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UNC-Chapel Hill GEOG 070 - GIS definition

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Slide 1Characteristics of GIS?Geographic DataWhat is Geodesy?The Earth is:Types of projectionsSlide 7Slide 8Slide 9Slide 10Universal Transverse MercatorUTMUTM vs SPCSTerminologyReview2 GIS Data ModelsRaster data model (details later)Vector Data ObjectsThought question:Slide 20Slide 21Raster Data Model - LinesRaster Data Model - AreasCell Size & ResolutionRules for Assigning Cell ValuesRaster Data Storage – Run Length EncodingVector Data Model - AdvantagesVector Data Model - DisadvantagesRaster Data Model - AdvantagesRaster Data Model - DisadvantagesIntended useVector setting: attribute data:Raster attributes: cell valuesAttribute dataSpectral Resolution of Landsat TMRemote SensingSlide 37Satellite ImagerySlide 39Multispectral Imagery DisplayColor composite imageDeforestationSlide 43Flood AssessmentDamage AssessmentsContrast Enhancement example:ScatteringSlide 48Atmospheric WindowsGeometric CorrectionSlide 51Uses of MapsThematic vs. ReferenceMap ElementsShapeHierarchical OrganizationChloroplethU.S. Census DataU.S. Census Data & TIGER/Line FilesTIGER line and address dataGIS is an information system that allows for capture, storage, retrieval, analysis and display of spatial data.Note that GIS is closely tied to the infrastructure that exists for creating spatial data – satellites, census protocols, weather stations, land survey techniques, and more.GIS definitionCharacteristics of GIS?GISGISComputer SoftwareDataPeopleComputerHardwareGeographic Data•Features must be referenced to some real world location•Known as georeferencingWhat is Geodesy?More simply, geodesy is the study of the Earth’s size and shape.The Earth is:A Spheroid An EllipsoidGeoid -- The surface on which gravity is the same as its strength at mean sea levelTypes of projections (a) Azimuthal (b) Cylindrical (c) Conic. Light source positions, also called perspective positions, play an especially important role in planar projections. (Different aspects, such as polar, equatorial, or oblique, will also affect the map projection)Tangent vs. Secant ProjectionsStandard lineStandard lineStandard linestandard point/lines: on a projected map, the location(s) free of all distortion at the exact point or lines where the surface (cylinder, cone, plane) touches the globe.latitudepositive in n. hemispherenegative in s. hemispherelongitudepositive east of Prime Meridiannegative west of Prime MeridianGeographic Coordinate System• This is a Global Coordinate System• Based on angles on the ellipsoidal EarthCartesian CoordinatesComputationally, it is much simpler to work with Cartesian coordinates than with spherical coordinatesx,y coordinatesreferred to as “eastings” & “northings”defined units, e.g. meters, feetTransverse Mercator Projection applied to each 6o zone to minimize distortionUTM Zone ProjectionUniversal Transverse Mercator•Each half has a different y-origin:•For the Northern half of the zone, the y-origin is at the Equator–Northing at equator is 0m•For the Southern half of zone the y-origin is 10,000,000 meters south of the Equator –Northing at equator is 10,000,000mUTM•The central meridian, which runs down the middle of the zone, is used to define the position of the origin•Distance units in UTM are defined to be in meters, and distance from the origin is measured as an Easting (in the x-direction) and a Northing (in the y-direction)•The x-origin is west of the zone (a false easting), and is placed such that the central meridian has an Easting of 500,000 metersUTM vs SPCS•SPCS–More accurate than UTM–used primarily for engineering applications, e.g. utility companies, local governments to do accurate surveying of facilities network (sewers, power lines)–Used for small areas–Difficult to use over larger areas (when multiple zones are necessary)•UTM–allows overlap between zones for mapping purposes–The UTM system is globalTerminology•Data Object -- digital representation of an entityPastureRoadHouseReview•The relationships between data objects in space.Topology•Entities in the real world are represented as one of the following in a GIS:–Vector data:•Points•Lines•Areas (or polygons)–Raster data•Pixels in an array2 GIS Data ModelsKey concept!Cell (x,y)The raster data model represents the Earth’s surface as a two-dimensional array of grid cells, with each cell having an associated value:1 2 3 5 84 6 8 3 93 5 3 3 17 5 4 3 92 2 4 5 2Cell valueCell size = resolutioncolumnsrowsRaster data model (details later)Vector Data ObjectsGeographic building blocks•Points–0 dimensional •Lines–1 dimensional•Polygons–2 dimensionalThought question: How are you going to represent the California OAK tree in digital format?A point? A polygon? Or a pixel?It will depend on:-Scale of observation-Purpose of your research-The type of data you have access to in the GISTopology: Connectivity10 11 12131415⑤①②③④Arc From-Node To-Node1 10 112 11 123 11 134 13 155 13 14Arc-node listConnected arcs are determined by searching through the list for common node numbers.Because of the common node 11, arcs 1, 2, and 3 all intersect. The computer can determine that it is possible to travel along arc 1 and turn onto arc 3. But it is not possible to turn directly from arc 1 to arc 5.Topology: ContiguityTwo geographic features which share a boundary are called adjacent. Contiguity is the topological concept which allows the vector data model to determine adjacency.An ArcFrom-NodeTo-NodeDirectionleftrightBCDE123456789Arc Left Right Polygon Polygon5 C B9 E C10 ? ? 1 ? ?A line = a series of connected cells that portray length Problems with this representation? • Line may be narrower than pixels show.• e.g. Road layer: how much of a road needs to be in a cell in order to be considered a ‘road’ cell?Raster Data Model - LinesArea = a group of connected cells that portray a shapeWhat problems could we have with this representation?• What if a lake’s edge falls in a pixel? Raster Data Model - AreasCell Size & Resolution•The size of the cells in the raster data model determines the resolution at which features can be represented•The resolution can have an effect on which features are represented in what locations:10 m Resolution1 m Resolution5 m ResolutionRules for Assigning Cell


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