truncation distance

[Gianpasquale Chiatante]

Hello list!

I'm analysing bird observations collected (not by me) through point transects with unlimited distances. Points were placed randomly in the study sites at a minimum distance of 300 m each other (average distance between points equal to 600 m). This distance was defined on the basis of a previous study showing that the average distance of detectability was equal to ~150 m; in this way the overlap between counts should be minimal avoiding double counts.

I'm wondering if I must inevitably truncate data to 150 m, apart 10% suggested. This doubt arises because there are a lot of observations around 250 m, falling in the overlap area. However, this overlap occurred only for 4% of the points.

Thank you in advance for your comments!

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Gianpasquale Chiatante, PhD

University of Pavia

Department of Earth and Environmental Sciences

Via Adolfo Ferrata 1,  27100 Pavia (PV) - Italy

http://ecoeto.unipv.it/chiatante

https://www.researchgate.net/profile/Gianpasquale_Chiatante2

[Eric Rexstad]

Gianpasquale

It is not a rule preventing "overlap" between points.  Have a look at this 2006 paper by Prof Buckland

Buckland, S. T. (2006). Point transect surveys for songbirds: Robust methodologies. The Auk, 123(2), 345–357. https://doi.org/10.1093/auk/123.2.345

indicating from simulation, very little bias arising from a point transect survey with considerable overlap between points.

Be guided instead by the shape of the radial distance histograms when choosing a truncation distance.  Remember, at large distances there is little information about the shape of the detection function near the point--that should be your guide in determining the truncation distance.

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Eric Rexstad
Centre for Ecological and Environmental Modelling
University of St Andrews
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