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R function to pick out high and low tides from a dataset of tidal water levels
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| ### Important note: I'm no longer maintaining this gist, since this code is incorporated into the | |
| ### VulnToolkit package. | |
| ### Because it's not being maintained, the code below might not work. The latest, updated version | |
| ### is available at https://github.com/troyhill/VulnToolkit/blob/master/R/HL.r | |
| ### Function to pick out high and low tides from a dataset of tidal water levels | |
| ### | |
| ### Arguments: | |
| ### - "level" is a numeric vector of water levels. | |
| ### - "time" is a vector (numeric or POSIX*) indicating the time of water level measurements. Units must be minutes. | |
| ### - "period" is a single numeric or integer estimate of tidal period (full tidal cycle). Units must be hours. | |
| ### - "phantom" (TRUE by default) is a protective measure taken to prevent the inclusion of an artificial | |
| ### high or low tide at the end of the dataset. If the water level measurements | |
| ### end precisely at a low or high tide, this can be changed to FALSE. | |
| ### - "tides" is used to optionally subset the output to include only high or low tides. | |
| ### This argument can be "all" (default), "H", or "L" | |
| ### | |
| ### Function returns a 3-column dataframe with date/time, water level, and a H/L indicator. | |
| ### | |
| ### Have a problem? Find a bug? Email Hill.Troy@gmail.com | |
| HL <- function(level, time, period = 13, phantom = TRUE, tides = "all") { | |
| # Check arguments | |
| if(is.numeric(level) == FALSE) | |
| stop("invalid entry: 'level' must be numeric") | |
| if(is.numeric(time) == FALSE & class(time)[1] != "POSIXlt" & class(time)[1] != "POSIXct" ) | |
| stop("invalid entry: 'time' must be numeric or POSIX*") | |
| if(is.numeric(period) == FALSE & class(period) != "integer") | |
| stop("invalid entry: 'period' must be numeric") | |
| if(tides != "all" & tides != "L" & tides != "H") | |
| stop("invalid entry: 'tides' must be 'all', 'H', or 'L'") | |
| # Set locals | |
| partial.tide <- period * 60 * 60 # seconds | |
| t.int <- as.numeric( time[2] ) - as.numeric( time[1] ) # seconds | |
| wll.2 <- data.frame(1:length(level), level, time) | |
| width <- partial.tide / t.int | |
| # Find high tides | |
| t.x <- which.max(level[1:(1 + width)]) | |
| for(i in 1:length(level)) { | |
| if(!is.na(t.x[i] + 0.5 * width)) { | |
| temp <- which.max(level[(t.x[i] + 0.5 * width) : (t.x[i] + 1.25 * width)]) | |
| val <- temp - 1 + (t.x[i] + 0.5 * width) | |
| t.x <- rbind(t.x, val) | |
| } else t.x <- t.x | |
| } | |
| ht <- wll.2[t.x[,1], ] # filters high tides from initial dataset, including dates/times | |
| ht$tide <- rep("H", nrow(ht)) | |
| # Repeat process for low tides | |
| t.y <- which.min(level[1:(1 + width)]) | |
| for(i in 1:length(level)) { | |
| if(!is.na(t.y[i] + 0.5 * width)) { | |
| temp <- which.min(level[(t.y[i] + 0.5 * width) : (t.y[i] + 1.25 * width)]) | |
| val <- temp - 1 + (t.y[i] + 0.5 * width) | |
| t.y <- rbind(t.y, val) | |
| } else t.y <- t.y | |
| } | |
| lt <- wll.2[t.y[,1], ] | |
| lt$tide <- rep("L", nrow(lt)) | |
| if(tides == "H") { | |
| hl <- ht[,-1] # removes index variable | |
| rownames(hl) <- seq(length=nrow(hl)) # re-numbers rows, mostly for aesthetics | |
| if(phantom == TRUE) { | |
| hl <- hl[-nrow(hl), ] | |
| } else if (phantom == FALSE) { | |
| hl <- hl | |
| } else stop("invalid entry: 'phantom' must be 'TRUE' or 'FALSE' ") | |
| invisible(hl) | |
| } else if(tides == "L") { | |
| hl <- lt[,-1] | |
| rownames(hl) <- seq(length=nrow(hl)) | |
| if(phantom == TRUE) { | |
| hl <- hl[-nrow(hl), ] | |
| } else if (phantom == FALSE) { | |
| hl <- hl | |
| } else stop("invalid entry: 'phantom' must be 'TRUE' or 'FALSE' ") | |
| invisible(hl) | |
| } else if(tides == "all") { | |
| hl <- rbind(ht, lt) | |
| hl <- hl[order(hl[,1]),] | |
| hl <- hl[,-1] | |
| rownames(hl) <- seq(length=nrow(hl)) | |
| if(phantom == TRUE) { | |
| hl <- hl[-nrow(hl), ] | |
| } else if (phantom == FALSE) { | |
| hl <- hl | |
| } else stop("invalid entry: 'phantom' must be 'TRUE' or 'FALSE' ") | |
| invisible(hl) | |
| } | |
| } |
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