Submitted to: Rangeland Ecology and Management
Publication Type: Peer reviewed journal
Publication Acceptance Date: 5/3/2005
Publication Date: 3/15/2006
Citation: Booth, D.T., Cox, S.E., Meikle, T.W., Fitzgerald, C. 2006. The accuracy of ground cover measurements. Rangeland Ecology and Management. 59:179-188. Interpretive Summary: Ground cover is a key indicator of rangeland condition but conventional methods for measuring ground cover are labor intensive and expensive. Cover measurement by image-analysis is a developing, less-expensive alternative but how image-analysis measurements compare to conventional measurements is a critical question. To answer that question we compared the results from conventional ground-cover measurements of a known population with results from measurement by image-analysis. We found that conventional techniques were more accurate but we also identified how image-analysis accuracy might be improved. The critical factor influencing accuracy was the area of the contact point. These findings provide an important measure of relative accuracy among methods and are particularly relevant for land managers with tight monitoring budgets and for research to improve rangeland monitoring methods.
Technical Abstract: Ground cover is a key indicator of rangeland condition and influences rangeland management decisions. Despite this, few critical advances in its measurement have been made over the past several decades. With the advent of digital photography and image processing techniques, however, automated methods for measuring ground cover may be feasible. In order to assess the potential for automation, we compared conventional and automated methods for measuring ground cover against a known population. The known population was represented by 20 nadir images of a Wyoming big sagebrush vegetation type acquired with a 5-megapixel Olympus E20 digital single lens reflex camera mounted on an aluminum camera frame at 2 meters above ground level. The images were converted to a simplified color 2-dimensional image with known color cover values and then printed to a 1 x 1 meter poster size. Posters were evaluated for color cover under laboratory conditions using the conventional techniques of steel-point frame, laser-point frame, line-point intercept, ocular estimation, and line intercept. Photographs of the posters were measured for color cover using VegMeasure image analysis software and the Digital Grid Overlay method. Results indicate that conventional techniques had significantly greater correlation (R > 92% agreement of measured to known) than measurements from the VegMeasure analysis (R = 70%). The critical factor influencing accuracy was the area of the contact point for the given method. These findings provide an important measure of relative accuracy among methods and are particularly relevant for land managers with tight monitoring budgets and for research to improve rangeland monitoring methods.