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Minnesota Department of Transportation
A study initiated in 2004/2005 in District 4 of Mn/DOT compared the use of 100 1/4-mi long segments against seven 3-mi segments on sampling precision for the estimation of weed infestations in highway rights-of-way for Canada thistle (Cirsium arvense (L.)(Scop.), leafy spurge (Euphorbia esula L.), and poison ivy (Toxicodendron radicans). The comparison showed that the 3-mi and 1/4-mi plans yielded equivalent estimates of mean weed population density, although the results indicated that at the district level precision was greater with the 1/4-mi plan. Further investigations suggested that additional improvements in precision and efficiency would likely occur if segment lengths were shortened to 125' or less. The project implementation phase reported here aimed to investigate efficiencies of two sampling plans, one with weed density measured over 100, 225-ft segments, and one with presence/absence of weeds measured over 150, 14-ft segments. Results showed that the presence-absence surveys almost consistently underestimated the area infestation derived from the 225-ft surveys. The 14-ft survey required substantially less time to conduct, but at this time they appear to be less precise than the 225-ft surveys with regard to quantifying infestation density. It is recommended that an additional season of survey data be analyzed to better quantify the precision of the 14-ft survey results.
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