Why so different? Examining the methodologies used in two old growth forest mapping projects

In 1990, two different projects simultaneously mapped old growth in National Forests in the northern spotted owl region in the Pacific Northwest. The two projects used significantly different technology and techniques, and found amounts of old growth acreage that differed by a factor of two. This paper investigates the procedures used in each project to determine the sources of the differing results, using four of the forests mapped as a case study. The project carried out by Pacific Meridian Resources (PMR) for the U.S. Forest Service (USFS) used Landsat TM imagery and sophisticated image processing techniques, whereas The Wilderness Society project relied primarily on photo-interpretation but also used Landsat MSS imagery. The USFS project quantitatively mapped different size and structure classes, but The Wilderness Society´s project attempted to qualitatively categorize several variations of old growth. The projects operated under tremendous deadline pressure, had vastly different budgets, used very different hardware and software systems and had very different techniques for ground truth and accuracy assessment. Thus, despite the fact that their goals were identical these factors caused significantly different results