Knowledge Base

Delete, Move, and Interpolate Points

Delete Points

Aim

To remove a sample (or some of the points of your sample) from your data set. Deleting points means that they are set to missing.

Important: If you delete a point, you cannot recover the original coordinates, you can only interpolate it. If you delete an entire sample, the surface area is also lost. If you are not sure about the consequences of setting points to missing, save the edits first. You can always return to your original track data.

Procedure

1. Select the sample (or range) that contains the points you want to set to missing.
2. Do one or more of the following:
   • If your track contains only center-points, click the Set to missing button on the toolbar or press Ctrl+Del.
   • If your data also contains nose-point and tail-base, to set all points to missing, click the Set to missing button or press Ctrl+Del.
   • To set only nose-points to missing, click the Set nosepoint to missing button or press Ctrl+Shift+M.
   • To set only tail-base points to missing, click the Set tailbase to missing button or press Ctrl+M.

What "Set To Missing" Means

• Missing points are shown as linear interpolations between the non-missing points of their own type. However, their coordinates are not in the track data and are not subject to analysis. For example, if sample n is missing, the distance moved between the two non-missing points n–1 and n+1 is given by the straight line connecting the two points of the same type (nose-point to nose-point, etc.).
• In the Samples List, the points' X,Y coordinates are removed. If you set the sample to missing, the surface area is also removed. Missing values are indicated by "-".
• If you set a nose-point to missing, the Head direction value for that sample is removed.

Move Points

Aim

To change the X,Y coordinates of a point on the track plot.

Procedure

1. Hover the mouse on the point you want to move. The mouse cursor changes to a four-headed arrow.
2. Press and hold the left mouse button and drag to the position you require.
3. Release the mouse button. The point is moved to the new position and its X,Y coordinates are updated in the Samples List.
4. To de-select the point, press Esc or click on the background image.

Notes

• If you move a missing sample, this becomes non-missing and therefore included in your data.
• You cannot move a sample outside the arena.
• If your experiment is set to Center-point, nose-point and tail-base detection, a sample is made of three points (no matter whether those are actual data points or missing). You cannot move a sample as a whole; you must move each point separately (center-, nose-, and tail-base points).
• If you move the nose-point, the corresponding value of Head direction for that sample is unchanged.

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.

Procedure

1. If you want to interpolate non-missing points that are 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 samples) 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.
   • 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. 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 not actually 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.
• 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.

More Information About Interpolation

Interpolation Range

To interpolate points, EthoVision XT needs complete samples immediately before and after the sample to be interpolated. "Complete samples" means that:

• For Only center-point detection, the center point of the adjacent samples must not be missing.
• For Center-point, nose-point and tail-base detection:
  • If you chose Interpolate the selection on the toolbar, all three points of the adjacent samples must not be missing.
  • If you chose Interpolate center points on the toolbar, the center-point of the adjacent samples must not be missing.

If the samples adjacent to the selected range are not complete, EthoVision XT searches for the first complete sample before and after that sample (or range). Therefore, all incomplete samples in the middle are included in the range to interpolate.

Example: The missing samples 5 and 6 have been selected for interpolation. Samples 4 and 7 are adjacent to the range, but not complete: sample 4 lacks the nose-point, while sample 7 lacks the tail-base point.

• If you choose to interpolate the entire selection, EthoVision XT needs the coordinates of the three points for both adjacent samples in order to calculate the coordinates of the three points of samples 5 and 6. Therefore, samples 4 and 7 are not used. Instead, the complete samples 3 and 8 are used as adjacent samples, so the interpolating range is extended to samples 4 and 7. The coordinates of the missing points of samples 4 to 7 are calculated.