R help - Mar 2012 - How to fit a line through the "Mountain crest", i.e., through the highest density of points - in a "loess-like" fashion.

Hi,

I'm trying to normalize data by fitting a line through the highest density
of points (in a 2D plot).
In other words, if you visualize the data as a density plot, the fit I'm
trying to achieve is the line that goes through the "crest" of the
mountain.

This is similar yet different to what LOESS does. I've been using loess
before, but it does not exactly that as it takes into account all points.
Although points farther from the fit have a smaller weight, they result in
the fit being a bit off the crest.

Do you know a package or maybe even an option in loess that would allow me
achieve this?
Any advice or idea appreciated.

Emmanuel

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On Mar 10, 2012, at 3:55 PM, Emmanuel Levy wrote:
> Hi,
>
> I'm trying to normalize data by fitting a line through the highest  
> density
> of points (in a 2D plot).
> In other words, if you visualize the data as a density plot, the fit  
> I'm
> trying to achieve is the line that goes through the "crest" of
the
> mountain.
Are you familiar with the kde2d  of bkde2D functions in various  
packages? If you then collected the max density for each X and Y you  
might want to see whether that 2-d function would follow a  
sufficiently regular path that would represent the projection of the  
ridge on the z=0 plane.
>
> This is similar yet different to what LOESS does.
Do you want a curve or a line?
> I've been using loess
> before, but it does not exactly that as it takes into account all  
> points.
> Although points farther from the fit have a smaller weight, they  
> result in
> the fit being a bit off the crest.
>
> Do you know a package or maybe even an option in loess that would  
> allow me
> achieve this?
I don't. I happen to have a dataset where I could test it. But you are  
likely to get better responses if you provide a test case.
> Any advice or idea appreciated.
>
> Emmanuel
>
> 	[[alternative HTML version deleted]]
Plain text is preferred.

-- 

David Winsemius, MD
West Hartford, CT