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

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

29 th International Geographical Congress 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

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

29 th International Geographical Congress 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”

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

29 th International Geographical Congress 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

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Results Areas of 1x1 degree squares vary according to: –Projection –Resolution –Latitude

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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

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

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Asia Land Cover in Lambert Azimuthal Equal Area Projection (8 km Pixels)

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

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

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

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

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Asia Land Cover by Projection, 1 km Pixels

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

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

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

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

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

29 th International Geographical Congress 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

29 th International Geographical Congress 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

29 th International Geographical Congress 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?

29 th International Geographical Congress 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

29 th International Geographical Congress Current Work 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

29 th International Geographical Congress 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