Table 1 Currently applied path characteristics. Different signals or parameters can be calculated either based on consecutive relocations within a trajectory (“stepwise”) or for multiple relocations such as identified path-segments (“across multiple steps”)
Characteristic | Description | Type | Calculation | Reference |
|---|---|---|---|---|
Displacement | Increment of the X and Y values between two consecutive relocations, change in absolute spatial position | primary | stepwise | |
Time lag | Duration / increment in time between consecutive relocations (usually determined by sampling regime) | primary | stepwise | |
Turning angles / heading | Relative and absolute turning angles between consecutive relocations, change in direction | primary | stepwise | |
Step length | Euclidean distance between two consecutive relocations | primary | stepwise | |
Velocity / speed | Distance traveled in a given time interval between two relocations; less sensitive to missing data than step length | primary | stepwise | |
Persistence / turning velocity | Transformations of speed and turning angle: persistence velocity represents the tendency and degree of a movement to persist in a certain direction. Turning velocity shows the tendency of a movement to turn in a perpendicular/opposite direction | secondary | stepwise | |
Net / mean squared displacement | Squared displacement between the first and current relocation of the trajectory; applied to characterize diffusion behavior or migration patterns | secondary | stepwise | |
First passage time | Time required for crossing a predefined endpoint based on a circle (radius) around a starting relocation. Sums the times of all forward and backwards relocations within the radius; index of area-restricted search behavior | secondary | stepwise | |
Residence time | Extension of the first passage time accounting for returns of the animal in a given area. Sums the times of all relocations (backwards and forwards) of a trajectory within a given vicinity around a relocation. | secondary | stepwise | [31] |
Pseudo-Azimuth | Recalculates the basic azimuth value at the midpoint between two consecutive steps to range within 0 and 360. Can be used as indicators for movements with same or parallel directions. | primary | stepwise | [124] |
Straightness index | Ratio of Euclidean distance between the beginning and end of a trajectory and the total path length (sum of all step lengths) | secondary | across multiple steps | |
Sinuosity / Tortuosity | Adaptions of the straightness index analyzing the probabilistic distributions of the changes in the turning angles and the beeline distance between the start and end points of the trajectory; index of path orientation | secondary | across multiple steps | |
Fractal dimension | Measure of path tortuosity; non-Euclidean dimension of the trajectory varying between one (completely straight) and two (tortuous, completely spanning two-dimensional space); different implementations exist | secondary | across multiple steps | |
Multi-scale straightness index | Repeated calculation of the straightness index of a trajectory over a range of different temporal scales | secondary | across multiple steps | [76] |
Area interest index | Repeated calculation of the straightness index for a limited size of a sliding window along the trajectory. With each repetition, the number of relocations within the trajectory is reduced | secondary | across multiple steps |