# Applying Savitzky-Golay filters in “R”

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When applying SG, we select a moving average window with an odd value

**“n”**for the number of data points. SG fit a polynomial of**“p”**degree to this data points and give the value to the central point (this is the reason to have an odd value).We apply also an smooth in the case of

**“m”**= 0, or the first (m=1), second (m=2) or third (m=3) derivatives.**Apply a Savitzky-Golay smoothing filter**

Description

Smooth data with a Savitzky-Golay smoothing filter.

Usage

sgolayfilt(x, p = 3, n = p + 3 – p%%2, m = 0, ts = 1)

## S3 method for class ‘sgolayFilter’

filter(filt, x, …)

Arguments

x | signal to be filtered. |

p | filter order. |

n | filter length (must be odd). |

m | return the m-th derivative of the filter coefficients. |

ts | time scaling factor. |

filt | filter characteristics (normally generated by sgolay). |

Playing with these three parameter we change the shape of the spectra.

Example:

The upper spectra was taken with p=2, n=7 and m=2

The lower one with p=2, n=31 and m=2

As we can see the size of the window has a big influence on the resolution of the peaks.

How to apply it in R:

(For the lower spectrum)

**>sgolay_2_31_2<-apply(fattyac$NITm,1,sgolayfilt,p=2,n=31,m=2)**

**>matplot(wavelengths,sgolay_2_31_2,lty=1,pch=21,**

**+ xlab=”nm”,ylab=”SG_2_31_2_abs”)**

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