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Comparison of raw versus post-differentially corrected GPS collar fixes in free-ranging goats

T. A. Gipson, S. P. Hart, and R. Heinemann

Journal of Animal Science 87(E-Supplement 2):309. 2009

Even though selective availability of GPS signals was discontinued on May 1, 2000, there remains some debate as to whether GPS fixes need to be post-differentially corrected. The objective of this study was to determine the effect of post-differential correction on fixes of GPS collars worn by free-ranging goats. Twenty-one wether goats (46 4.7 kg) were fitted with GPS collars that recorded a fix every 5 min and released into a novel environment (35?53'40"N, 94?45'21"W) of 4.6 ha. Collars were downloaded after 1 wk and 41,744 raw (R) GPS fixes were post-differentially corrected (C). For fix status, C decreased 3-D fixes and increased 2-D fixes and No-fix compared with raw fixes (R: 95,8, 4.0, and 0.2%; C: 69.1, 28.4, and 2.5% for 3-D, 2-D, and No-fix status, respectively; ?2 = 10,270, P < 0.01). A higher percentage of C fixes were located within the boundary of the study compared with R (89.7 vs. 86.3%, P < 0.01). The correcting distance between R and C fixes was greater in daylight hours than at night (23.4 vs. 16.9 m; P < 0.01). With distance calculations restricted to fixes within the boundary, the minimum (straight-line) distance traveled between consecutive fixes was greater for R than for C (29.5 vs. 27.6 m, P < 0.01). Therefore, the calculation of daily total minimum distance traveled per goat was greater for R than for C (4.16 vs. 3.82 km, P < 0.01). Inter-goat distance was greater for R than for C (19.9 vs. 15.4 m, P < 0.01). Analysis using R vs. C fixes may affect conclusions because more C than R fixes were within study area boundary, corrections were greater during daylight hours when animals were most active, and intra/inter-animal distance calculations were greater for R than for C. These differences may be especially important for researchers studying spatial distribution of grazing animals or calculating distance traveled such as in energy expenditure experiments.


 

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