spatstat residuals.ppm with an intensity given as an image?

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spatstat residuals.ppm with an intensity given as an image?

Seth Flaxman
I've got the intensity of an inhomogeneous Poisson process (fit using
some new methods I'm working on, so not created by spatstat) as an
image object, the observed point pattern as a ppp object, and I'd like
to call residuals.ppm to compute residuals. Is there a simple way to
create a fitted point process model (ppm) from an image and points so
I can pass this directly to residuals.ppm?

Thanks,
Seth

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Re: [FORGED] spatstat residuals.ppm with an intensity given as an image?

Rolf Turner

On 07/06/17 22:22, Seth Flaxman wrote:

> I've got the intensity of an inhomogeneous Poisson process (fit using
> some new methods I'm working on, so not created by spatstat) as an
> image object, the observed point pattern as a ppp object, and I'd like
> to call residuals.ppm to compute residuals. Is there a simple way to
> create a fitted point process model (ppm) from an image and points so
> I can pass this directly to residuals.ppm?

Sorry for the long delay in responding.

There are two ways to do this:

[1] Fit a model using the putative intensity as an offset:

              fit1 <- ppm(X ~ offset(log(lam)))

where 'lam' is the pixel image of intensity, and 'X' is the point pattern.

Then do:

            res <- residuals(fit1)

[2] Fit some other model (e.g. just a constant intensity model) and use
the argument 'fittedvalues' of residuals.ppm to specify the fitted
intensity values.

            fit2 <- ppm(X ~ 1, forcefit=TRUE)

            res  <- residuals(fit2, fittedvalues=lam[quad.ppm(fit2)])

cheers,

Rolf Turner

--
Technical Editor ANZJS
Department of Statistics
University of Auckland
Phone: +64-9-373-7599 ext. 88276

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