A Guide to 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.

Comparison of EASE Grid versions

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 Unpublished report to the National Snow and Ice Data Center, Boulder, Colorado USA.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. 

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.

Map projection details

The four EASE Grid projections comprise two polar azimuthal equal-area projections for the Northern and Southern hemispheres, a global cylindrical equal-area projection, and a temperate zone cylindrical equal-area projection (figure below); the temperate projection is a subset of the global projection with different latitudinal bounds. More details about these four EASE Grid projections are available in Brodzik and Knowles 2002, Brodzik et al. 2012, and Brodzik et al. 2014. 

EASE-Grid map projections: The Southern Hemisphere Azimuthal (top left), the Northern Hemisphere Azimuthal (top right), the Global Cylindrical (bottom), and the Temperate Cylindrical (bottom)

Grid definitions

Each EASE Grid projections can be covered by many different grids. These grids are, essentially, the parameters necessary to define a transparent piece of graph paper that is overlaid on a flat map and then anchored to it at the map origin. The following four elements completely describe a grid:

• the map projection
• the numbers of columns and rows
• the area of each grid cell
• the location of the map's origin

Any number of grid definitions can therefore be used to describe the effect of changing the "graph paper." For example, doubling the number of columns or rows doubles the resolution; see Figure 2 for an example of how this scaling can produce a family of multi-resolution grids that nest within one another. Though the user has complete flexibility to define a custom grid for any EASE Grid projection, the tables below describe some common EASE Grids in detail. 

1 km resolution EASE Grids

3 km resolution EASE Grids

3.125 km resolution EASE Grids

6.25 km resolution EASE Grids

9 km resolution EASE Grids

12.5 km resolution EASE Grids

25 km Resolution EASE Grids

36 km resolution EASE Grids

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