[R] Spatstat, markcorr, max. radius limited??

[Rolf Turner]

On 21/11/2009, at 12:01 AM, rudi1978 at gmx.de wrote:

> Hello,
>
> could please somebody help me. I want to apply the mark correlation  
> function but for radii up to 75 meters (in 75 individual 1m steps).  
> Unfortunately, "There is a sensible default for the values of the  
> argument r at which the mark correlation function kf (r) should be  
> evaluated", so that I do not get results for r > 25m. Can I change  
> something in the function to get the required values as written above?

Questions about contributed packages should be addressed, in the  
first instance,
to the maintainer(s) of the package rather than to the R-help list.

I am one of the maintainers, but I'm afraid that I don't know/ 
understand a lot about
the mark correlation function, and Adrian --- the other maintainer,  
who does know a lot ---
is off on holidays, and won't be back till 7 December.

In the mean time let me just say that ``There is a sensible default''  
is probably
an understatement.  The default is what you ***should*** use.  Trying  
to set
your own value of ``r'' will probably give you nonsense.  This is my  
understanding
of the situation, anyway.

That being said --- how do you *know* that you do not get results for  
r > 25?
Did you just plot the output from markcorr()?  There is also a  
``sensible default''
for the x-axis limits, which is used *unless xlim is specified* in  
your call to
plot().  Did you try specifying xlim to extend to something about r =  
25?

And ***that*** being said, you probably shouldn't!  Given that the  
``sensible
default'' for xlim is c(0,25) then it seems to me very likely that  
***it is simply
not meaningful*** to consider the mark correlation function for r > 25.

I cannot explain to you *why* it is not meaningful, but I'm sure that  
were this
not the case Adrian would not have written the function this way.   
When he gets
back from hols Adrian may be able to explain to you just why it is  
not meaningful
to consider r > 25 in your setting.

Finally let me draw to your attention a quote from the late great  
John Tukey:

> The combination of some data and an aching desire for an answer does
> not ensure that a reasonable answer can be extracted from the given
> body of data.


The fact that you have an aching desire to know the value of the mark
correlation function at values of r > 25 does not guarantee that  
reasonable
estimates of these values can be found from the data that you have
available.

	cheers,

		Rolf Turner

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