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1 October 2011 Efficiency and Accuracy of Wildland Weed Mapping Methods
Stephanie D Christensen, Corey V Ransom, Kimberly A Edvarchuk, V. Philip Rasmussen
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Land managers must set weed management priorities if limited resources are to be utilized effectively. Weed surveys and inventories assist land managers in this process by providing information regarding the identity, location, and relative abundance of weeds on their land. Although this information is vital, it can be challenging to select a method that provides the necessary data to meet management objectives while remaining accurate and cost effective. This paper critically evaluates four wildland weed mapping methods. These methods were defined as (1) paper-drawn, (2) buffered-point, (3) screen-drawn, and (4) perimeter-walked. Polygons were drawn by hand on topographic maps in the paper-drawn method. The other methods utilized handheld geographic positioning system (GPS) technology to digitally record infestations. Six experienced weed mappers independently recorded the location and size of eight sagebrush patches using each method. Time and accuracy were evaluated for each method based upon mapping time, distance walked, horizontal precision error, estimated size error, and shape error. The paper-drawn method was significantly less accurate than GPS-based methods at recording patch size and location. There was no significant difference in the accuracy of the buffered-point, screen-drawn, and perimeter-walked methods at reporting patch size and location. The need to cover land area quickly and efficiently favors the selection of the buffered-point or screen-drawn method because of time and distance factors. However, if patch shape is an important factor, the perimeter-walked or buffered-point methods may be the best choices of methods tested.

Interpretive Summary: Weed inventories provide land managers with valuable information regarding the distribution, identity, location, and relative abundance of weeds on the landscape. This information can then be used to guide the development of successful management strategies. It can be difficult to select a mapping method that will provide necessary data while remaining efficient and cost effective. This study demonstrates that mapping weed patches using paper-drawn features provides the least accurate data. Although a degree of error remains, the use of GPS technology can significantly improve the accuracy of data collected. If weed patch size and location are important factors, there is no accuracy advantage in using the screen-drawn or perimeter-walked methods over the buffered-point technique. Further, the need for most weed mapping programs to cover land quickly and efficiently favors the selection of the buffered-point or screen-drawn methods. However, if accurately depicting the shape of a patch is important, either the perimeter-walked or the buffered-point method would be the best choice. It is important to remember that the accuracy of any data collected is dependent upon the proficiency of the weed mapper in using the selected method. The screen-drawn method will require more training than the buffered-point and perimeter-walked methods. This technique requires a mapper to evaluate and illustrate patch size, shape, and location on a GPS screen, which provides more opportunities for mistakes. Overall, the buffered-point method may be the best technique for obtaining useful, accurate, and cost effective weed data.

Weed Science Society of America
Stephanie D Christensen, Corey V Ransom, Kimberly A Edvarchuk, and V. Philip Rasmussen "Efficiency and Accuracy of Wildland Weed Mapping Methods," Invasive Plant Science and Management 4(4), 458-465, (1 October 2011).
Received: 24 February 2011; Accepted: 1 September 2011; Published: 1 October 2011

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