Wieslander vegetation type data (VTM) dating from 1930 for Yosemite National Park

Wieslander vegetation type map (VTM) dating from 1930 for Yosemite National Park. The VTM survey began in 1928 and continued until the early 1940s. The VTM maps for the central Sierra Nevada were made in 1935 and 1936 (Walker 2000). The survey field work was done before aerial photographs were available; crews sketched vegetation patches from terrain high points that afforded views of the landscape (Keeler-Wolf 2007). Observations were recorded directly on photographic enlargements of US Geological Survey (USGS) topographic maps, mostly 30 minute quadrangles. First edition maps were used, based on topographic surveys in the late 1800s (Walker 2000). The high quality of the fieldwork on which the VTM map is based has been noted. Dr. Jim Thorne (UC Davis) cites Wieslanders unpublished Instructions for the preparation of the vegetative type map of California (on file in the library at YNP) to show Wieslanders determination to obtain the best possible data, and Wieslander's appreciation for the value of the VTM maps to future researchers (Thorne et al. 2008). The detailed protocol prescribed by Wieslander is summarized by Keeler-Wolf (Keeler-Wolf 2007). The consensus of the modern experts is that the VTM maps were produced in a consistent way using a well-defined, detailed protocol, which is available to current researchers. However, Keeley (2004) noted the difficulty to relocate and resurvey VTM plots. Seven decades after they were first made by Wieslander's U.S. Forest Service crews, the hand-drawn VTM maps for the central Sierra Nevada, including YNP, were digitized by Thorne and colleagues at the University of California-Davis. Benefiting from Walker's earlier experience in attempting to convert the first edition topographical maps to current editions (Walker 2000), Thorne's group chose instead to retain the projection used in the original USGS topo maps.