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GIS Brownbag Series

GIS Brownbag Series. Part 3: Georeferencing and Projections. Or… Why does my data not line up?. G is for Geographic. Tied to a location on Earth. Georeferencing : tying data to a location on Earth. 87 meters. Georeferencing Methods. Where in the world is IDWR?. Address.

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GIS Brownbag Series

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  1. GIS Brownbag Series Part 3: Georeferencing and Projections Or… Why does my data not line up?

  2. G is for Geographic • Tied to a location on Earth Georeferencing: tying data to a location on Earth

  3. 87 meters Georeferencing Methods Where in the world is IDWR? • Address 322 East Front Street Boise, Idaho USA 2. Linear Referencing IDWR is about 2.5 miles north of the Boise Airport

  4. Where in the world is IDWR? Georeferencing Methods 3. Geographic Coordinates 43° 36' 49" N 116° 12' 09" W

  5. T19N T18N T17N T16N T15N T14N R2W R1W R1E R2E Where in the world is IDWR? Georeferencing Methods 4. Public Land Survey System (PLSS) Township 03N Range 02E Prime meridian

  6. Shape of the Earth?

  7. Sphere? “perfect ball” Distance from center to surface is the same everywhere

  8. More mass + higher speeds: Earth bulges out at the equator Ellipsoid (a.ka. Spheroid)

  9. Ellipsoid (a.k.a. Spheroid) Different spheroids have been used to estimate the Earth’s shape.

  10. Spheroids Spheroid covered with long + lat Can we mix layers that are using different spheroids?

  11. WGS 84 Clarke 1866 Boise NO!! Layers will be misaligned Spheroids Can we mix layers that are using different spheroids?

  12. Shape of the Earth??? Spheroid? “bumps and dimples” : use a geoid

  13. Why bother with spheroids when Earth is shaped like a geoid? Model the correct shape of the Earth Conclusion: we need a geoidand a spheroid To draw longitude and latitude grid

  14. Geoids and Spheroids Good fit Spheroid Bad fit Geoid Datum: describes the geoid and how it relates to a spheroid Does this datum work everywhere?

  15. Datums Many datums to choose from…

  16. Many datums to choose from…

  17. We have not solved a basic problem yet: Earth is round, paper is flat!

  18. Y X Y latitude X longitude Earth is round, paper is flat! On flat surface need to turn round surface in Cartesian Coordinate System Easiest way: simply plot longitude (X) vs latitude (Y)

  19. Geographic Coordinate System Does this look good? No: some areas are distorted

  20. Projections GIS uses complex mathematical models. • 3 main types of projections: • Cylindrical • Conic • Planar

  21. Cylindrical Projection

  22. IDTM Idaho Transverse Mercator

  23. Conic Projection

  24. Planar Projections

  25. All projections distort reality. Why? Going from 3D to 2D • Which projection is this? How is it distorted? Mercator • Where would this projection be useful? • 1. Shape • 2. Area • 3. Distance • 4. Direction Antarctica isHUGE!

  26. Comparison of different projections

  27. Projecting Geographic Coordinates Example: Use GPS to collect spatial data at BSU GIS does not know in which coordinate system those coordinates fit…

  28. The same coordinates in different projections… Idaho StatePlane East UTM Zone 10 UTM Zone 11 UTM Zone 12

  29. Defining a Projection SO…We need to tell the GIS which coordinate system we are using!!!

  30. Projecting Data Sometimes you may want to change the projection of a layer For example: Geographic IDTM

  31. Coming Soon: GIS Workshop September 2 from 8:30-4:00 Room 150 in the Water Center There is still space! Next brownbag GIS lecture on September 18 Topic TBA

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