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Projections

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Introduction to the use of map projections in GIS

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Projections

  1. 1. Projections Dr. Hans van der Kwast OpenCourseWare ocw.unesco-ihe.org
  2. 2. Learning objectives • After this course you will be able to • Understand why we use projections • Know the advantages and disadvantages of projections • Choose the right projection for your purpose • Understand the difference between on-the-fly projection and the projection of layers
  3. 3. Why projections? • Map projections portray the surface of the earth or portion of the earth (3D) on a flat peace of paper or on a screen (2D) • A Coordinate Reference System (CRS) defines, with the help of coordinates, how the 2D projected map in a GIS is related to real places on the earth 3 By Globcal International (http://globcal.net/globcal.png) By Crates (Own work)
  4. 4. Latitude and longitude • Latitude and longitude in degrees • Geographic Coordinate Reference Systems • WGS-84 • Location of Kampala: 0°18’49” North 32°34’52” East 4
  5. 5. Converting Lat/Lon to decimal degrees • 32°34’52” (32 degrees, 34 minutes, 52 seconds) • 32 + 34’/60 + 52”/3600 = 32.5811 degrees • How much is 0°18’49” North in decimal degrees? 5
  6. 6. Map projections • Problem: from a 3D world to a 2D map • Have you ever peeled an orange? • Properties of geographic objects that are distorted: • Area • Scale • Shape • Direction 6 Source: Carol
  7. 7. Three families of map projections a) Cylindrical projections b) Conical projections c) Planar projections • All projections have advantages and disadvantages • Distortions of angular conformity, distance and area 7
  8. 8. Projections that compromise distortions • Robinson projection: • Compromises distortions of area, angular conformity and distance • Winkel Tripel projection 8 Robinson projection
  9. 9. Projections with angular conformity • Conformal or orthomorphic projections: • Mercator projection • Lamber Conformal Conic • Results in distortion of areas • Larger the area the larger the distortion • Used by USGS topographical maps • Used for: navigation, meteorology 9 Mercator projection
  10. 10. Projections with equal distance • Equidistant projection • Constant scale • Maintains accurate distances from the centre of the projection or along given lines • Examples: Plate Carree Equidistant Cylindrical, Equirectangular, Azimuthal Equidistant projection • Use: radio and seismic mapping, navigation 10 Plate Carree Equidistant Cylindrical projection
  11. 11. Projections with equal distance 11 United Nations logo uses the Azimuthal Equidistant projection
  12. 12. Projections with equal areas • Equal area projection • Preserves proportions of areas • Results in distortions in angular conformity • Examples: Alber’s equal area, Lambert’s equal area, Mollweide Equal Area Cylindrical projection • Use: general reference, education 12 Mollweide Equal Area Cylindrical projection
  13. 13. Universal Transverse Mercator (UTM) • UTM is a global map projection • Divided in 60 equal zones, 6 degrees wide in longitude from East to West • UTM zones numbered 1-60 starting at the international date line • Origin on the equator at a specific longitude • N or S are used to distinguish between Northern and Southern hemisphere • E.g. Uganda: • UTM Zone 36N • Kampala: 452611 Easting, 36127 Northing 13
  14. 14. Universal Transverse Mercator (UTM) 14
  15. 15. Some terminology • Datum • Spheroid • Geoid • False Northing, False Easting 15
  16. 16. Datum, spheroid, geoid 1. Ocean 2. Reference ellipsoid 3. Local plumb line 4. Continent 5. Geoid • Datum: localised approximation of earth’s ellipsoid. Global: e.g. WGS-84 16 By MesserWoland (Own work)
  17. 17. Example False Northing, False Easting Dutch projection: Rijksdriehoekstelsel • Origin originally in Amersfoort (O.L.- Vrouwetoren) • Since 1970 moved to: False Northing 155000 m, False Easting 463000 m 17 "RDbounds" by Hans Erren
  18. 18. Which projection to use? • Depends on: • Regional extent • Type of analysis • Availability of data (national data, global data) 18
  19. 19. GIS and Projections • Decide on the projection of your model data before you start preprocessing! • You need a common reference system (per project or for your organisation): • Local coordinate system (e.g. Amersfoort/RD new) • Global coordinate system (e.g. UTM Zone 31N/WGS-84) • Geographic Coordinate Reference System (Lat/Lon, WGS-84)
  20. 20. Coordinates, more practical • Use EPSG codes to standardise projections within a project! • Supported by most open source GIS desktop and server applications, incl. QGIS, GDAL • EPSG codes (European Petroleum Survey Group), examples: • Amersfoort RD/New: 28992 • UTM Zone 31 North, datum WGS-84: 32631 • Google Earth (Lat/Lon WGS-84): 4326 • Online reference: • http://spatialreference.org/
  21. 21. On-the-fly reprojection (OTF) • All layers visualised in a GIS application need to be in the same projection • Instead of reprojecting all layers to the same projection, GIS applications use On-the-Fly reprojection. 21 Beware! OTF reprojection does not change the projection of layers!
  22. 22. 3 Cases with projections 1. Projection is known AND projection is assigned  No action needed 2. Projection is known BUT NOT assigned  Assign projection to layer 3. Projection is unknown  Georeference layer (register/rectify) 22
  23. 23. Acknowledgements • Examples were taken from: A Gentle Introduction to GIS http://docs.qgis.org/2.6/en/docs/gentle_gis_introduction/ 23

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