derivedDsurface: Derived Absolute Density

In secr: Spatially Explicit Capture-Recapture

Derived Absolute Density

Description

A relative density model (one fitted by maximising the conditional likelihood) has a zero intercept for density on the default log scale (the intercept is 1.0 for the identity link). The constant k that relates absolute density to relative density D^\prime = k^{-1} D is obtained as described in Efford (2025).

These functions infer the value of k and use this to construct the predicted density surface (predictDsurface is the equivalent function for full-likelihood models).

The derived method for ‘secr’ objects performs the same calculation as derivedDcoef and also back-transforms the intercept and computes a delta-method estimate of its variance and confidence limits. However that variance estimate is unreliable.

Usage

derivedDcoef(object, sessnum = 1, groups = NULL, se = FALSE)

derivedDsurface(object, mask = NULL, sessnum = NULL, groups = NULL)

Arguments

object	fitted secr relative density model
mask	new mask for which to compute densities
sessnum	integer session number
groups	character vector of covariate names to define groups (optional)
se	logical; if TRUE the variance of beta0 is estimated by the delta method

Details

The theory is provided by Efford in prep.

derivedDcoef(object, se = TRUE) is equivalent to derived(object, Dweight = TRUE), although derivedDcoef returns estimates on the link scale.

Value

For derivedDcoef –

A dataframe mimicking coef(object) with an initial row for the derived density intercept \beta_0. For an identity link, subsequent density coefficients are scaled by the derived \beta_0.

For derivedDsurface –

Dsurface object with class c("Dsurface", "mask", "data.frame"). Multi-session if object is multi-session and sessnum = NULL.

If groups are defined the result is a list of Dsurfaces.

References

Efford, M. G. In prep. SECR models for relative density.

See Also

derived

predictDsurface

secr documentation built on April 4, 2025, 2:06 a.m.