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A Comparison of Equal-Area Map Projections for Regional and Global Raster Data

A Comparison of Equal-Area Map Projections for Regional and Global Raster Data . E. Lynn Usery and Jeong-Chang Seong. Outline. Objectives Hypotheses Approach Theoretical Results Empirical Results with Land Cover Conclusions. Objectives.

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A Comparison of Equal-Area Map Projections for Regional and Global Raster Data

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  1. A Comparison of Equal-Area Map Projections for Regional and Global Raster Data E. Lynn Usery and Jeong-Chang Seong 29th International Geographical Congress

  2. Outline • Objectives • Hypotheses • Approach • Theoretical Results • Empirical Results with Land Cover • Conclusions 29th International Geographical Congress

  3. Objectives • Determine effects of various map projections on regional and global raster data • Assess problem mathematically • Test empirically • Long-term goal -- develop specialized projection, if necessary, to optimize projection of raster data 29th International Geographical Congress

  4. Hypotheses • Projection of raster data will produce variable results dependent on three factors: • Projection type and specific projection • Raster resolution • Latitude 29th International Geographical Congress

  5. Approach • Theoretical • Use vector representation of 1x1 degree squares at various latitudes to determine actual areas • Convert squares to raster and transform using exact projection equations (rigorous transformation) to various projections • Tabulate resulting areas of cells and compare to the vector “truth” 29th International Geographical Congress

  6. Projections Used • Equal Area • Goode Homolosine (Goode) • Equal Area Cylindrical (Eq-Cyl) • Mollweide (Mw) • Pseudocylindircal -- compromise • Robinson (Rob) 29th International Geographical Congress

  7. Resolutions Examined • 500 m – MODIS sensor IFOV • 1 km – AVHRR IFOV, NDVI base • 4 km – LAC, GAC temporal composites • 8 km – LAC, GAC temporal composites • 16 km, 25 km – Extent of largest features • 50 km – Larger than most geographic features used in modeling applications 29th International Geographical Congress

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  9. Results • Areas of 1x1 degree squares vary according to: • Projection • Resolution • Latitude 29th International Geographical Congress

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  12. Approach • Empirical • Transform land cover of 1 km raster pixels of Asia to various projections with resampling to different pixel sizes • Tabulate land cover percentages and compare among projections and among raster resolutions of the same projection 29th International Geographical Congress

  13. Asia Landcover • Downloaded from EDC • Lambert Azimuthal Equal Area Projection • Goode Homolosine Projection • USGS Land Cover Classes (24 categories) 29th International Geographical Congress

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  15. Asia Land Cover in Lambert Azimuthal Equal Area Projection (8 km Pixels) 29th International Geographical Congress

  16. Asia Land Cover in Goode Projection (8 km Pixels) 29th International Geographical Congress

  17. Asia Land Cover in Equal Area Cylindrical Projection (8 km Pixels) 29th International Geographical Congress

  18. Asia Land Cover in Mollweide Projection (8 km Pixels) 29th International Geographical Congress

  19. Asia Land Cover in Robinson Projection (8 km Pixels) 29th International Geographical Congress

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  21. Asia Land Cover by Projection, 1 km Pixels 29th International Geographical Congress

  22. Asia Land Cover by Projection, 4 km Pixels 29th International Geographical Congress

  23. Asia Land Cover by Projection, 8 km Pixels 29th International Geographical Congress

  24. Asia Land Cover by Projection, 16 km Pixels 29th International Geographical Congress

  25. Asia Land Cover by Projection, 25 km Pixels 29th International Geographical Congress

  26. Asia Land Cover by Projection, 50 km Pixels 29th International Geographical Congress

  27. Asia Land Cover by Projection • Verifies theoretical analysis • Robinson overestimates except at 50 km • 16 km • Lam, Mw, Eq-Cyl retain almost identical % • Mw same at 50 km • Goode doesn’t maintain between 16 and 50 km 29th International Geographical Congress

  28. Asia Land Cover by Projection • Latitudinal results verified by examining specific land covers which occur at unique latitudes • Deciduous needleleaf forests occur in high latitudes • Order of areas lowest to highest • Mw, Eq-Cyl, Robinson • Goode anomaly because different source 29th International Geographical Congress

  29. Summary • Empirical results at 1, 4, 8, 16, 25, and 50 km verify the theoretical results shown in the graphics. • Visually which is most pleasing? 29th International Geographical Congress

  30. Conclusions • Regional and global raster data yield varying areas when projected in different equal area projections. • Variance is by projection, resolution, latitude • 1 km or less, any equal area is okay • 1 to 8 km, Mw shows best accuracy • 16 to 25 km, Eq-Cyl and Goode better • 50 km, Mw best • Overall, Mw a good alternative 29th International Geographical Congress

  31. Global datasets Land Cover (1 km) Elevation (30 arc-sec and 5 min) Vegetation (1 degree) Precipitation (30 min) Temperature (30 min) Projections Equal Area Cylindrical Eckert IV Hammer Mollweide Quartic Authalic Sinusoidal Robinson Van der Grinten Current Work 29th International Geographical Congress

  32. Future Work • Correct problems with raster projection • DSS for use with current software based on empirical base developed • Develop dynamic projection for raster data • Implement error correction procedures • Prefect resampling from one projection to another 29th International Geographical Congress

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