Fiksel
The Fiksel Interaction
Creates an instance of Fiksel's double exponential pairwise interaction point process model, which can then be fitted to point pattern data.
Usage
Fiksel(r, hc=NA, kappa)
Arguments
- r
- The interaction radius of the Fiksel model
- hc
- The hard core distance
- kappa
- The rate parameter
Details
Fiksel (1984) introduced a pairwise interaction point process with the following interaction function $c$. For two points $u$ and $v$ separated by a distance $d=||u-v||$, the interaction $c(u,v)$ is equal to $0$ if $d < h$, equal to $1$ if $d > r$, and equal to $$\exp(a \exp(-\kappa d))$$ if $h \le d \le r$, where $h,r,\kappa,a$ are parameters. A graph of this interaction function is shown in the Examples. The interpretation of the parameters is as follows.
- $h$is the hard core distance: distinct points are not permitted to come closer than a distance$h$apart.
- $r$is the interaction range: points further than this distance do not interact.
- $\kappa$is the rate or slope parameter, controlling the decay of the interaction as distance increases.
- $a$is the interaction strength parameter,
controlling the strength and type of interaction.
If$a$is zero, the process is Poisson. If
a
is positive, the process is clustered. Ifa
is negative, the process is inhibited (regular).
ppm()
, which fits point process models to
point pattern data, requires an argument
of class "interact"
describing the interpoint interaction
structure of the model to be fitted.
The appropriate description of the Fiksel
pairwise interaction is
yielded by the function Fiksel()
. See the examples below.
The parameters $h$, $r$ and $\kappa$ must be
fixed and given in the call to Fiksel
, while the canonical
parameter $a$ is estimated by ppm()
.
To estimate $h$, $r$ and$\kappa$
it is possible to use profilepl
. The maximum likelihood
estimator of$h$ is the minimum interpoint distance. If the hard core distance argument hc
is missing or NA
,
it will be estimated from the data when ppm
is called.
The estimated value of hc
is the minimum nearest neighbour distance
multiplied by $n/(n+1)$, where $n$ is the
number of data points.
See also Stoyan, Kendall and Mecke (1987) page 161.
Value
- An object of class
"interact"
describing the interpoint interaction structure of the Fiksel process with interaction radius $r$, hard core distancehc
and rate parameterkappa
.
References
Baddeley, A. and Turner, R. (2000) Practical maximum pseudolikelihood for spatial point patterns. Australian and New Zealand Journal of Statistics 42, 283--322.
Fiksel, T. (1984) Estimation of parameterized pair potentials of marked and non-marked Gibbsian point processes. Electronische Informationsverabeitung und Kybernetika 20, 270--278.
Stoyan, D, Kendall, W.S. and Mecke, J. (1987) Stochastic geometry and its applications. Wiley.
See Also
Examples
Fiksel(r=1,hc=0.02, kappa=2)
# prints a sensible description of itself
data(spruces)
X <- unmark(spruces)
fit <- ppm(X ~ 1, Fiksel(r=3.5, kappa=1))
plot(fitin(fit))