[R-sig-Geo] [FORGED] Re: [FORGED] kernel density: units of bandwidth

[Rolf Turner]

On 23/02/16 10:34, Tadaishi Yatabe-Rodriguez wrote:

> Thanks, Rolf. This ,makes it clearer. I've another question though: the
> ppp object I get the Kernel density from is not projected, then what
> would be the unit of projection of this un-projected object? The
> projection of the object from which it was created from (e.g.
> SpatialPoints object)?
>
> Thanks again,

You should probably keep this discussion on-list; others who subscribe 
to the list are much more knowledgeable than I and may have useful 
contributions to make (and may correct my errors).  I have taken the 
liberty of including the list in my reply.

I think that your question is not well-posed.  The density.ppm() 
function treats the coordinates of points as Euclidean coordinates (with 
the same units on both the x and y axes).  If your point pattern is 
"un-protected" then I presume that x and y are in longitude and latitude 
in which case the coordinates are not Euclidean and the x and y axis 
units are different, since longitude and latitude are constructed in 
different ways.

If the region in question is "small" (and not too near either pole, I 
guess) then this probably doesn't make much difference, and the units of 
sigma would, roughly speaking, be "degrees".  Otherwise it seems to me 
there is no meaningful answer to your question.

It would probably be best to project your data before forming the ppp 
object.

Ege has been doing some work on point patterns on the sphere which might 
possibly be of relevance here.  He may feel like chiming in on this issue.

cheers,

Rolf

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Technical Editor ANZJS
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