Hello Jessica,

Thank you for the screenshot of your video.

Maybe I am not too experienced Tracker user nor I do not know all Tracker features but if your camera is fixed and does not follow the bike (i.e. the curb lies on the same place in the video frame during the record and so its position is not time-dependent) I would try to read x and y coordinates of the curb in your co-ordinate system, put them for example into a spreadsheet to plot its graph and use for example a polynomial regression function for its approximation. This way you obtain a regression equation of curb_y=f(curb_x) i.e. curb vertical position=f(horizontal position), i.e. the curb reference curve. Then in the Tracker you can use item "Define..." to display a Data Builder where you can define new custom variable for calculating your bike distance from the curb, something like dist=curb_y(x)-y.

For more details about the data builder see paragraph 8. Defining new data columns with Data Builder in the Plot View chapter (https://physlets.org/tracker/help/plotview.html) of the Tracker Help (https://physlets.org/tracker/help/frameset.html).

Further interesting Tracker feature is the External model (https://physlets.org/tracker/help/datatrack.html).

I hope it is not over-engineered. ;-)

Best regards,

Vratislav

Hi Jessica, here's something that might work:

1. Open the video in Tracker, set its clip properties (start frame, step size, etc), and set the scale. Leave the origin and angle fixed. Save this tab as a TRK file.
2. Open the saved TRK in a second tab.
3. In the first tab, track the cyclist with a point mass and save the changed TRK. Each y-value is the point's perpendicular distance from the (fixed) x-axis (not yet what you want!).
4. In the second tab, uncheck the fixed origin and angle boxes in the Coordinate System menu. Then in each frame set the origin and angle such that the x-axis is right on the curb. Save this tab as a second TRK file.
5. In the second tab, choose Edit|Copy Object|Coordinate System.
6. In the first tab, choose Edit|Paste Coordinate System. The y-values are now measured from the curb in each frame!
7. You can choose Edit|Undo Edit Coordinate System and Edit|Redo Edit Coordinate System to switch between the fixed axis and the curb axes.

Does this work for you?  Doug



>  I'd like to track the distance between the side of the
> road (curb/kerb) and the bike to analyse the position
> on the road and the influence of "door-dashing" fear.
>
>
> I'd love to define the curb as a reference line
> instead of referencing to the X-Axis.