georeferencing & map projections...summary georeferencing geometry plane projection (flat earth...
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Overview
•Map projections
• properties
• projection types
• UTM
• coordinate systems
Geo�information process•Georeference
• systems
• ellipsoid / geoid
• datums / reference surfaces
• sea level
Geo�reference systems
Geo � Reference � Systems
earth something to refer to coordinates
physical reality geometrical abstractions< relation >
History
� Local (for at least 21 centuries)
� National (since mid 19th century (NL))
� Continental (since mid 20th century)
� Global (since 1970 / GPS, 1989)
Geo�referencing (in brief)
� Georeferencing:
� Geometrically describing locations on the earth surface by means of earth�fixed 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 � East�West
� Its not based on a Cartesian plane but on location on the earth surface (spherical coordinate system)
Geographic coordinates
� Angular measures
� Degrees�minutes�second
� Lat 51o’ 59’ 14.5134”
� Lon 5o’ 39’ 54.9936”
� Decimal Degrees (DD)
� Lat 51.98736451427008
� Lon 5.665276050567627
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
� Horizontal datum: (ellipsoid) for position
� mathematical model
� Vertical datum: (geoid) for elevation
� physical model
One location:
‘egg’
‘potato’
Geoid undulation (global)
–120 m 0 m 80 m
http://www.csr.utexas.edu/grace/gravity/gravity_definition.html
Two different abstract models
� Two different positions
� Two different ‘heights’:� orthometric (related to
geoid) = H
� geodetic (related to ellipsoid) = h = H+N
� geoid undulation = N (‘potato minus egg’)
One location, but yet:
Difference in ‘Mean Sea Levels’ 2
� Average tide IJmuiden (North Sea)
� Average low tide Oostende (Dover Channel)
Netherlands — Belgium
A visible elevation jump of
+2.34 m
from Netherlands to Belgium ????
Many different ellipsoids (a small selection)
Ellipsoid Major axis. Unit of Flattening
name a measure 1/f
Clarke 1866 6 378 206.4 m 294.978 698 2
Bessel 1841 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, p35�37
Datum: mathematical model of the Earth to serve as reference
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 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
Equidistant ...
� 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
… a confusing concept, because:
An equidistant projection has NO uniform scale
Dutch map grid
� Datum point: Amersfoort
� Bessel 1841 ellipsoid
� Projection: Planar
� Conformal
� Azimuthal
� False origin:
� X = – 155.000 m
� Y = – 463.000 m
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 ...
� 1. UTM projection
� can be defined with different datums (ellipsoids)
� 2. UTM grid
� can be defined on other projections than UTM
… a source of much confusion
as UTM stands for different things:
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
� Geographic coordinates
� angle East/West from 0�meridian (longitude)
� angle North/South from Equator (latitude)
� Cartesian coordinates
� distance from Y�axis (X�coordinate)
� distance from X�axis (Y�coordinate)
Coordinates in a map projection plane:
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:
© Wageningen UR
Theory Chang, 2006
Chapter 2: Coordinate systems
Practical: Exercise Module 3: ‘Map projections’
Georeferencing is about … (1)
� Positions via� angles (triangulation)
� lengths (distances)
� time (GPS)
� Elevations via� vertical distances
(between gravity level surfaces)
Measurements in the real world (material)
to acquire:
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