[R] loess line predicting number where the line crosses zero twice

[Peter Alspach]

Tena koe Stephen

You'll need to use loess(w[,"bkf_depths"]~w[,"measure"])) and predict with a sufficiently dense sequence of w[,'measure'] to interpolate to an 'accuracy' that meets your requirements.  Actually, if I understand your data correctly, it could be that simple linear interpolation (i.e., without loess) would be sufficient (even better).

HTH ....

Peter Alspach

> -----Original Message-----
> From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-
> project.org] On Behalf Of stephen sefick
> Sent: Friday, 16 July 2010 11:33 a.m.
> To: r-help at r-project.org
> Subject: [R] loess line predicting number where the line crosses zero
> twice
> 
> These data represent stream channel cross-sectional surveys.  I would
> like to be able to find the measurement on the tape (measure) where
> the Bank Full Depth (bkf_depths) is 0.  This will happen twice because
> the channel has two sides.  I thought fitting a loess line to these
> data and then predicting the measurment number would do it.  I was
> wrong.  Below is my failed attempt.  My naive thought is this - I
> would like to run my finger along this line and when it hits zero I
> would like to pick out the value of measure. This should happen twice.
>  Any help would be greatly appreciated!
> 
> w <- (structure(list(measure = c(0, 0.2, 0.4, 0.6, 0.8, 1, 1, 1.2,
> 1.4, 1.6, 1.8, 2, 2.2, 2.4, 2.6, 2.8, 3, 3.2, 3.4, 3.6, 3.8,
> 4, 4.2, 4.4, 4.6, 4.8, 4.8, 4.84, 5, 5.2, 5.4, 5.6, 5.68), bkf_depths =
> c(0,
> 0.44, 0.46, 0.66, 0.9, 1.1, 1.1, 1.2, 1.3, 1.33, 1.36, 1.36,
> 1.36, 1.38, 1.38, 1.36, 1.36, 1.38, 1.37, 1.37, 1.34, 1.32, 1.36,
> 1.35, 1.36, 1.4, 1.4, 1.3, 0.7, 0.57, 0.21, -0.05, -0.12)), .Names =
> c("measure",
> "bkf_depths"), row.names = c(32L, 1L, 2L, 3L, 4L, 5L, 29L, 6L,
> 7L, 8L, 9L, 10L, 11L, 12L, 13L, 14L, 15L, 16L, 17L, 18L, 19L,
> 20L, 21L, 22L, 23L, 31L, 24L, 30L, 25L, 26L, 27L, 28L, 33L), class =
> "data.frame")
> )
> 
> plot(w[,"bkf_depths"]~w[,"measure"])
> lines(loess(w[,"bkf_depths"]~w[,"measure"]))
> 
> #this is what I tried and there should be two 0s one at the left side
> of the graph and one at the right side
> predict(loess(w[,"measure"]~w[,"bkf_depths"]), 0)
> 
> 
> kindest regards,
> 
> 
> --
> Stephen Sefick
> ____________________________________
> | Auburn University                                   |
> | Department of Biological Sciences           |
> | 331 Funchess Hall                                  |
> | Auburn, Alabama                                   |
> | 36849                                                    |
> |___________________________________|
> | sas0025 at auburn.edu                             |
> | http://www.auburn.edu/~sas0025             |
> |___________________________________|
> 
> Let's not spend our time and resources thinking about things that are
> so little or so large that all they really do for us is puff us up and
> make us feel like gods.  We are mammals, and have not exhausted the
> annoying little problems of being mammals.
> 
>                                 -K. Mullis
> 
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