[R] Calculate focal values for neighboring cells that are located at the raster edge from the function "focal" in the R package "raster"

[Jeremie Juste]

Hello,

>> I am using the function "focal" in the R package "raster" but I don’t
>> understand how the function calculates values for neighboring cells that
>> are located at the raster edge. Here is an example reproducible:

focal is a method for the S4 object RasterLayer. You can have access to
this method by using 

> findMethods("focal")

and you can trace the method by using
> trace(what='focal', tracer=browser, at=1, signature='RasterLayer')

HTH,

Jeremie






>> }
>>
>> r <- raster(ncol=3, nrow=3)
>> vals <- 1:ncell(r)
>> r <- setValues(r, vals)
>> plot(r)
>>
>> func_f  <- focal(r, w=matrix(1,nrow=3,ncol=3),
>>                    fun= func, pad = TRUE, padValue = NA)
>> text(func_f )
>> getValues(func_f)
>>
>>
>> From the example, I manage to find the value “2.06066”:
>>
>> c <- 5
>>
>> (abs(1-c)*(1/sqrt(2)) + abs(2-c)*1 + abs(3-c)*(1/sqrt(2)) +
>>
>>   abs(4-c)*1 + abs(5-c)*0 + abs(6-c)*1 +
>>
>>   abs(7-c)*(1/sqrt(2)) + abs(8-c)*1 + abs(9-c)*(1/sqrt(2))) / 8
>>
>>
>>  but I don’t manage to find the value “2.18566”. How can I find this value
>> ? In addition, why does the function with “pad = TRUE” and without
>> “pad=TRUE” give the same result ?
>>
>>  Thank you very much for your time.
>>
>> Have a nice day
>>
>> Marine
>>
>>
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>>
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>>
>
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> ______________________________________________
> R-help using r-project.org mailing list -- To UNSUBSCRIBE and more, see
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> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.