Motivation and History
A task fundamental to advancing global change research is the availability of a standard reference system for direct digital comparison and interuse of remote sensing data sets on varying spatial and temporal scales. The availability of a standard gridding scheme is an essential requirement for systematic time-series studies. Such a scheme also supports the direct comparison of different remote sensing products, as well as the validation of algorithms, through digital comparison with surface station and other ancillary data sets which have been processed into the common grid.
In 1989, NASA Headquarters requested guidance from the scientific community on how to best construct geophysical products over land surfaces derived from Defense Meteorological Satellite Program (DMSP) Special Sensor Microwave Imager (SSM/I) data. For this purpose, a science working group was formed known as the SSM/I Products Working Team (SPWT). Among other tasks, the SPWT was asked to recommend specific data formats that the National Snow and Ice Data Center (NSIDC) might use to distribute SSM/I data with global coverage.
The SPWT recommended that NSIDC/University of Colorado and the University of Michigan Radiation Laboratory (RADLAB) collaborate to develop a prototype earth grid, specific to the needs of SSM/I data, but with a potential for general application to any global scale data set. The result of this collaboration was the Equal-Area SSM/I [sic] Earth Grid (EASE-Grid) which was distributed as a prototype in early 1993 to a broad user evaluation group, including the NOAA/NASA SSM/I Pathfinder Science Working Group (SWG), the NASA/EOS Polar DAAC Advisory Group (PoDAG), and members of the MIMR Science Team. User response formed the basis for the development of the final format, which was selected by the NOAA/NASA Pathfinder SSM/I SWG for the generation of Level-3 products (Armstrong and Brodzik, 1995).
Since the first application of the EASE-Grid included an interpolation method (the Backus-Gilbert method developed by RADLAB) that was specific to the SSM/I sensor, the "S" in the acronym originally stood for SSM/I. However, other polar data set producers quickly recognized the flexibility and adaptability of the EASE-Grid projection and gridding abstractions and adopted these for use in other data sets, including the AVHRR, SMMR, and TOVS Path-P Polar Pathfinder data sets, and the Arctic and Antarctic Research Institute (AARI) Sea Ice Observations. It soon became evident to staff at NSIDC that there was nothing SSM/I-specific about the EASE-Grid format: the format was proving itself adaptable to many more remote sensing and in situ applications than we had originally envisioned. By this point, the acronym enjoyed a valuable measure of name recognition, so we decided to keep the acronym, but we changed its definition to the more accurate Equal-Area Scalable Earth Grid. Some early publications, dated in the early to mid 1990s, include the obsolete reference to SSM/I, but the current definition should replace SSM/I with Scalable.
In 2011, NSIDC defined a new version, EASE-Grid 2.0, and recommended that it be used for production of gridded data for Soil Moisture Active Passive (SMAP) and MEaSUREs snow and ice data. These respective projects have subsequently adopted EASE-Grid 2.0. The most important change in the revised format is the choice of WGS 84 as the coordinate system projection ellipsoid, rather than the International 1924 Authalic Sphere (Brodzik et al., 2012). The original EASE-Grid definition required that data be reprojected in order to distribute as GeoTIFF, which is understood and used in many popular software packages (Haran, 2008; Billingsley and Haran, 2008). Data referenced to the WGS 84 datum and stored in EASE-Grid 2.0 projections can now be formatted directly as GeoTIFF without any reprojection. Other details of EASE-Grid 2.0 are described on the EASE-Grid 2.0 Format Description page.
As funding allows, NSIDC is committed to supporting data sets in both EASE-Grid and EASE-Grid 2.0 format for the foreseeable future. Due to easier distribution options for EASE-Grid 2.0, we recommend that producers of new gridded data seriously consider EASE-Grid 2.0. We also recommend that any reprocessing that is performed on legacy EASE-Grid data sets also make the change to EASE-Grid 2.0 as part of the reprocessing effort.
For detailed projection information, please see the Format Description page.
Armstrong, R. L. and M. J. Brodzik.1995. An Earth-Gridded SSM/I Data Set for Cryospheric Studies and Global Change Monitoring. Advances in Space Research 10:155-163.
Billingsley, B. and T. Haran. 2008. Using GeoTIFFs for Data Sharing: Limitations and Solutions. Eos Trans. AGU, 89(53), Fall Meeting Suppl., Abstract IN23B-1078. Accessed 2011-11-27 at ftp://sidads.colorado.edu/pub/ppp/conf_pppBillingsley/Using_GeoTIFFs_for_Data_Sharing_Limitations_and_Solutions.pdf.
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 Int. J. Geo-Inf. 1(1):32-45, doi:10.3390/ijgi1010032.
Haran, Terry. 2008. HDF-EOS vs. GeoTIFF: GIS Problems when Projection and Datum Spheroids are Different. Presentation at MODIS Science Team Meeting, May, 2008. Accessed 2011-11-27 at ftp://sidads.colorado.edu/pub/ppp/conf_ppp/Haran/HDF-EOS_vs._GeoTIFF_GIS_Problems_when_Projection_and_Datum_Spheroids_are_Different.pdf.