ESW  R Documentation 
Returns effective strip width (ESW) from an estimated
line transect detection
functions. This function applies only to line transect information.
Function EDR
is for point transect data. Function
effectiveDistance
accepts either point or line transect data.
ESW(obj, newdata)
obj 
An estimated detection function object. An estimated detection
function object has class 'dfunc', and is usually produced by a call to

newdata 
A data frame containing new values of
the covariates at which ESW's are sought. If NULL or missing and

Effective strip width (ESW) of a distance function is its
integral. That is, ESW is the area under the distance function from its
lefttruncation limit (obj$w.lo
) to its righttruncation limit
(obj$w.hi
).
If detection does not decline with distance, area under the detection
function is the entire halfwidth of
the strip transect (i.e., obj$w.hi  obj$w.lo
).
In this case density is the number sighted targets
divided by area surveyed, where area surveyed is
obj$w.hiobj$w.lo
times
total length of transects.
When detection declines with distance, less than the total halfwidth is effectively covered. In this case, Buckland et al. (1993) show that the denominator of the density estimator is total length of surveyed transects times area under the detection function (i.e., this integral). By analogy with the nondeclining detection case, ESW is the transect halfwidth that observers effectively cover. In other words, if ESW = X, the study effectively covers the same area as a study with nondeclining detection out to a distance of X.
A technical consideration: Rdistance uses the trapezoid rule to numerically
integrate under the distance
function from obj$w.lo
to obj$w.hi
. Twohundred
trapezoids are used in the approximation to speed calculations. In some
rare cases, two hundred trapezoids may not be enough. In these cases,
users should modify this function's code and bump seq.length
to
a value greater than 200.
If newdata
is not missing and not NULL and
covariates are present in obj
, the returned value is
a vector of ESW values associated with covariates in the
distance function and equal in length to the number of rows in newdata
.
If newdata
is missing or NULL and covariates are present
in obj
, an ESW vector with length equal to
the number of detections in obj$detections
is returned.
If obj
does not contain covariates, newdata
is ignored and
a scalar equal to the (constant) effective strip width for all
detections is returned.
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.
dfuncEstim
, EDR
,
effectiveDistance
# Load example sparrow data (line transect survey type)
data(sparrowDetectionData)
dfunc < dfuncEstim(formula=dist~1
, detectionData = sparrowDetectionData)
# Compute effective strip width (ESW)
ESW(dfunc)
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