Forest resources in Washington and Oregon were surveyed in the early 1930s by employees of the Pacific Northwest Experimental Station (the original name of the current Pacific Northwest Research Station). This was the first of many periodic forest surveys conducted nationwide by the USDA Forest Service. Many publications and maps were produced from the Washington and Oregon 1930s survey data. Forest cover maps created from that data (at an original scale of 1:253, 440) have recently become available in digital formats, but little documentation was provided with the electronic files, and the older publications are not readily available to most users.

The 27 forest cover types defined are the result of the combination and generalization of some 50 types used by the Forest Survey in mapping all forest land in Oregon and Washington. In addition to the 50 primary cover types, all immature stands were differentiated according to age in 10 year classes and according to their density of three degrees of stocking, namely, good, medium and poor. None of this detail of age of stocking is shown on this map; such types may include any or all degrees of stocking. The complete type mapping, using all the detail of the original field work, has been reproduced for every county in Oregon and Washington in the form of uncolored blue line prints, scale one inch to the mile, which show type boundaries, type numbers and age and stocking symbols. These county maps may be obtained from the PNW Forest Experiment Station, US Court House, Portland, Oregon.