A partial component of ddfilter, although works as a stand-alone function.
This function removes locations by speed, inner angle, and quality index as described in Shimada et al. (2012).
ddfilter_loop(sdata, qi = 4, ia = 90, vmaxlp = 1.8)A data frame containing columns with the following headers: "id", "DateTime", "lat", "lon", "qi".
The function filters the input data by the unique "id".
"DateTime" is date & time in class POSIXct.
"lat" and "lon" are the latitude and longitude of each location in decimal degrees.
"qi" is the numerical quality index associated with each location fix where a greater number indicates a higher accuracy
(e.g. number of GPS satellites used for estimation).
An integer specifying a threshold of quality index, which is used to evaluate the locations of loop trips. Default is 4.
An integer specifying a threshold of inner angle, which is used to evaluate the locations of loop trips. Default is 90 degrees.
A numeric value specifying a threshold of speed, which is used to evaluate the locations of loop trips.
Default is 1.8 km/h.
If this value is unknown, it can be estimated from sdata using the function vmaxlp.
The input data is returned without locations identified by this filter. The following columns are added: "pTime", "sTime", "pDist", "sDist", "pSpeed", "sSpeed", "inAng". "pTime" and "sTime" are hours from a previous and to a subsequent fix respectively. "pDist" and "sDist" are straight distances in kilometres from a previous and to a subsequent fix respectively. "pSpeed" and "sSpeed" are linear speed from a previous and to a subsequent fix respectively. "inAng" is the degree between the bearings of lines joining successive location points.
This function removes locations if all of the following criteria apply:
the number of source satellites are less than or equal to qi,
the inner angle is less than and equal to ia and the speed either from a previous or to a subsequent location exceeds vmaxlp.
If vmaxlp is unknown, it can be estimated using the function vmaxlp.
Shimada T, Jones R, Limpus C, Hamann M (2012) Improving data retention and home range estimates by data-driven screening. Marine Ecology Progress Series 457:171-180 doi:10.3354/meps09747