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Home , Georeferencing

Georeferencing & projections

2009/2010 CGIGIRS ©

Overview •Georeference Geoinformation process

• systems • ellipsoid / geoid • datums / reference surfaces • sea level •Map projections

• properties • projection types • UTM • coordinate systems Georeference systems Geo Reference 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 ) Georeferencing (in brief)

 Georeferencing:  Geometrically describing on the earth surface by means of earthfixed coordinates Geographic coordinate systems

 on the earth in and (e.g. 51°58' N 5°40' E )

 Latitude  parallels  North South  Longitude  meridians  EastWest

 Its not based on a Cartesian plane but on location on the earth surface (spherical coordinate system) Latitude Longitude

Geographic coordinates

 Angular measures  Degreesminutessecond 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 61866 378 206.4 294.978 m 698 2 Bessel 61841 377 397.155 299.152 m 812 85 Everest 1830 (India) 6 377 276.3458 300.801 m 7 GRS80 (New Intern’l) 6 378 137 298.257 m 222 100 882 7 WGS84 6 378 137 298.257 m 223 563

Various ellipsoids; selection adopted from M. Hooijberg, Practical Geodesy, 1997, p3537 Question

 Is it possible to have different coordinates for the same location? Examples (Bellingham, Washington)

 NAD 1927  Lat 122.466903686523  Lon 48.7440490722656  NAD 1983  Lat 122.46818353793  Lon 48.7438798543649  WGS 1984  Lat 122.46818353793  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 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 (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 0meridian (longitude)  angle North/South from Equator (latitude)

 Cartesian coordinates  distance from Yaxis (Xcoordinate)  distance from Xaxis (Ycoordinate) Dutch example Meta data of Dutch Topographic data

 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, 2006 Chapter 2: Coordinate systems

Practical: Exercise Module 3: ‘Map projections’

© Wageningen UR Georeferencing is about … (1) Measurements in the real world (material) to acquire:

 Positions via  angles (triangulation)  lengths (distances)  time (GPS)

 Elevations via  vertical distances (between gravity level surfaces) ‘Good’ old days Combination of reference systems ‘Good’ new days