Thank you very much Roman and Barry,
problem solved

by the way, for plotting purpose, I actually used:
> plot(gBoundary(gBuffer(MyMap , width=-0.15)))
in order to get only the boudaries, not the all shrinked polygon
and then the over() function to check wether each store was actually inside
the shrinked polygon
> OKstores <- over(Stores , ShrinkMap)

problem solved.
Up to the next one...

Sylvain



2014-03-14 15:49 GMT+01:00 Barry Rowlingson <b.rowlingson@lancaster.ac.uk>:

>
>
>
> On Fri, Mar 14, 2014 at 12:11 PM, Roman Luštrik <roman.lustrik@gmail.com>wrote:
>
>> Can you plot a circle (or some other shape) around each store and find the
>> difference between your polygon and the 'store buffer'? Alternatively, you
>> can shrink your entire polygon. See `rgeos` package, say `gBuffer`
>> function.
>>
>
> Yes, you use gBuffer with a negative distance.:
>
>  > require(rgeos)
>  > p2 = readWKT("POLYGON((2 2,-2 2,-2 -2,2 -2,2 2),(1 1,-1 1,-1 -1,1 -1,1
> 1))")
>  > plot(p2)
>  > plot(gBuffer(p2,width=-0.2),add=TRUE,col="red")
>
>

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