TrajAngles: Turning angles of a Trajectory

Description

Calculates the step angles (in radians) of each segment, either relative to the previous segment or relative to the specified compass direction.

Usage

TrajAngles(trj, lag = 1, compass.direction = NULL)

Value

Step angles in radians, normalised so that -pi < angle <= pi. If compass.direction is NULL (the default), the returned vector will have length nrow(trj) - 2, i.e. one angle for every pair of adjacent segments. If compass.direction is not NULL, the returned vector will have length nrow(trj) - 1, i.e. one angle for every segment.

Arguments

trj: the trajectory whose angles are to be calculated.
lag: Angles between every lag'th segment are calculated. Only applies to non-directed walks, i.e. compass.direction is NULL.
compass.direction: If not NULL, step angles are calculated relative to this angle (in radians), otherwise they are calculated relative to the previous step angle.

Details

Note that since turning angles are circular quantities, i.e. 360° == 0°, it is incorrect to treat them as linear quantities. In particular, do not calculate arithmetic means or standard deviations of turning angles. See Batschelet, (1981) for a detailed explanation and techniques for dealing with circular quantities.

The turning angle before and after every zero-length segment will be NA, since the angle of a zero-length segment is undefined. This behaviour began in trajr version 1.5.0 (or development version 1.4.0.9000). Prior to this fix, the angle of a zero-length segment was assumed to be 0, which led to incorrect turning angles being returned. One approach to dealing with zero-length segments is to simply remove them from the trajectory. See TrajFromTrjPoints for a means to achieve this.

References