011 002, 003,100, 101 …110, 111 998, 999•Everything is represented as 0s and 1s in a computer. These two-state forms correspond to yes/no, on/off, open/closedOne to one correspondenceDecimal Binary0 01 12 103 114 1005 1016 ?Binary NotationDavid Tenenbaum – GEOG 070 – UNC-CH Spring 2005Binary NotationDecimal:72,479 = 70,000 = 7×1042,000 = 2 ×103400 = 4×10270 = 7×1019 = 9×1001001011021031042021222324Binary:1 0 1 0 01× 24+0 × 23+1 × 22+0 × 21 +0 × 20= 16 + 0 + 4 + 0 + 0 = 20Note: In binary1010+ 11010000David Tenenbaum – GEOG 070 – UNC-CH Spring 20050 1 1 0 0 0 1•Binary information can be represented by electrical pulses passing along wires with two different voltage levels as shown in the following sequence of pulsesElectronic Representation of BitsDavid Tenenbaum – GEOG 070 – UNC-CH Spring 2005101110108 bits = 1 byte1024 bytes = 1 Kb1024 Kb = 1 Mb1024 Mb = 1 Gb1024 Gb = 1 Tb1024 Tb = 1 Pb1 bit = 1 binary digit1 byte = 8 bitsBits and BytesDavid Tenenbaum – GEOG 070 – UNC-CH Spring 2005•If computers store everything using 0s and 1s, then how are characters represented?•The ASCII (American Standard Code for Information Interchange) code assigns the numbers 0 through 127 to 128 characters, including upper and lower case alphabets plus various special characters, such as white space etc.•e.g. decimal 85 is assigned to represent upper case U. In binary, 01010101 = 85. Thus the computer represents U using 01010101.•Files which contain information encoded in ASCII are easily transferred and processed by different computers and programs. These are called “ASCII” or “text” files.ASCII EncodingDavid Tenenbaum – GEOG 070 – UNC-CH Spring 2005representationRepresenting the Real WorldEarth surface Digital data•You now have some familiarity with how a digital representation works fundamentally. But now we encounter some key issues:•What do we represent?•What model should we use?David Tenenbaum – GEOG 070 – UNC-CH Spring 2005Representing the Real World•We can think of geographic data as consisting of individual facts about the geographic world that link a place, a time, and some descriptive property, which we usually refer to as an attribute.•A full and complete representation of the Earth would consists of an infinite number of geographic facts becauseThe world is infinitely complex, but computer systems are finite. Representation is all about the choices that are made in capturing knowledge about the world (p. 65 of the text)David Tenenbaum – GEOG 070 – UNC-CH Spring 2005Two


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