What are the EASE Grids?

The Equal-Area Scalable Earth (EASE) Grids are intended to be versatile formats for global-scale gridded data, including remotely sensed data. Data can be expressed as a digital array with one of many possible grid resolutions, which are defined in relation to one of four possible projections: Northern / Southern Hemispheres (Lambert's equal-area, azimuthal), temperate zones (cylindrical, equal-area), or global (cylindrical, equal-area). 

Knowles (1993) states that "Two of the most important characteristics of maps are whether they are conformal or equal-area. No map projection is both, and some are neither." On conformal maps, angles within a small area are reproduced accurately so a small circle on the globe will look like a small circle on the map; by definition, the aspect ratio remains 1:1 everywhere, while the areal distortion varies, e.g. from -6% at the pole to +276% at the equator. On equal-area maps, a small circle placed anywhere on the map will always cover the same area on the globe, though the shape may be distorted. A very popular map that is neither conformal nor equal-area is the cylindrical equidistant map, also known as the lat-lon grid. This map suffers from both areal and shape distortion. The EASE Grids utilize an equal-area projection because that minimizes the amount of distortion over the poles, using the Northern and Southern Hemisphere projections, and on other key areas of the globe, using the temperate and global projections. Since areas don't change between grid cells, visualization and intercomparison operations are greatly simplified and analysis is more convenient.

There are currently two EASE Grids, referred to as EASE-Grid and EASE-Grid 2.0. Table 1 summarizes the differences between them.

    Table 1. Comparison of EASE Grid Versions
    Feature EASE-Grid EASE-Grid 2.0
    Date Created 1992 2011
    Pole location Center of cell, polar grids cannot be nested Intersection of the four center cells, grids can be nested
    Nested Grids Cells can be nested if the grid extent is changed Coverage can stay the same, only the number of cells changes
    Corner Points Corner cell locations in azimuthal grids are undefined No undefined "off-the-Earth" corner cells 
    EPSG North: 3408
    South: 3409
    Global/Temperate: 3410    		 North: 6931
    South: 6932
    Global/Temperate: 6933
    Datum International 1924 Authalic Sphere WGS 84
    GeoTIFF Requires reprojection1 Supported without reprojection
    Software Issues Usually requires user to understand custom projection settings1 Supported by most software
    1 Projection difficulties arise because the projection ellipsoid and reference datum are different, and most software assume they are the same

    References

    Knowles, Kenneth W. 1993. A Mapping and Gridding Primer: Points, Pixels, Grids, and Cells. Unpublished report to the National Snow and Ice Data Center, Boulder, Colorado USA.

    EASE-Grid

    Brodzik, M. J. and K. W. Knowles. 2002. “Chapter 5: EASE-Grid: A Versatile Set of Equal-Area Projections and Grids.” in Michael F.Goodchild (Ed.) Discrete Global Grids: A Web Book. Santa Barbara, California USA: National Center for Geographic Information & Analysis. https://escholarship.org/uc/item/9492q6sm.

    EASE-Grid 2.0

    Brodzik, M. J., B. Billingsley, T. Haran, B. Raup, M. H. Savoie. 2012. EASE-Grid 2.0: Incremental but Significant Improvements for Earth-Gridded Data Sets. ISPRS International Journal of Geo-Information, 1(1):32-45, doi:10.3390/ijgi1010032. http://www.mdpi.com/2220-9964/1/1/32.

    Brodzik, M. J., B. Billingsley, T. Haran, B. Raup, M. H. Savoie. 2014. Correction: Brodzik, M. J. et al. EASE-Grid 2.0: Incremental but Significant Improvements for Earth-Gridded Data Sets. ISPRS International Journal of Geo-Information 2012, 1, 32-45. ISPRS International Journal of Geo-Information, 3(3):1154-1156, doi:10.3390/ijgi3031154. http://www.mdpi.com/2220-9964/3/3/1154