dealing with zero counts

[D Bowler]

Hello,

I have just started using the dsm package - it looks pretty useful for a question I am tackling. I have distance-sampling data for birds at a set of sites and I wish to upscale the data to estimate the population size of the species over the larger region. I only have one distance sampling transect per site.

I would like to check how to treat transects with zero counts, since these contain information relevant for the upscaling section of my analysis.

(1) where do I add these transects: I tried fitting the the ds model to only the positive detections and then adding in the transects with zero observations to the observation.data object by setting size as 0 and distance as NA, but the model doesn't like these NAs. Alternatively, I can just add them to the segment.data object and these transects with zero observations would not be found in the observation.data????

(2) how these zeros would be treated: I don't think the model can account for total non-detection at a transect, since I don't have replicate surveys, so these zeros are still treated as zeros in the gam model when using dsm, right?

Cheers,

Diana

[Eric Rexstad]

Diana

Sampling only a single transect within your sites does represent a challenge to you understanding of bird abundance. I'm not sure that density surface modelling will resolve those challenges, but here's some thoughts about data organisation for the dsm​ package.

Have a look through the case study about density surface models with line transect sampling:

http://examples.distancesampling.org/dsm-line-dolphins/mexico-analysis.html

Example dsm analysis: pantropical dolphins in the Gulf of Mexico Typically (i.e. for other datasets) it will be necessary divide the transects into segments, and allocate observations to the correct segments using a GIS or other similar package 1, before starting an analysis using dsm.. Shapefiles and converting units examples.distancesampling.org

​You will note that four data frames are needed: segment data, distance data, observation data and prediction grid.  Information from transects without detections only appear in the segment data data frame.  Segment data contains location of the segment, a unique label for the segment, the transect to which it belongs, importantly-the size of the segment (effort) and whatever covariates you recorded at the segment.

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From: distance...@googlegroups.com <distance...@googlegroups.com> on behalf of D Bowler <diana.e...@gmail.com>
Sent: 17 July 2022 11:31
To: distance-sampling <distance...@googlegroups.com>
Subject: [distance-sampling] dealing with zero counts

 

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Y