A question about the "extract" function in package "raster"

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Hi Robert,

If I understood correctly, the mechanism behind the "extract" function is
that,  ever cell has a center with a coordinate. The "extract" function will
calculate the distance between the sampling point and nearby cell center. If
the distance less than the buffer, then this cell will be include to
calculate the area mean or other "fun" argument defined functions. 

This function is really helpful for me.

Thanks for your explanation.

Cheers,
Richard

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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-----
Richard Ma
PhD student, Ecology & Remote Sensing
Climate Change Cluster, Department of Environment Science
University of Technology, Sydney
http://everydropr.wordpress.com
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