[R] approxfun returning value higher than I would expect

[stephen sefick]

Below is the code that I am using in a much larger function.  I would
expect a bankfull measure at zero to be between 0.6 and 0.8 approxfun
is returning 0.8136986.  I am sure that I am missing something.

measure_bkf <- (structure(list(measurment_num = c(0, 0.2, 0.4, 0.6,
0.8, 1, 1.2,
1.4, 1.6, 1.8, 2, 2.2, 2.4, 2.6, 2.8, 3, 3.2, 3.4), bankfull_depths_m
= c(-0.48,
-0.48, -0.42, -0.26, 0.58, 0.48, 0.47, 0.54, 0.5, 0.52, 0.52,
0.56, 0.58, 0.61, 0.68, 0.62, 0.67, 0.66)), .Names = c("measurment_num",
"bankfull_depths_m"), row.names = c("6124", "612", "613", "614",
"615", "616", "617", "618", "619", "620", "621", "622", "623",
"624", "625", "626", "627", "628"), class = "data.frame"))


measure_bkf_not_zero <- subset(measure_bkf, measure_bkf$bankfull_depths_m!=0)

bkf_min <- which.max(measure_bkf_not_zero[,"bankfull_depths_m"]<0)

bkf_max <- which.max(measure_bkf_not_zero[,"bankfull_depths_m"])

#bkf_min <- ifelse(length(bkf_min)>1, bkf_min[1], bkf_min)
#bkf_max <- ifelse(length(bkf_max)>1, bkf_max[1], bkf_max)

#s <- with(measure_bkf_not_zero, approx(measurment_num,
bankfull_depths_m,
xout=seq(measure_bkf_not_zero[bkf_min,"measurment_num"],
measure_bkf_not_zero[bkf_max,"measurment_num"], length=2000)))
#int_bkf <- with(s, x[which.min(y[y>0])])

s <- with(measure_bkf_not_zero[bkf_min:bkf_max,],
approxfun(bankfull_depths_m, measurment_num), ties=mean)

int_bkf <- s(0)

-- 
Stephen Sefick
____________________________________
| Auburn University                                   |
| Department of Biological Sciences           |
| 331 Funchess Hall                                  |
| Auburn, Alabama                                   |
| 36849                                                    |
|___________________________________|
| sas0025 at auburn.edu                             |
| http://www.auburn.edu/~sas0025             |
|___________________________________|

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