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Geo Referencing & Map Projections

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Geo Referencing & Map Projections Geoĉreferencing & Map projections 2009/2010 CGIĉGIRS © Overview •Georeference GeoĈinformation process • systems • ellipsoid / geoid • datums / reference surfaces • sea level •Map projections • properties • projection types • UTM • coordinate systems Geoĉreference systems Geo ĉ Reference ĉ Systems earth something to refer to coordinates physical reality< relation > geometrical abstractions Garden maintenance objects need a reference Y X History Local (for at least 21 centuries ) National (since mid 19 th century (NL) ) Continental (since mid 20 th century ) Global (since 1970 / GPS, 1989 ) Geoĉreferencing (in brief) Georeferencing: Geometrically describing locations on the earth surface by means of earthĉfixed coordinates Geographic coordinate systems Location on the earth in Longitude and Latitude (e.g. 51°58' N 5°40' E ) Latitude parallels North South Longitude meridians EastĉWest Its not based on a Cartesian plane but on location on the earth surface (spherical coordinate system) Latitude Longitude Geographic coordinates Angular measures Degreesĉminutesĉsecond o Lat 51 ’ 59’ 14.5134” o Lon 5 ’ 39’ 54.9936” Decimal Degrees (DD) Lat 51.98736451427008 Lon 5.665276050567627 Model of the earth Spheroid and datum Spheroid (ellipsoid) approximates the shape of the earth Geodetic datums define the size and shape of the earth and the origin and orientation of the coordinate systems used to map the earth. Datum WGS 1984 (world application) Horizontal and vertical models One location : Horizontal datum: (ellipsoid ) for position ‘egg’ mathematical model Vertical datum: (geoid ) for elevation physical model ‘potato’ Rotating potato Mean gravity level at mean sea level Tom & Jerry Geoid undulation (global) http://www.csr.utexas.edu/grace/gravity/gravity_definition.html –120 m 0 m 80 m Two different abstract models One location , but yet: Two different positions Two different ‘heights’: orthometric (related to geoid) = H geodetic (related to ellipsoid) = h = H+N geoid undulation = N (‘potato minus egg’) Map ‘Jumping’ Difference in ‘Mean Sea Levels’ 2 Netherlands — Belgium Average tide Average low tide IJmuiden (North Oostende (Dover Sea) Channel) A visible elevation jump of +2.34 m from Netherlands to Belgium ???? Difference in ‘Mean Sea Levels’ Differences between Height Reference Levels within Europe Many different ellipsoids (a small selection) Datum: mathematical model of the Earth to serve as reference Ellipsoid Major axis. Unit of Flattening name a measure 1/f Clarke 1866 6 378 206.4 m 294.978 698 2 Bessel1841 6 377 397.155 m 299.152 812 85 Everest 1830 (India) 6 377 276.3458 m 300.801 7 GRS80 (New Intern’l) 6 378 137 m 298.257 222 100 882 7 WGS84 6 378 137 m 298.257 223 563 Various ellipsoids; selection adopted from M. Hooijberg, Practical Geodesy, 1997, p35ĉ37 Question Is it possible to have different coordinates for the same location? Examples (Bellingham, Washington) NAD 1927 Lat ĉ122.466903686523 Lon 48.7440490722656 NAD 1983 Lat ĉ122.46818353793 Lon 48.7438798543649 WGS 1984 Lat ĉ122.46818353793 Lon 48.7438798534299 Projections Attemp to portray (a portion of) the earth on a flat surface From spherical coordinate system to a planar (Cartesian) coordinate system. Always lead to distortions Map projection 16 th century Waldseemuller Type of projection (projection surface) Projection plane Planar Cylindrical Conical Map projections Mathematical projections (abstract) from an ellipsoid to a map plane Numerous projections Projection plane always flat Cartesian coordinates Countries uses own projections Always purposely designed Type of map projections Grouping by preserved properties: conformal : preserves local angles and shapes – global equivalent : represents areas in correct relative size – global equidistant : maintains consistency of scale for certain distances ĉ local azimuthal : retains certain accurate directions – local … but never conformal and equivalent Properties Tissot indicatrices: to show the distortion of parts of a map Cylindrical projections Conformal Equidistant Equivalent Cylindrical projections Equal area conformal at Equator conformal at higher latitudes (N & S) What is the projection type? What is the projection type? What is the projection type? Conical projections ... … defined for USA Conformal ( Lambert ) Equal area ( Albers ) Equidistant ... … a confusing concept , because: means “equal in distance ” distance on earth surface equal to distance in map projection plane (scale 1:1) but only applied to specific directions “all” directions to a single point , or “all” perpendiculars to a single line An equidistant projection has NO uniform scale Great Circle (azimuthal) Great Circle (equidistance) Dutch map grid Datum point: Amersfoort Bessel 1841 ellipsoid Projection: Planar Conformal Azimuthal False origin: X = – 155.000 m Y = – 463.000 m UTM 1 Universal Transverse Mercator 60 zones 6 degrees UTM zones UTM 2 M: Mercator projection T: transverse (cylinder axis in Equator plane) U: universal (60 projection zones of 6 degree latitude) 1 Central line per zone 2 standard lines per zone (180 km to the west and the east of central line) False Easting and False Northing UTM ... … a source of much confusion as UTM stands for different things: 1. UTM projection can be defined with different datums (ellipsoids) 2. UTM grid can be defined on other projections than UTM With UTM coordinates always check ellipsoid and projection Dutch topographic map (1996) Civil Bessel ellipsoid RD map grid Military WGS 84 ellipsoid (formerly Hayford) UTM map grid UTM background http://www.dmap.co.uk/utmworld.htm UTM Grid Zones of the World http://www.maptools.com/UsingUTM/ Using UTM Coordinate system Coordinates Coordinates in a map projection plane : Geographic coordinates angle East/West from 0ĉmeridian (longitude) angle North/South from Equator (latitude) Cartesian coordinates distance from Yĉaxis (Xĉcoordinate) distance from Xĉaxis (Yĉcoordinate) Dutch example Meta data of Dutch Topographic data maps PROJCS["Rijksdriehoekstelsel_New", GEOGCS["GCS_Amersfoort", DATUM["D_Amersfoort", SPHEROID["Bessel_1841",6377397.155,299.1528128]], PRIMEM["Greenwich",0.0], UNIT["Degree",0.0174532925199433]], PROJECTION["Double_Stereographic"], PARAMETER["False_Easting",155000.0], PARAMETER["False_Northing",463000.0], PARAMETER["Central_Meridian",5.38763888888889], PARAMETER["Scale_Factor",0.9999079], PARAMETER["Latitude_Of_Origin",52.15616055555555], UNIT["Meter",1.0]] longitude of center of projection 5 23 15,5006 DMS latitude of center of projection 52 09 22,1841 DMS radius of sphere of reference 6370997 datum WGS 1984 Summary Georeferencing Geometry Plane projection (flat earth model) vs. Spherical projection (round earth model) Coordinate systems Geographic coordinates (latitude and longitude) Geocentric coordinates (X, Y, Z – mass centre of the earth) Cartesian coordinates Datums Horizontal and Vertical references Ellipsoid / Geoid / Mean Sea Level Vertical elevation / Geoid undulation Role of Gravity Map projections Properties: shape, area, distance, angle UTM, RD, false origin Study materials: Theory Chang, 2008 Chapter 2: Coordinate systems Practical: Exercise Module 3: ‘Map projections’ © Wageningen UR.
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