[R] Finding Lower Envelope of Points on a Plot?

[Stephan Kolassa]

Hi,

one possibility would be to calculate the convex hull using chull(). I 
believe that the hull points are returned by chull() in a clockwise 
order (?), so the points between the rightmost and the leftmost point in 
the chull() result are the lower half of the convex hull. Remove these 
points from the original dataset (a variant of "peeling convex hulls") 
and iterate until you have removed your prespecified percentage of 
points - these all lie outside of the final lower hull (though the 
percentage will, of course, only be approximated, but you should be able 
to modify this to taste).

HTH
Stephan


David Winsemius schrieb:
> 
> On Jun 30, 2010, at 2:05 PM, Asha Sharma wrote:
> 
>> Hi,
>>
>> I am looking for a way to find the lower envelope of points on a plot, 
>> preferably specifying what percentage of points should be allowed to 
>> lie outside the envelope. There must be a straightforward way to do 
>> this, but I do not seem to be able to find it. I would greatly 
>> appreciate any help.
> 
> You probably want something like the "lower half" of the convex hull. 
> You should find quite a bit of code with your favorite r search engine 
> on the topic of "convex hull".