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Introduction to Coordinate Systems and Projections

Introduction to Coordinate Systems and Projections. Rob Juergens, Annette Locke. Introduction. We want to give you a basic understanding of Coordinate systems Projecting data Transforming data. Coordinate Systems. Geographic (GCS) Projected (PCS). What is a coordinate system?.

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Introduction to Coordinate Systems and Projections

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  1. Introduction to Coordinate Systems and Projections Rob Juergens, Annette Locke

  2. Introduction • We want to give you a basic understanding of • Coordinate systems • Projecting data • Transforming data

  3. Coordinate Systems Geographic (GCS) Projected (PCS)

  4. What is a coordinate system? A way of describing points to answer questions about distance, location, and direction

  5. Distance • Q: How far is Redlands? • A: 32 million 32 million what? Seems like a lot. • 32 million feet from Beijing. What were you expecting? • Miles from where I am right now. • A coordinate system defines what is expected

  6. Location • Q: Where is San Diego? • A: (-116.53, 33.82), no wait, it is (-12973635.720, 4006201.638) What do these numbers mean? • (-116.53, 33.82) are longitude and latitude in decimal degrees • (-12973635.720, 4006201.638) are xy-coordinates in meters • How am I supposed to know that? • A coordinate system defines what these numbers mean

  7. Direction • Q: I am looking at a map and want to place a marker at the North Pole. Where do I put it? • A: At the top of the map. Hmm, or at the center. Or … hang on, it’s impossible. The North Pole isn’t on the map! • A coordinate system defines where a particular location is on the map or specifies that it isn’t on the map at all

  8. Geographic Coordinate System (GCS) • Global – 3D spherical surface • Point referenced by longitude and latitude values

  9. Projected Coordinate System (PCS) • Flat – 2D surface based on a GCS • Point referenced by x, y coordinates on a grid Y X < 0 Y > 0 X > 0 Y > 0 (0,0) X X < 0 Y < 0 X > 0 Y < 0

  10. Coordinate System Projected Coordinate System Geographic Coordinate System Projection Linear Unit Prime Meridian Datum Projection Parameters Spheroid Angular Unit

  11. Two ways to specify a coordinate system • Well-known ID (WKID) • Predefined coordinate systems only • Well-known text (WKT) • Predefined or custom

  12. Well-Known ID (WKID) • Every predefined coordinate system has a WKID • For example, GCS_WGS_1984, WKID = 4326 • WKID < 32767 is EPSG assigned • EPSG Geodetic Parameter Dataset, http://www.epsg-registry.org/ • WKID > 32767 is Esri assigned • Esri WKID may change • Esri  EPSG • Old WKID will still work • Example, Web Mercator 102100  3857

  13. Geographic Coordinate SystemWell-Known Text (WKT) GEOGCS[ "GCS_WGS_1984", DATUM[ "D_WGS_1984", SPHEROID[ "WGS_1984", 6378137.0, 298.257223563] ], PRIMEM[ "Greenwich", 0.0], UNIT[ "Degree", 0.0174532925199433] ]

  14. Projected Coordinate SystemWell-Known Text (WKT) PROJCS[ “World_Mercator", GEOGCS[ "GCS_WGS_1984", DATUM[ "D_WGS_1984", SPHEROID[ "WGS_1984", 6378137.0, 298.257223563] ], PRIMEM[ "Greenwich", 0.0], UNIT[ "Degree", 0.0174532925199433] ], PROJECTION[ "Mercator " ], PARAMETER[ "Central_Meridian", 0.0], PARAMETER[ "Standard_Parallel_1", 0.0], PARAMETER[ "False_Easting", 0.0], PARAMETER[ "False_Northing", 0.0], UNIT[ “Meter", 1.0] ]

  15. SADD = Shape Area Direction Distance • Web Mercator distorts area and distance • Which is bigger? • Greenland • South America • Antarctica • South America is 17 million sq km • Antarctica is 14 million sq km • Greenland is 2 million sq km

  16. Preserve Shape and Direction Stereographic

  17. Preserve Area Albers Equal Area Conic

  18. Preserve Direction Gnomonic

  19. Preserve Direction and Distance Azimuthal Equidistant

  20. What is happening when we project data? Case 1: Both PCSs contain the same GCS PCS A1 PCS A2 (x, y) Projection Projection GCS A (lon, lat) (λ, φ)

  21. What is happening when we project data? Case 2: Each PCS contains a different GCS PCS A1 PCS B1 (x, y) Projection Projection (lon, lat) GCS A GCS B (λ, φ) Geographic Transformation Or Datum Transformation

  22. Why do we need to transform our data? European Datum 1950 vs. World Geodetic System 1984

  23. Changing datums My data Earth’s surface Earth-centered datum (WGS 84) Local datum (NAD 27)

  24. Geographic (Datum) Transformation • Converts one GCS to another GCS • Defined in a particular direction • For example, NAD27 to WGS84 • All are reversible • Suitable for a particular area • May be more than one applicable GT

  25. 33 transformations between NAD 27 and WGS 84 Which is best? Depends on the region covered by your data

  26. How do I find transformations? Demo

  27. Now you understand … • Coordinate Systems • Projecting your data • Transforming your data

  28. Where is my data?

  29. Buffers Demo

  30. That’s all folks!Don’t forget to fill out the surveyIntroduction to Coordinate Systems and Projections

  31. Resources • http://resources.arcgis.com/en/help • Desktop → Guide Books → Map projections • Developer Help • List of ArcGIS APIs • Lining Up Data in ArcGIS, Margaret Maher • ESRI Technical paper: Understanding Coordinate Management in the Geodatabase • ESRI Technical paper: Understanding Geometric Processing in ArcGIS

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