Hi @SophieElena,
if you want to get a more or less accurate indication of real units you need to make sure, that your reference measuring tape is rather at the front of the hoove (which in your first image is already behind the hoove) or make sure that the position of the measureing tape is always consistent.
Second, it needs to be exactly perpendicular to the axis between hoove and camera.
Otherwhise you introduce already alterations.
The camera distance to the hoove should also be standardized to minimize the measurement inaccuracies.
You need to include that measuring tape in all images and set the scale in all images separately if you apply different zoom etc. (while consistent imaging setting would be preferrable e.g. always the same zoom and distance to the object)
The vertical measurement line you want to draw should best be also straight. You can achieve this by pressing Shift while you draw a line selection.
To keep all the lines you want to draw, you best use the ROI Manager. Therefore, after drawing a new line just press T on the keybord and the line will be added to the ROI Manager. Thus you can first draw all the lines and adjust them if necessary (therefore, you need to select them in the ROI Manager, adjust size and/or position and press Update in the ROI manager)
Finally, You can press Measurein the ROI Manager to get all the length in the scaled units in a results table (this will be the last column in the table).
Besides the necessity of a consistent imaging setup, also the measurement lines drwn manually will have different distances which introduces variation. There is surely a more automatic way to do that, but for the moment this might help you in getting started.
