distfilter function

Filter locations by distance

This function removes locations that are located beyond a specified distance.

distfilter(sdata, max.dist = 100, method = 1, ia = NA)

Arguments

• sdata: A data frame containing columns with the following headers: "id", "DateTime", "lat", "lon". See the data turtle for an example. The function filters the input data by a unique "id" (e.g. transmitter number, identifier for each animal). "DateTime" is the GMT date & time of each location in class POSIXct or character with the following format "2012-06-03 01:33:46". "lat" and "lon" are the latitude and longitude of each location in decimal degrees.
• max.dist: A numeric value specifying a threshold of distance between successive locations. Default is 100 km.
• method: An integer specifying how locations should be filtered with max.dist. A location is removed if the distance from a previous and(1)/or(2) to a subsequent location exceeds max.dist. Default is 1 (both way).
• ia: An integer (0 to 180) specifying an inner angle (in degrees) between consecutive locations, beyond which the locations are considered potential outliers. Default (NA) ignores this option. See details.

Returns

The input data is returned without locations identified by this filter. The following columns are added: "pDist", "sDist", 'inAng'. "pDist" and "sDist" are straight distances in kilometres from a previous and to a subsequent fix respectively. "inAng" is the degree between the bearings of lines joining successive location points.

Details

This function removes locations if the distance from a previous and/or to a subsequent location exceeds max.dist and the inner angle is less than ia. If ia is NA (default), inner angles are not considered in the filtering.

Examples

#### Load data sets
## Fastloc GPS data obtained from a green turtle
data(turtle)

## A Map for the example site
data(Australia)
data(SandyStrait)

#### Filter temporal and/or spatial duplicates
turtle.dup <- dupfilter(turtle, step.time=1/60, step.dist=0.001)
 

#### distfilter
turtle.dist <- distfilter(turtle.dup, max.dist = 50, ia = 20)

#### Plot data removed or retained by ddfilter
## Entire area
p1 <- to_map(turtle.dup, bgmap=Australia, point.size = 2, line.size = 0.5, axes.lab.size = 0, 
            multiplot = FALSE, point.bg = "red",
            title.size=15, title="Entire area")[[1]] + 
  geom_point(aes(x=lon, y=lat), data=turtle.dist, size=2, fill="yellow", shape=21)+
  geom_point(aes(x=x, y=y), data=data.frame(x=c(154, 154), y=c(-22, -22.5)), 
             size=3, fill=c("yellow", "red"), shape=21) + 
  annotate("text", x=c(154.3, 154.3), y=c(-22, -22.5), label=c("Retained", "Removed"), 
           colour="black", size=4, hjust = 0)

## Zoomed in
p2 <- to_map(turtle.dup, bgmap=SandyStrait, xlim=c(152.7, 153.2), ylim=(c(-25.75, -25.24)), 
            axes.lab.size = 0, point.size = 2, point.bg = "red", line.size = 0.5, 
            multiplot = FALSE, title.size=15, title="Zoomed in")[[1]] + 
geom_path(aes(x=lon, y=lat), data=turtle.dist, linewidth=0.5, colour="black", linetype=1) + 
geom_point(aes(x=lon, y=lat), data=turtle.dist, size=2, colour="black", shape=21, fill="yellow")

gridExtra::marrangeGrob(list(p1, p2), nrow=1, ncol=2)

Author(s)

Takahiro Shimada