How to use Distance Sampling along transect with obscured sightline at 0m

[Margaret Swift]

Hello! I am running a distance sampling in R on a survey that was taken via airplane; there is a dip in detection at 0m for most animals in the survey (see attachment). How would I modify the Distance method to account for this? Would I have to make these adjustments before or after running the model? I couldn't find an appropriate example in the catalog on your website (there are so many now! how exciting!), but if i've missed one that addresses this question, please let me know.

Thanks,

Maggie

[Eric Rexstad]

Maggie

It seems that the bins of your histogram are 17m in width if my eyes don't deceive me.  That's not uncommon for aerial surveys when detectability isn't perfect under the plane, near the trackline.

One option could be to do nothing; the fitted detection function will smooth through the "few" detections in the first bin and the "considerable" detections in the second and subsequent bins.  Goodness of fit statistics will suffer a bit as a result, but perhaps not drastically.

The other option is to left-truncate data, mentioned in Section 10.3.2.1 of Buckland et al. (2015) and Section 8.4.3 in Buckland et al. (2001).  That could involve removing the first bin such that detectability is assumed perfect at 20m.  As an aside, I would probably also impose right truncation on the data in the histogram as well, perhaps back as far as 500-600m.

There is an example of left truncation associated with a camera trap analysis found at

http://examples.distancesampling.org/Distance-cameratraps/camera-distill.html#truncation-decisions

Analysis of camera trapping data - Distance Sampling Truncation decisions. As described by Howe, Buckland, Després-Einspenner, & Kühl ():. a paucity of observations between 1 and 2 m but not between 2 and 3 m, so we left-truncated at 2 m. Fitted detection functions and probability density functions were heavy-tailed when distances >15 m were included, so we right truncated at 15 m. examples.distancesampling.org

this shows the syntax to be used with the ds()​ function to produce left (and right) truncation.

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Subject: {Suspected Spam} [distance-sampling] How to use Distance Sampling along transect with obscured sightline at 0m

 

Hello! I am running a distance sampling in R on a survey that was taken via airplane; there is a dip in detection at 0m for most animals in the survey (see attachment). How would I modify the Distance method to account for this? Would I have to make these adjustments before or after running the model? I couldn't find an appropriate example in the catalog on your website (there are so many now! how exciting!), but if i've missed one that addresses this question, please let me know.

Thanks,

Maggie

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[Margaret Swift]

Hey Eric, thanks so much! I agree right-truncation is appropriate; the methods for collection stipulated that counts should only be made to about 400m anyway, so I've been truncating to 500 to allow for a bit of overlap since these things are imperfect when conducted from the air.

As for left-truncation, I'll have to give it some thought, but thanks for pointing me in the right direction! I'll take a look at the Buckland et al papers. 

Thank you for all your help!

Maggie