R help - Mar 2012 - Creating polygons from scattered points

Hello,

I have a distribution dataset for species consisting of xy coordinates at
the 1km resolution, with only presence data. So a simplified example of a
species distribution might be:

y <- rbind(as.integer(rnorm(100,50,20)), as.integer(rnorm(200,100,30)),
as.integer(rnorm(100,180,15)))
x <- rbind(as.integer(rnorm(200,50,20)), as.integer(rnorm(200,100,20)),
as.integer(rnorm(100,200,15)))
plot(y~x)

I would like to create polygons for each species distribution, where if an
island is present (as I have tried to show in the example), it would be a
seperate polygon, and the jagged edges of coastlines etc are maintained. I
have spent ages trying to find a package that will allow me to convert
scattered point distributions to polygons but haven't found anything that
works, the functions I have found require the data already to be in the
format where the only xy coordinates present are the outline of the
polygon.

Can anyone please recommend a function I can use here, or suggest a way of
extracting the outline points? I have tried this manually but cannot seem
to write a code that will effectively take account of jagged edges and
islands.

Thanks very much for your help,

Louise.

	[[alternative HTML version deleted]]

This might be a question for the R-SIG-Geo or R-SIG-Ecology lists.

Best,
Michael

On Wed, Mar 14, 2012 at 7:13 AM, Louise Mair <lm609 at york.ac.uk>
wrote:
> Hello,
>
> I have a distribution dataset for species consisting of xy coordinates at
> the 1km resolution, with only presence data. So a simplified example of a
> species distribution might be:
>
> y <- rbind(as.integer(rnorm(100,50,20)), as.integer(rnorm(200,100,30)),
> as.integer(rnorm(100,180,15)))
> x <- rbind(as.integer(rnorm(200,50,20)), as.integer(rnorm(200,100,20)),
> as.integer(rnorm(100,200,15)))
> plot(y~x)
>
> I would like to create polygons for each species distribution, where if an
> island is present (as I have tried to show in the example), it would be a
> seperate polygon, and the jagged edges of coastlines etc are maintained. I
> have spent ages trying to find a package that will allow me to convert
> scattered point distributions to polygons but haven't found anything
that
> works, the functions I have found require the data already to be in the
> format where the only xy coordinates present are the outline of the
> polygon.
>
> Can anyone please recommend a function I can use here, or suggest a way of
> extracting the outline points? I have tried this manually but cannot seem
> to write a code that will effectively take account of jagged edges and
> islands.
>
> Thanks very much for your help,
>
> Louise.
>
> ? ? ? ?[[alternative HTML version deleted]]
>
> ______________________________________________
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> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.

No time to really think about this, but:

a) to "convert scattered point distributions to polygons" you might
look at
convex hulls; e.g. convhulln {geometry}
b) to identify islands some kind of cluster analysis

Hope that helps a little.

KJ

"Louise Mair" <lm609 at york.ac.uk> wrote in message 
news:CAMkzT57pep1zKPgm2DG5TN6r0P-_O5Tf+wSnURXdOHF8zVMnpw at
mail.gmail.com...
> Hello,
>
> I have a distribution dataset for species consisting of xy coordinates at
> the 1km resolution, with only presence data. So a simplified example of a
> species distribution might be:
>
> y <- rbind(as.integer(rnorm(100,50,20)), as.integer(rnorm(200,100,30)),
> as.integer(rnorm(100,180,15)))
> x <- rbind(as.integer(rnorm(200,50,20)), as.integer(rnorm(200,100,20)),
> as.integer(rnorm(100,200,15)))
> plot(y~x)
>
> I would like to create polygons for each species distribution, where if an
> island is present (as I have tried to show in the example), it would be a
> seperate polygon, and the jagged edges of coastlines etc are maintained. I
> have spent ages trying to find a package that will allow me to convert
> scattered point distributions to polygons but haven't found anything
that
> works, the functions I have found require the data already to be in the
> format where the only xy coordinates present are the outline of the
> polygon.
>
> Can anyone please recommend a function I can use here, or suggest a way of
> extracting the outline points? I have tried this manually but cannot seem
> to write a code that will effectively take account of jagged edges and
> islands.
>
> Thanks very much for your help,
>
> Louise.
>
> [[alternative HTML version deleted]]
>