Submitted to: Rangeland Ecology and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/16/2007
Publication Date: 3/20/2008
Citation: Ganskopp, D.C., Johnson, D.D. 2008. Gps collar sampling frequency: affects on measures of resource use. Rangeland Ecology and Management. Rangeland Ecol Manage 61:226-231. Interpretive Summary: The use of Global Positioning System (GPS) collars for accurate renderings of animal movements and activities in remote areas has furnished researchers and biologists with a revolutionary tool displaying very high accuracy. Researchers can determine an animal’s position several times a day, but there have been no studies to evaluate how often positions should be acquired to accurately measure how far an animal travels or which portions of the landscape it visited. Battery life and internal memory restrict the amount of data one may acquire. We used GPS collars to determine the positions of cattle at several time intervals from as frequently as once every 5 minutes to once per day over 15 days. As sampling times were extended from once every 5 minutes to 10, 20, 80, and 1440 minute intervals, travel distances were underestimated by 20, 36, 59, and 78 percent, respectively. Expanding sampling intervals from once every 5 minutes to 10, 20, 80 and 1440 minutes misrepresented measures of travel corridors or areas of the landscape actually used by cattle by 17, 30, 60, and 92 percent, respectively. Determination of an animal’s travels and measures of what portions of the landscape are actually used are best established by using the most frequent sampling interval possible. One will most likely have to settle on a compromise schedule by considering their specific objectives and the memory and battery life limitations of their GPS collars. These findings will help biologists and scientists determine which GPS collar sampling schedules are best fitted to their project objectives.
Technical Abstract: One challenge of animal behavior studies employing global position system (GPS) collars is selecting a sampling frequency to attain project goals. High data resolution (i.e. frequent sampling) is generally appealing because it maximizes the amount of behavioral information garnered. However, one may also need extended sampling to adequately describe long term behavior. Because tradeoffs often exist between high data resolution and sampling duration, we evaluated the effects of variable GPS sampling intervals on measures of distance traveled, proportions of pasture accessed, and paths traversed by cattle. This was accomplished with GPS collars configured to integrate cattle positions every 5 minutes for 15 days in three 800-ha pastures. Differentially corrected datasets were iteratively reduced to simulate increasingly longer GPS integration intervals. Increasing sampling intervals from 5 to 10 minutes decreased estimated total distance cattle traveled by 20% from 136 to 109 km over 15 days. The relationship between length of integration interval and total distance cattle traveled was relatively constant up to a breakpoint in the curvilinear function (approximately 72 minutes) with estimated travels being reduced by about 0.92 km/minute. When integration intervals were expanded from 72 minutes to once daily, estimates of mean distance traveled decreased linearly at a more gradual rate of 0.033 km/minute. Reducing GPS integration frequency linearly decreased estimates of the proportion of pastures visited by cattle. Spatial errors accompanying less frequent sampling intervals, however, were extremely large and seriously misrepresented resources visited by cattle. If travel corridors or accurate assessments of resources accessed are of critical concern, then longer GPS integration intervals should be avoided, because they propagate seriously flawed spatial interpretations. Similarly, if accurate measures of travel distances are critical, then we suggest a relatively frequent GPS integration interval (i.e. 5 minutes) be used.