argosfilter (version 0.70)

vmask: Filter locations for speed

Description

This function filters location data obtained from Argos, using the MacConnell al. (1992) algorithm.

Usage

vmask(lat, lon, dtime, vmax)

Value

Returns a vector with the following elements: "removed" (location removed by the filter),

"not" (location not removed) and "end_location" (location at the end of the track where the algorithm could not be applied).

Arguments

lat

a numeric vector of latitudes, in decimal degrees

lon

a numeric vector of longitudes, in decimal degrees

dtime

a vector of class POSIXct with date and time for each location

vmax

speed threshold, in m/s

Author

Carla Freitas

Details

Locations are filtered using the algorithm described in MacConnell et al. (1992). Since this algorithm calculates for each location the root mean square (rms) of the speeds to the previous, 2nd previous, next and 2nd next location, high swimming speeds can be obtained for points that are adjacent to outlier locations. Therefore, when the algorithm is applied to a set of locations, only the peaks in rms (that are above the maximum speed) are removed. Other locations are not removed even if above the speed limit. rms is then recalculated n times until all locations are below the speed threshold.

References

McConnell, B.J., Chambers, C. and Fedak, M.A. (1992) Foraging ecology of southern elephant seals in relation to the bathymetry and productivity of the Southern Ocean. Antarctic Science 4:393-398.

See Also

sdafilter

Examples

data(seal)
lat<-seal$lat
lon<-seal$lon
dtime<-seal$dtime
lc<-seal$lc

# filter by speed
mfilter<-vmask(lat,lon,dtime,2)

# plot unfiltered (grey) and filtered data (green)
plot(lon,lat,col="lightgrey",type="l",xlim=c(5,18), 
ylim=c(77.1,79.1),xlab="Longitude",ylab="Latitude")
lines(lon[which(mfilter=="not")],lat[which(mfilter=="not")],col="darkgreen")