[R] Fit a smooth closed shape through 4 points
Thierry Onkelinx
thierry.onkelinx at inbo.be
Mon Mar 21 16:44:55 CET 2016
Dear Allie,
You could use elliptic fourier analysis
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")
shapepoints$Theta <- seq(0, 2 * pi, length = nrow(shapepoints))
model <- lm(cbind(x, y) ~ I(sin(Theta)) + I(cos(Theta)), data = shapepoints)
model2 <- lm(cbind(x, y) ~ I(sin(Theta)) + I(cos(Theta)) + I(sin(2 *
Theta)) + I(cos(2 * Theta)), data = shapepoints)
newdata <- data.frame(Theta = seq(0, 2 * pi, length = 41))
plot(predict(model, newdata = newdata), type = "l", xlim = c(-10, 10), ylim
= c(-10, 10))
points(shapepoints, col = "red")
plot(predict(model2, newdata = newdata), type = "l", xlim = c(-10, 10),
ylim = c(-10, 10))
points(shapepoints, col = "red")
Best regards,
Thierry
ir. Thierry Onkelinx
Instituut voor natuur- en bosonderzoek / Research Institute for Nature and
Forest
team Biometrie & Kwaliteitszorg / team Biometrics & Quality Assurance
Kliniekstraat 25
1070 Anderlecht
Belgium
To call in the statistician after the experiment is done may be no more
than asking him to perform a post-mortem examination: he may be able to say
what the experiment died of. ~ Sir Ronald Aylmer Fisher
The plural of anecdote is not data. ~ Roger Brinner
The combination of some data and an aching desire for an answer does not
ensure that a reasonable answer can be extracted from a given body of data.
~ John Tukey
2016-03-21 15:04 GMT+01:00 Alexander Shenkin <ashenkin op ufl.edu>:
> 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 op ufl.edu
>> <mailto:ashenkin op 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)
>>
>> ______________________________________________
>> R-help op r-project.org <mailto:R-help op r-project.org> mailing list --
>> To UNSUBSCRIBE and more, see
>> https://stat.ethz.ch/mailman/listinfo/r-help
>> PLEASE do read the posting guide
>> http://www.R-project.org/posting-guide.html
>> and provide commented, minimal, self-contained, reproducible code.
>>
>>
>>
> ______________________________________________
> R-help op r-project.org mailing list -- To UNSUBSCRIBE and more, see
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
> http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>
[[alternative HTML version deleted]]
More information about the R-help
mailing list