# [R] Issue with asin()

Sarah Goslee sarah.goslee at gmail.com
Mon Mar 19 15:31:17 CET 2012

```Hi,

You're not following the algorithm as given. The asin step shouldn't
be done for all values, but only for the ones that don't meet the
previous conditions. You're trying to calculate that step for ALL
values, then only use certain ones. You must instead subset the
values, THEN calculate that step. I would guess that your "working"
Excel version does follow the correct algorithm, but it's hard to know
for certain.

Here's a version that more closely follows the given reference:

MaxDailyTemp <- values_max
MinDailyTemp <- values_min
k <- 0

GDD <- rep(0, length(Tmin))
AvgDailyTemp <- (MaxDailyTemp + MinDailyTemp)/2

# if MaxDailyTemp < k
# GDD = GDD + 0

# if MaxDailyTemp > k & MinDailyTemp > k
# GDD = GDD + AvgDailyTemp - k
GDD[MaxDailyTemp > k & MinDailyTemp > k] <- AvgDailyTemp[MaxDailyTemp
> k & MinDailyTemp > k] - k

# if MaxDailyTemp > k & MinDailyTemp < k
# GDD = GDD + (1/pi) * [ (AvgDailyTemp – k) * ( ( pi/2 ) – arcsine(
theta ) ) + ( a * cos( arcsine( theta ) ) ) ]
a <- (MaxDailyTemp - MinDailyTemp)/2
theta <- ((k - AvgDailyTemp)/a)
GDD[MaxDailyTemp > k & MinDailyTemp < k] <- (1/pi) * (
(AvgDailyTemp[MaxDailyTemp > k & MinDailyTemp < k] - k) * ( ( pi/2 ) -
asin( theta[MaxDailyTemp > k & MinDailyTemp < k] ) ) + (
a[MaxDailyTemp > k & MinDailyTemp < k] * cos( asin( theta[MaxDailyTemp
> k & MinDailyTemp < k] ) ) ) )

sum(GDD)

Sarah

On Mon, Mar 19, 2012 at 7:42 AM, Letnichev <chatelain.phil at gmail.com> wrote:
> Hello everyone,
>
> I am working for a few days already on a basic algorithm, very common in
> applied agronomy, that aims to determine the degree-days necessary for a
> given individual to reach a given growth stade. The algorithm (and context)
> is explained here:  http://www.oardc.ohio-state.edu/gdd/glossary.htm , and
> so I implemented my function in R as follows:
>
> DD <- function(Tmin, Tmax, Tseuil, meanT, method = "DDsin")
> ### function that calculates the degree-days based on
> ### minimum and maximum recorded temperatures and the
> ### minimal threshold temperature (lower growth temperature)
>        {
>        ### method arcsin
>        if(method == "DDsin"){
>                cond1 <- (Tmax <= Tseuil)
>                cond2 <- (Tmin >= Tseuil)
>                amp <- ((Tmax - Tmin) / 2)
>                print((Tseuil-meanT)/amp)
>                alpha <- asin((Tseuil - meanT) / amp)
>                DD_ifelse3 <- ((1 / pi) * ((meanT - Tseuil) * ((pi/2) - alpha)) +
> amp*cos(alpha))
>
>                DD <- ifelse(cond1, 0, ifelse(cond2, (meanT - Tseuil), DD_ifelse3))
>        }
>
>        ### method (Tmin + Tmax) / 2
>        else if(method == "DDt2"){
>                cond1 <- (meanT > Tseuil)
>                DD <- ifelse(cond1,(meanT - Tseuil),0)
>                }
>
>        else{
>                stop("\nMethod name is invalid.\nMethods available = DDsin (sinus) or DDt2
> (mean)\n")
>                }
>        return(DD)
> }
>
> BUT! When I try to process random data:
>
> library(reshape2)
> library(plyr)
>
> station <- rep(c("station1","station2","station3"), 20)
> values_min <- sample(-5:20, size = 60, replace = T)
> values_max <- sample(20:40, size = 60, replace = T)
> meanT <- ((values_min+values_max)/2)
> d <- data.frame(station,values_min,values_max,meanT)
> names(d) <- c("station", "values_min","values_max","meanT")
>
> x<-ddply(d, .(station), transform, t1 =
> cumsum(DD(values_min,values_max,0,meanT)))
>
> I get a warning on my alpha calculation (NaN produced); indeed, the values I
> give as argument to asin() are out of the range [-1:1], as the print()
> reveals. I can't figure out how to solve this issue, because the same
> algorithm works in Excel (visual basic).
> It is very annoying, especially because it seems that no occurence of such
> error using that algorithm can be found on Internet.
> Any help is welcome :) Thanks for your time
>
> P.
>

--
Sarah Goslee
http://www.functionaldiversity.org

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