Nine estimates of the coal resources of the United States have been published in the past 71 years. Although many details of these estimates differ markedly, the 1913, 1922, and 1974 estimates are surprisingly similar. Some differences are due to increased geologic data, others reflect changes in terminology, definitions, criteria, guidelines, and methodologies used for estimating coal resources and reserves during the last 100 years. Because of the increased data and changes, many of the early estimates are not particularly useful in modern resource assessments.

Preliminary definitions that are being prepared in 1980 by the U.S. Geological Survey are compared with those published in 1976 and currently in use. Anticipated results of the new definitions are to lessen existing confusion about estimation procedures, to make such procedures easier and more precise, and to promote use of a commonly accepted terminology accompanied by standardized definitions, criteria, guidelines, and methodologies for estimating coal resources.

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