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Applying a variable-shape spatial filter to map relative abundance of manatees (Trichechus manatus latirostris)

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Abstract

Presented is a modified spatial filter, called a variable-shape filter, that was used to transform a map of point locations of Florida manatees (Trichechus manatus latirostris) to a contoured surface illustrating relative abundance. Rather than having a fixed polygon shape and size as conventional filters do, this method preserves constant filter area but conforms polygon shape to include only the desired land-cover categories–in this study, water. Variable-shape filter polygons are formed by starting with the cell that the point is located in and then adding the nearest contiguous cells of the desired land-cover category to the polygon until the area requirement is reached. Surfaces generated using the variable-shape filter were compared to those created with a conventional, circular, fixed-shape filter. Four filter sizes, based on an analysis of manatee hourly travel rates estimated from satellite and radio telemetry data, were used. Filter sizes, defined in terms of a circle's radius, were 125 m, which was the 25th percentile of the cumulative manatee travel-rate distribution; 325 m, the 50th percentile; 800 m, the 75th percentile; and 3,950 m, the 99th percentile. The fixed-shape and variable-shape filters differed principally in how their results were influenced by land. The variable-shape filter, programmed to maintain constant area, estimated animals to occur farther from shore than the fixed-shape filter did. Fixed-shape filter polygons were occasionally divided by land barriers, such as peninsulas, resulting in calculations of relative abundance estimates that were near the visual sighting in terms of euclidian distance but far in terms of manatee travel. The variable-shape filter was preferable primarily because it was more sensitive to manatee ecology: only cells contiguous to the animal's mapped location were included in the filter calculations.

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Authors and Affiliations

  1. Florida Marine Research Institute, 100 Eighth Avenue S.E., St. Petersburg, Florida, 33701-5095, USA

    Richard O. Flamm, Leslie I. Ward & Bradley L. Weigle

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  1. Richard O. Flamm
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  2. Leslie I. Ward
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  3. Bradley L. Weigle
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Flamm, R.O., Ward, L.I. & Weigle, B.L. Applying a variable-shape spatial filter to map relative abundance of manatees (Trichechus manatus latirostris). Landscape Ecology 16, 279–288 (2001). https://doi.org/10.1023/A:1011182302522

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