There are quite a few threads on here discussing that. Here are six of the best ways to easily demonstrate that the horizon is below eye level:I have a question about [...] the entire "horizon always rises to eye level" argument.
1. Use a professional theodolite. Eye level is where the crosshair is:
2. Download and properly calibrate a theodolite app. Eye level is where the crosshair is:
3. Make your own theodolite using a spirit level:
4. Use parallel lines to find the vanishing point (which is always at eye level):
5. Take a picture of eyes, with the camera at eye height, and see where the horizon is:
6. Use a water level - after all, all flat earthers know "water always lies flat".
Number 5, by the way, is an example rather than a demonstration - the camera in that photo is probably slightly below eye level, therefore his eyes appear even more above the horizon than they normally would..