R/utils-outliers.R

In AirSensor: Process and Display Data from Air Quality Sensors

#' @keywords internal
#' @title Flag outliers in vectorized data
#' 
#' @description This function uses Hampel filter outlier detection to flag
#' outliers in \code{parameter} column of the incoming dataframe. The 
#' \code{windowSize} and \code{thresholdMin} parameters as passed on to the
#' \code{seismicRoll::findOutliers} function.
#' 
#' An additional boolean column named \code{<parameter>_outlierFlag} is added
#' to the dataframe. This column will have \code{TRUE} whenever an outlier is
#' detected for the chosen \code{parameter}.
#' 
#' See \code{seismicRoll::findOutliers} for further details.
#' 
#' @param df A data frame.
#' @param parameter The data frame parameter to use for outlier detection.
#' @param windowSize The size of the rolling window.
#' @param thresholdMin The minimum threshold value used to detect outliers.
#' 
#' @return A dataframe with an additional column identifying outliers.
#' 

.flagOutliers <- function(
  df = NULL, 
  parameter = NULL,
  windowSize = 23,
  thresholdMin = 8
) {
  
  # ----- Validate parameters --------------------------------------------------
  
  MazamaCoreUtils::stopIfNull(df)
  MazamaCoreUtils::stopIfNull(parameter)
  
  # ----- Flag outliers --------------------------------------------------------
  
  data <- df[[parameter]]
  outlierFlagName <- paste0(parameter, "_outlierFlag")
  
  # Identify outliers
  result <- try({
    outlierIndices <- 
    seismicRoll::findOutliers(
      x = data, 
      n = windowSize,
      thresholdMin = thresholdMin
    )}, silent = TRUE)
  
  if ( 'try-error' %in% class(result) ) {
    df[[outlierFlagName]] <- FALSE # if error with hampel filter ignore outlier flagging
  } else {
    # Make a new logical column
    df[[outlierFlagName]] <- FALSE 
    df[[outlierFlagName]][outlierIndices] <- TRUE
  }
  return(df)
  
}

#' @keywords internal
#' @title Replace outliers with rolling median
#' 
#' @description Perform smoothing by replacing outliers with the window median 
#' value. Using this technique, you can create an highly smoothed, artificial 
#' dataset by setting \code{thresholdMin = 1} or lower (but always above zero).
#' 
#' @param df A data frame.
#' @param parameter The data frame parameter to check and replace outliers.
#' @param medWin The size of the rolling median window.
#' @param ... Parameters to extend \code{flagOutliers}.
#' 
#' @return A \code{data.frame} with replaced outliers
#' 

.replaceOutliers <- function(
  df = NULL,
  parameter = NULL, 
  medWin = 7,
  ...
) { 
  
  # ----- Validate parameters --------------------------------------------------
  
  MazamaCoreUtils::stopIfNull(df)
  MazamaCoreUtils::stopIfNull(parameter)
  
  df <- .flagOutliers(
    df = df, 
    parameter = parameter,
    ...
  )
  
  outlierFlagName <- paste0(parameter, "_outlierFlag")
  
  roll_med <- function(x, n) seismicRoll::roll_median(x, n)
  
  flagged <- which(df[[outlierFlagName]])
  
  df[[outlierFlagName]] <- NULL
  
  df[[parameter]][flagged] <- 
    roll_med(df[[parameter]], medWin)[flagged]
  
  return(df)
  
}


#' @keywords internal
#' @title Plot flagged outliers
#' 
#' @description An internal capability to quickly plot outlier data. 
#'
#' @param df data frame that contains datetime, PM2.5, and outlier boolean flags
#' @param parameter the parameter with the associated outliers flag
#' @param ylim y axis limits
#' @param xlab x axis label
#' @param ylab y axis label
#' @param title plot title
#' @param subtitle plot substitle
#' @param data_shape shape of points 
#' @param data_size size of points
#' @param data_color color of points
#' @param data_alpha alpha of points
#' @param outlier_shape outlier point shape
#' @param outlier_size outlier point size
#' @param outlier_color outlier point color
#' @param outlier_alpha outlier point alpha
#'
#' @return gg object
#' 
.plotOutliers <- function(
  df = NULL, 
  parameter = "pm25",
  ylim = NULL,
  xlab = "Date", 
  ylab = "ug / m3", 
  title = "default", 
  subtitle = NULL,
  data_shape = 18, 
  data_size = 1, 
  data_color = "black",
  data_alpha = 0.5,
  outlier_shape = 8, 
  outlier_size = 1, 
  outlier_color = "red",
  outlier_alpha = 1.0
) {
  
  # ----- Validate parameters --------------------------------------------------
  
  MazamaCoreUtils::stopIfNull(df)

  # ----- Create df_outliers ---------------------------------------------------
  
  col_flag <- names(df)[which(stringr::str_detect(names(df), "_outlierFlag"))]
  col_param <- names(df)[which(stringr::str_detect(names(df), parameter))][1]
  
  df_ind <- which(df[[col_flag]])
  
  # Create the outliers first (just for fun)
  df_outliers <-   
    ggplot2::geom_point(
      data =  df[df_ind,],
      mapping = ggplot2::aes(x = .data$datetime, y = .data[[col_param]]), 
      shape = outlier_shape, 
      size = outlier_size, 
      color = outlier_color,
      alpha = outlier_alpha
    )

  # ----- Create plot ----------------------------------------------------------
  
  # Now create the entire plot, adding outliers at the end
  df_plot <- 
    ggplot2::ggplot(
      data = df,
      mapping = ggplot2::aes(x = .data$datetime, y = .data[[col_param]])) + 
    ggplot2::geom_point(
      shape = data_shape,
      size = data_size,
      color = data_color,
      alpha = data_alpha
    ) + 
    ggplot2::ylim(ylim) +
    ggplot2::labs(
      x = xlab, 
      y = ylab, 
      title = 
        ifelse(
          title == "default", 
          paste0(
            "Channel ",
            ifelse(stringr::str_detect(col_param, "A") ,"A ", "B "),
            "PM2.5"
          ), 
          title
        ), 
      subtitle = subtitle
    ) + 
    ggplot2::theme(
      plot.title = ggplot2::element_text(size = 11),
      plot.subtitle = ggplot2::element_text(size = 8)
    ) + 
    df_outliers
  
  # ----- Return ---------------------------------------------------------------
  
  return(df_plot) # No sampling will occur
  
}