Rdistance contains functions and associated routines to analyze
distance-sampling data collected on point or line transects.
Some of Rdistance's features include:
Accommodation of both point and line transect analyses
in one routine (dfuncEstim).
Regression-like formula for inclusion of distance function
covariates (dfuncEstim).
Automatic bootstrap confidence intervals
(abundEstim).
Availability of both study-area and site-level abundance
estimates (help("predict.dfunc")).
Classical, parametric distance functions
(halfnorm.like, hazrate.like,
negexp.like), and
expansion functions (cosine.expansion,
hermite.expansion, simple.expansion).
Automated distance function fits and selection
autoDistSamp.
print, plot, predict, coef,
and summary methods for distance function objects and
abundance classes.
Distance-sampling is a popular method for abundance estimation in
ecology. Line transect surveys are conducted by traversing
randomly placed transects in a study area with the objective of
sighting animals and estimating density or abundance. Data collected
during line transect surveys consists of sighting records for
targets, usually either individuals or groups of individuals. Among
the collected data, off-transect distances are recorded or computed from
other information (see perpDists). Off-transect distances
are the perpendicular distances from the transect to the location of
the initial sighting cue.
When groups are the target, the number of individuals in the group is
recorded.
Point transect surveys are similar except that observers stop one or more times along the transect to observe targets. This is a popular method for avian surveys where detections are often auditory cues, but is also appropriate when automated detectors are placed along a route. Point transect surveys collect distances from the observer to the target and are sometimes called radial distances.
A fundamental characteristic of both line and point-based distance sampling analyses is that probability of detecting a target declines as off-transect or radial distances increase. Targets far from the observer are usually harder to detect than closer targets. In most classical line transect studies, targets on the transect (off-transect distance = 0) are assume to be sighted with 100% probability. This assumption allows estimation of the proportion of targets missed during the survey, and thus it is possible to adjust the actual number of sighted targets for the proportion of targets missed. Some studies utilize two observers searching the same areas to estimate the proportion of individuals missed and thereby eliminating the assumption that all individuals on the line have been observed.
A detailed comparison of
Rdistance to other options for distance sampling analysis (e.g.,
Program DISTANCE, R package Distance, and R package unmarked)
is forthcoming. While some of the functionality in Rdistance
is not unique, our aim is to provide an easy-to-use, rigorous,
and flexible analysis option for distance-sampling data.
We understand that beginning
users often need software that is both easy to use and easy to understand,
and that advanced users often require greater flexibility and customization.
Our aim is to meet the demands of both user groups. Rdistance is
under active development, so please contact us with issues, feature
requests, etc. through the package's GitHub website
(https://github.com/tmcd82070/Rdistance).
Rdistance contains four example data sets: two collected using
line-transect methods (i.e., sparrowDetectionData and
sparrowSiteData) and two collected using point-transect
methods (i.e.,
thrasherDetectionData and thrasherSiteData).
Main author and maintainer: Trent McDonald <trent@mcdonalddatasciences.com>
Coauthors: Ryan Nielson, Jason Carlisle, and Aidan McDonald
Contributors: Ben Augustine, James Griswald, Joel Reynolds, Pham Quang, Earl Becker, Aaron Christ, Brook Russelland, Patrick McKann, Lacey Jeroue, Abigail Hoffman, Michael Kleinsasser, and Ried Olson
Buckland, S.T., Anderson, D.R., Burnham, K.P. and Laake, J.L. 1993. Distance Sampling: Estimating Abundance of Biological Populations. Chapman and Hall, London.
Useful links: