Don't you use "observation" as the basic premise for your belief that the earth is flat?
Nope. The basic premise is based on the controlled water convexity experiments which have been repeated numerous times over 150 years, under a variety of atmospheric conditions, with human eye sight and with lasers, by multiple researchers, on multiple types of water environments, published in multiple mediums including a scientific journal seeking to peer review such experiments, and sometimes with barometric pressure instrument controls at request.
I have never seen the results of such a study. If the surface of water is truly flat as you say, then why do images of Chicago from across Lake Michigan show only the top portion of the tallest buildings? If the surface of the water is truly flat, shouldn't I be able to see more of the building?
Firstly, "Lake" Michigan is not really a lake, it is an inland sea. Secondly, for an explanation, read the chapter Perspective on the Sea in the book Earth Not a Globe by Samuel Birley Robotham.
But Tom - why do you accept Rowbotham's result rather than Ulysses Grant Morrow's very similar experiment which "proved" that the Earth is concave and Henry Yule Oldham's result which "proved" that the Earth is convex?
There are NUMEROUS results from these kinds of test that show all three outcomes.
(See:
https://en.wikipedia.org/wiki/Bedford_Level_experiment - which describes the whole horrible fiasco.)
Why dismiss Morrow and Oldham out of hand and accept Rowbotham?
The intellectually honest conclusion is that because these "visibility over water" experiments clearly produce mixed results, that we have to conclude that this is an (at best) unreliable way to decide the issue. We all see things like mirages and Fata Morgana over water - we KNOW that light bends in close proximity to a water surface - so why...WHY...are you happily ignoring those effects and taking Rowbotham's result as the gospel truth when other, equally accomplished, experimenters came to two completely opposite conclusions?
The rational way to deal with these varying results is to try to explain what was different between them (air temperature, pressure and humidity seem like reasonable candidates) - and look for an explanation for their wildly differing conclusions rather than to pick one of the three conflicting results and determine that the whole of physics, geography, etc is incorrect.
To simply pick one of the three as "correct" and ignore the others is simply not a reasonable approach.
Now - if you wish to deny the "refraction" explanation - that's fine...it's a tough thing to prove.
But if you do that, then the Bedford Level experiment and it's successors proved (at best) an inconclusive result - we should not base our entire discussion upon them.
Instead - take the evidence of things like the phases of the moon - the rising and setting of the sun/moon/stars - the orientation of the moon as seen from different locations - the variation in the force of gravity over the surface of the earth. All of those observations are a perfect fit for a round earth - and do not require multiple layers of increasingly bizarre theories.
So - remind us again what is responsible for the phases of the moon and the experimental evidence you have for that?