interp function

Interpolate Biogas Composition or Cumulative Production

interp interpolates (or extrapolates) biogas composition (methane concentration) or cumulative production data to a wanted time using one of several possible methods.

interp(times, y, time.out, method = "linear", extrap = FALSE)

Arguments

• times: measurement times. Numeric vector or POSIX.
• y: response variable at times, e.g., mole fraction of methane in biogas, or biogas cumulative production. Numeric vector.
• time.out: time or times at which interpolated values are needed. Numeric vector or POSIX.
• method: method used for interpolation. Default is "linear", which uses the approx function. Use "f1" for to always use nearest value to right (see f = 1 argument in approx). For spline interpolation, use "ffm" or the other options available (see spline).
• extrap: should y be exptrapolated? Logical. Default is FALSE. See Details .

Details

interp is really a wrapper for the interpolation functions approx and spline. For cumulative production, which (usually) must monotonically increase, method = "hyman" is the best choice. Extrapolation behavior depends on method. For method = "linear", extrap = TRUE simply returns the value of the closest y. For manometric measurement with mix of venting/no venting with composition measurements only when venting use method = "f1". See approx and spline for more information.

Returns

Interpolated estimates of y at given times.

Author(s)

Sasha D. Hafner and Charlotte Rennuit

See Also

cumBg

Examples

# Fake composition data 
dat <- data.frame(time = c(1, 7, 14, 28), 
                  xCH4 = c(0.3, 0.5, 0.61, 0.65))
interp(dat$time, dat$xCH4, time.out = 10)
interp(dat$time, dat$xCH4, time.out = 10, method = "natural")
interp(dat$time, dat$xCH4, time.out = c(10, 30))
interp(dat$time, dat$xCH4, time.out = c(10, 30), method = "natural")
interp(dat$time, dat$xCH4, time.out = c(10, 30), extrap = TRUE)

# Actual data
data(comp)
# Work with one reactor
bgc <- subset(comp, id=="2_1")

# With numeric time, interpolate to 1, 7, and 30 days
interp(bgc$days, bgc$xCH4, time.out = c(1, 7, 30))

# If extrapolation is OK
interp(bgc$days, bgc$xCH4, time.out = c(1, 7, 30), extrap = TRUE)

# Or POSIXct
interp(bgc$date.time, bgc$xCH4, 
       time.out = as.POSIXct("2014-07-12 13:00:00"))

# For cumulative gas production
data(vol)
# Work with one reactor
bgv <- subset(vol, id=="2_1")
# Calculate cumulative volume
bgv <- cumBg(bgv, time.name = "days")
# The interpolate of cumulative production to 1, 7, and 30 days
interp(bgv$days, bgv$cvBg, time.out = c(1, 7, 30), method = "hyman")