Fit a smooth closed shape through 4 points

-----Original Message-----
From: R-help [mailto:r-help-bounces at r-project.org] On Behalf Of Alexander
Shenkin
Sent: 21 March 2016 14:04
To: r-help
Subject: Re: [R] Fit a smooth closed shape through 4 points

Thanks for your reply, Charles.  spline() doesn't seem to fit a closed shape;
rather, it's producing a parabola.  Perhaps I'm missing an argument I should
include?

grid.xspline() seems to get close to what I need, but it returns a grob object -
not sure how to work with those as shapes per se.

My goal is to produce a 2D shape from which I can calculate area, average
widths, and other such things.  The context is that we have measured tree
crowns in a manner that has produced 4 points such as these from two offset
axes.  We want to use the resulting shapes for our calculations.

(incidentally, my original points were off - here are the correct ones)

shapepoints = structure(list(x = c(8.9, 0, -7.7, 0, 8.9), y = c(0, 2, 0, -3.8, 0)),
.Names = c("x", "y"), row.names = c(NA, -5L), class = "data.frame")

plot(spline(shapepoints))

Thanks,
Allie

On 3/21/2016 1:10 PM, Charles Determan wrote:

Hi Allie,

What is you goal here?  Do you just want to plot a curve to the data?
Do you want a function to approximate the data?

You may find the functions spline() and splinefun() useful.

Quick point though, with so few points you are only going to get a
very rough approximation no matter the method used.

Regards,
Charles


On Mon, Mar 21, 2016 at 7:59 AM, Alexander Shenkin <ashenkin at ufl.edu
<mailto:ashenkin at ufl.edu>> wrote:

    Hello all,

    I have sets of 4 x/y points through which I would like to fit
    closed, smoothed shapes that go through those 4 points exactly.
    smooth.spline doesn't like my data, since there are only 3 unique x
    points, and even then, i'm not sure smooth.spline likes making
    closed shapes.

    Might anyone else have suggestions for fitting algorithms I could
    employ?

    Thanks,
    Allie


    shapepoints = structure(c(8.9, 0, -7.7, 0, 0, 2, 0, 3.8), .Dim = c(4L,
    2L), .Dimnames = list(NULL, c("x", "y")))

    smooth.spline(shapepoints)

    # repeat the first point to close the shape
    shapepoints = rbind(shapepoints, shapepoints[1,])

    smooth.spline(shapepoints)