Surely it depends on the reliability of the witness? If I asked a drug addled drunk the same question as a police officer trained in observation, I would trust the police officer. If there were several witnesses, all sober and responsible people, I would find their accounts compelling.
The key word is "so-called". An eye witness is an eye witness and sees what they see. Now they may lie about what they see, or misinterpret what they see, which is why evidence is tested.
In the case of global earth, the evidence is overwhelming that it is, in fact, a globe. Eyewitnesses are just one more support to the global earth case. It was known to be a globe about 2,300 years ago. Nothing has come to light that would prove that the earth is not a globe.