This map published in 1873 gives a general idea of how much of the United States was covered with forests and other woodlands in 1870. Compiled by Yale College professor William H. Brewer from statistics gathered from federal, state, and local government sources, it was the first such map that attempted to map this information for the entire country. The map was part of The Statistical Atlas of the United States Based on the Results of the Ninth Census, 1870, edited by Francis A. Walker, who was then head of the U.S. Census Office (now the U.S. Census Bureau). This remarkable atlas of thematic maps was the first atlas to provide a comprehensive geographical view of the social and economic statistics of the United States and its inhabitants in convenient map form. The 54 maps in the atlas cover a variety of topics in the physical, social, and economic geography of the United States, including climatic conditions, the distribution of foreign born Americans, racial statistics, and the location of key industries and natural resources.

When it ordered the preparation and publication of the Statistical Atlas in 1870, Congress expressed its hope that the atlas would be distributed "to public libraries, learned societies, colleges and academies" throughout the United States "with a view to promote that higher kind of political education which has hitherto been so greatly neglected in this country…." In other words Congress viewed the atlas as a tool that would help make Americans into better citizens well versed in knowledge of their own country, its inhabitants, industries, and natural characteristics. Since the country had just passed through a devastating civil war, Congress's concern about the political education of Americans was understandable, and goes a long way towards explaining why it was willing to support the considerable expense of compiling and publishing the Statistical Atlas. But why was it important in 1870 to map the distribution of woodlands in the United States?

In 1870, forests were one of the country's most important natural resources. As Prof. Brewer explained in his note accompanying the map, the industry was the second most important industrial sector in the country (behind the iron industry). Wood was "the main ingredient in the majority of buildings and the principal ingredient in the vast majority of ships, boats, [railroad] cars, carriages, etc., for transportation, so too for our furniture…. It is the sole household fuel of at least two-thirds [of] the inhabitants and the partial fuel of perhaps nine-tenths of the remaining third. For making steam, reducing metals and the various processes in the arts, it is used immense quantities." The rapid growth of the United States after 1850 meant that industrial and household consumption of American forests was also rapidly increasing. Whereas 25% of the country was densely forested in 1850, Brewer's map shows that only 15% of the country was densely forested in 1870. Dr. Brewer and other scientists and foresters were concerned that measures would to soon have to be taken on a national scale to ensure the survival of this vital resource.

Brewer's map thus may be seen as part of the beginning of the conservation movement in the United States in which forest management and the preservation of natural forest environments would play a key role. In 1872, while this map was being compiled, the first national park in the United States (and in the World), Wyoming's Yellowstone National Park, was created by an act of Congress. In 1885, out of concern for the cutting of trees for the lumber, paper, and iron mining industries, the New York state legislature created Adirondack Forest Preserve, the first state forest preserve in the nation. In 1905, the U.S. government created the U.S. Forest Service within the Department of Agriculture, charged with the task of managing industrial and recreational use of the nation's forests.

The core map is a special type of map called a dasymetric map, which uses various color shadings to indicate the relative value (intensity or number) of a particular physical or human geographical characteristic throughout a given area. In order to interpret the map it is essential to look carefully at the key at lower left. Note that the shading shows how many acres within any given square mile are covered with woodland (not how dense with trees these woodlands are). For example, in the areas on the map colored the darkest shade of green (also marked with a Roman numeral "V") at least 560 acres out of every scale mile were forested. There are 640 acres per square mile. Hence, if you traveled through one of these areas in 1870 (such as northern Michigan, Wisconsin, and Minnesota) at least 7 out of every 8 (=560/640) acres you encountered would be covered with some sort of woodland. If you traveled through one of the areas colored with a slightly lighter shade of green and marked with Roman numeral "IV" (for example, the southern Appalachian Mountains) between 360 and 560 acres out of every square mile (or 9/16 to 7/8 of the land area) would have been covered by woodlands in 1870. Class "III" lands (such as southeastern Missouri) had 240 to 360 wooded acres per square mile. Class "II" lands (such as the Colorado Rockies) had 120 to 240 wooded acres per square mile. Class "I" lands (such as eastern Pennsylvania) had 40 to 120 wooded acres per square mile. The unshaded portions of the map (such as almost all of Nevada) had less than 40 acres per square mile.