On Sat, Aug 13, 2011 at 9:35 PM, Xuanlong Ma <Xuanlong.Ma@uts.edu.au> wrote:

> Hi all,
>
> Suppose I have a raster object, its resolution is 0.25km. Now I want to
> extract the value with a spatial point. There is a argument in "extract"
> function called "buffer" to set the sampling "radius". If I want to
> sampling
> a 1 km^2 area in my 0.25km resolution raster. How to set the "buffer"? Is
> that "buffer = 500"?
>
>
Richard,

A circular buffer with a radius of 500 m gives you an area of   pi * 500^2
If you want an area of 1000m2 you  should use a radius of  sqrt(1000 / pi)

Note that in this case, a cell is "inside" the buffer if the buffer overlaps
 with the cell center.

Hope this helps,
Robert



> Just very curious how the "buffer" works in grid image. I have read the
> document and still very confused.
>
> Thanks for your kindly help!
>
> BTW, "raster" is really a great package with lots of useful functions for
> image processing. Thank you Robert!
>
> Regards,
> Richard
>
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