The fundamental approach undertaken for the global mapping effort was to stratify the Earth into physically distinct areas with their associated land cover. The approach is ecophysiographic in that it emphasizes both ecological (e.g. bioclimates and land cover) and physiographic (e.g. landform and lithology) properties of landscapes. This ecophysiographic concept also demonstrates that when only land cover is mapped with its physical environment context, the resulting areas are better conceptualized as ecological land units rather than ecosystems, since less is known about the vegetation. In other words, when the description of an area emphasizes its biophysical features, and also notes associated image-derived land cover, that area is better regarded as an ecological land unit than an ecosystem.

The development of the global ecological land unit (ELUs) map, produced from a partnership between USGS and Esri, was based on the premise that in the absence of rigorous, high biotic content ecosystem maps, global ELUs would be useful for a variety of global ecosystem assessments. The ecophysiographic stratification identifies ecological patterns at a global scale, which provides a context that is important to the subsequent mapping of ecological and geographic units at finer scales. It also supports the synthesis and comparison of disparate ecological studies at local and regional levels, and it provides a geospatial accounting framework for assessments of ecosystem service values.

This ELU stratification process is based on the geospatial combination of four global input layers - bioclimate, landform, lithology, and land cover – reconciled into a standard 250 meter raster framework. The bioclimates layer was a modified version of the Global Environmental Stratification (GEnS) dataset recently produced by Metzger in another GEOSS-commissioned effort. Since no DEM-derived global landforms layer existed, one was initially generated using the Missouri Resource Assessment Partnership (MoRAP) methodology applied to 250 meter global digital elevation data. The MoRAP algorithm, which uses slope and relative relief parameters, was subsequently improved with the addition of a profile parameter which then improved the delineation of tablelands. The lithology layer is the recently produced Global Lithology Map (GLiM) that identifies 16 lithological classes at its most general level of classification. For the global land cover input, the GlobCover 2009 product, collaboratively produced by the European Space Agency (ESA) and the Université Catholique de Louvain, was initially used. That product was then upgraded by the ESA to a Global Land Cover datalayer, which represents the global distribution of 23 land cover classes as interpreted from 300 m spatial resolution data from the MERIS satellite. In the most recent version of the ELUs, the categorical data were as follows: bioclimates (37), landforms (17), lithology (16), and land cover (36).

The input data (bioclimate region, landform type, surficial lithology, and land cover) were combined into a single 250 meter raster layer, called the ecological facets (EFs), which resulted in 106,959 unique combinations. These EFs represent the finest spatial resolution, globally comprehensive biophysical stratification yet attempted, and a detailed geospatial delineation of unique physical environments and their associated land cover. Although very rich in detail, the large number of EFs precludes meaningful cartographic display, and is essentially an unmanageable number of ecosystems from a practical and management perspective. Therefore as a final step, a generalized product termed ecological land units (ELUs) was aggregated from the EFs by generalizing the number of input attribute classes. This aggregation within classes resulted in a reduction to 3,639 global ELUs, which are cartographically feasible.

The final ELU map represents an ecophysiographic classification of the Earth's surface based on the geographic coincidence of climate, landforms, geology, and land cover. Climate, landforms and geology represent the physical setting that gives rise to ecological process, pattern, and the distribution of living things. Land cover represents a biotic response to the physical setting and is a key element of the physical and organic cycles that continue to shape the environment.

For additional information: A New Map of Global Ecological Land Units – An Ecophysiographic Stratification Approach.