speedfilter: Filter track data for speed
Description
Create a filter of a track for "bad" points implying a speed of motion
that is unrealistic.Usage
speedfilter(x, max.speed=NULL, test=FALSE)
Arguments
max.speed
speed in kilometres per hour
test
cut the algorithm short and just return first pass
Value
- Logical vector matching positions in the coordinate records that pass
the filter.
Warning
This algorithm is not considered to be particularly relevant to the
problems involved with location uncertainty in animal tracking. It is
provided merely as an illustrative benchmark for further work.
It is possible for the filter to become stuck in an infinite loop,
depending on the function passed to the filter. Several minutes is
probably too long for hundreds of points, test on smaller sections if
unsure.Details
Using an algorithm (McConnnell et al, 1992), points are tested for
speed between previous / next and 2nd previous / next points.
Contiguous sections with an root mean square speed above a given
maximum have their highest rms point removed, then rms is
recalculated, until all points are below the maximum. By default an
(internal) root mean square function is used, this can be specified by
the user.
If the coordinates of the trip data are not projected, or NA
the distance calculation assumeds longlat and kilometres (great
circle). For projected coordinates the speed must match the units of
the coordinate system. (The PROJ.4 argument "units=km" is suggested).References
The algorithm comes from McConnell, B. J. and 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