The Minimal Distance Moved smoothing method

Aim

To filter out small movements of the subject’s center point that are caused by random noise, not to the subject’s spatial displacement.

Working principle

With the Minimal Distance Moved method, you can pick out the data when the subject moved a minimal distance from one sample to the next.

▪When the distance moved between the current and the previous sample is above (or equal to) the threshold for Minimal Distance Moved, the current sample is used for analysis.

▪If the distance moved between the current and the previous sample does not exceed the threshold, the current sample is “moved back” to the previous sample.

The result of this filter is that small movements of the body points, supposedly due to noise, are removed, and the trajectory of the subject becomes simpler.

 

Procedure

1.Choose Acquisition > Track Smoothing Profile > Open.

2.Select the Minimal Distance Moved check box and click the Edit button.

 

3.Under Select, enter the Minimal Distance Moved threshold.

4.Choose one of the two options (see below for an example):

Direct (A > MDM). To select samples on the basis of the shortest distance (beeline distance) between samples. Select Direct to exclude movements such as breathing, when the animal is sitting still.

Along the path (B + C + D > MDM). To select samples on the basis of the actual path between samples. Select Along the path whenever you want to filter samples according to the distance moved along the path.

5.Click OK.

Notes

▪Minimal Distance Moved is only applied to the center-point.

▪Under some circumstances, using the Minimal Distance Moved filter might affect dependent variables other than Distance Moved and Velocity. To apply Minimal Distance Moved to some dependent variables, not others, create two Track Smoothing profiles: one with Minimal Distance Moved selected, the other without Minimal Distance Moved selected. Run batch calculations using both smoothing profiles.

Direct vs. Along the path Minimal Distance Moved

As shown by the case of Distance moved in the figure, calculation of the dependent variables is affected by what Minimal Distance Moved option you choose.

▪Direct (left). The program calculates the shortest distance between a sample and the next (AB, dashed line). If this distance is shorter than the threshold distance MDM, sample B is set to sample A. Then the program calculates the distance to the second next sample (AC), etc. until it finds a segment (AD) longer than MDM. Samples B and C are both set to sample A. This procedure is then repeated up to the end of the track.

▪Along the path (right). EthoVision XT calculates the distance between a sample and the next (AB, dashed line). If this distance is shorter than the distance threshold, sample B is set to sample A. Then the program calculates the cumulative distance along the path to the second next sample (AB + BC, etc.) In this example, sample B is set to sample A. However, the distance AB + BC is greater than MDM, so C stays in its place. Next, the sample D is set to sample C because CD < MDM. However, CD + DE is longer than MDM, so sample E stays in its place. The procedure is repeated up to the end of the track.

 

In both cases, all five samples are selected for analysis. However, some samples have been “moved” to a previous sample in such a way that their own value of distance moved is zero.

▪With the Direct method, samples B and C overlap with A, and E with D.

▪With Along the path, sample B overlaps with A, and D with C.

The resulting total distance moved can therefore differ depending on which option is chosen.