# Calibrate the arena

Aim

To convert the subject’s coordinates expressed in pixels to real-world coordinates.

important Avoid barrel and spherical distortion by using correct lenses. Place the camera above the middle of the arena, and perpendicular to the arena surface.

important If you use Track3D, do not do calibration within EthoVision XT. Instead, make sure that you set Pixel calibration to ON. For more information, see the Track3D Reference Manual.

Prerequisites

▪You grabbed a background image.

▪You know the real-world size of the arena. Measure this distance at the height of the animal, not, for example, at the top of the arena.

Procedure

1.In the Arenas and Zones section, click 1. Draw Scale to Calibrate.

2.Click the mouse button on the first a point in the video image, hold the button down, and drag to the second point in the image. Next, release the mouse button.

3.In the Calibration Distance window, type in the real-world distance between the two points.

Notes

▪Repeat the steps above for all calibration lines. EthoVision XT uses the average of the scale factors obtained from each calibration line. This factor is used for analyze movement of the subject in all directions. Draw calibration lines on both directions, horizontal and vertical.

▪See also Modify the coordinate system of the arena.

▪For each calibration line, EthoVision XT calculates a scale factor (real world distance entered divided by the length of the line in pixels). If this factor differs by more than 20% from the average of the other lines, a message is shown: “Calibration scale substantially larger or smaller then other existing calibration scales”.

Click No to edit the real world distance. Or click No and then Cancel to re-draw the last calibration line.

Click Yes to accept the value. Beware that very different scale factors may produce false results.

▪If you enter a real world distance smaller than 10 mm (0.39 in) or greater than 10 m (32 ft 9 45/64 in), a message is shown. Click Yes to confirm the value or No to edit it.

▪If you have clicked Cancel and you want to repeat the calibration procedure, click again 1. Draw Scale to Calibrate.

Calibrating with the Noldus Calibration Card

▪From the dialogue ‘Grab Background Image’ (also accessible by going to Arena Settings -> Arenas and zones -> right-click on ‘Background’ -> ‘Grab Background Image)

▪Place the ‘Noldus Calibration Card’ in view inside of the arena. Make sure that the black rectangular outline within the card is clearly visible in the camera view. If you are using a dark arena, you can place a piece of white paper behind the card.

▪Click ‘Calibrate’

▪The Calibration dialogue will open

▪Click ‘Calibrate’ to accept or decline the result

▪If calibration by the use of the Noldus Calibration Card fails, you can always use the manual calibration as defined in the procedure above