# Interpolate points

Aim

To replace one or more missing points (or samples) or points clearly misplaced with points calculated on the basis of the position of the first previous and next non-missing points.

If your experiment is set to Center-point, nose-point and tail-base detection, you can choose between two types of interpolation: interpolate all selected body points or the center-points only (see below).

Procedure

1.If you want to interpolate non-missing points clearly misplaced, set them first to missing (see Delete points). In other cases go to step 2.

2.Select the missing sample (or the range of) you want to interpolate.

tip To select missing samples automatically, click the Auto Select button on the toolbar, then choose Missing samples. Click the Next>> button to move the video to the next missing sample.

3.Click one of the following buttons on the toolbar:

Interpolate the selection (or press Ctrl+I). Click this button if you want to interpolate all the body points of the selected samples. See the notes below for how points are interpolated.

Interpolate center points (or press Ctrl+Shift+I). Click this button if you want to interpolate the Center points for the selected samples, and leave the nose-point and tail-base as they are currently (no matter whether missing or not).

If your experiment is set to Only center-point detection, only the first button is available. Click that button to interpolate the center-points.

4.A message appears after you have clicked the interpolate buttons, informing you that EthoVision XT has extended the range of samples to be interpolated, if needed (see the note below). Click Yes if you want to continue. If you are not sure about the result of the interpolation, click Cancel.

Notes

▪The interpolated points are highlighted in the track plot and get the color of non-missing points. In the Samples List, the points obtain X,Y coordinates.

▪The Subject area is interpolated linearly.

▪When you visualize the data, missing points are shown as linear interpolation between the first previous and next valid points. However, their coordinates are not calculated (you can check this in the Samples List). This means that missing points are actually not in the data set. If you want to include them in your data set, you must first interpolate them.

▪In some cases interpolation may result in points being located outside the arena. In those cases, they are automatically set to missing. This could be the reason why you still see missing points after you have carried out interpolation.

▪If you are not sure whether interpolation gives good results, save the edits first. See Saving the track edits

▪If you interpolate the nose- and tail-base points, the Head direction line is recalculated as the prolongation of the segment that joins the center-point and the nose-point after interpolation.

This has consequences on all variables based on the Head direction line: Head direction, Head directed to zone and Turn angle and Angular velocity when they are based on head direction.