You're a geodetic surveyor, right?
Not exactly. I'm an student of the wonderful world of observable science - electronics, optics, chemistry, computer science, robotics, etc.
I'm also not Jesse Z who is an honest to goodness surveyor.
Earlier this year when a new real-life friend popped the news to me that the earth was flat and challenged me to check into it, I did. And it wasn't.
But I did realize that I didn't at that time know of a single way to personally observe the curve.
Thus began my journey of looking for ways to show the curve in ways people (some more than others) can understand.
And one of those ways was to hit up eBay for old theodolites. That was a big mistake for a person on my budget and with my space limitations. Now I've got about 6 old digital electronic theodolites, over half of which work.
In any case, I found youtube videos that showed how to use a theodolite, and with my background in electronics design engineering I was able to repair most of them. (because I've been getting old cheap ones. Some of them worked fine, some had problems.)
Ironically, one of them from like the 70's or 80's actually has a NASA inventory sticker on it. Of course that's not the one I bring out when I'm doing a demo for a flat earther, because, well, yeah, for obvious reasons.
Pictured in my avatar is one of my two Pentax ETH-20F units which seems to be a really nice entry level theodolite (and they are old enough they were made in Japan, not China which is where Pentax now makes their stuff).
The ETH-20F seems to be as low as $150 to $350 if you watch and wait, otherwise it's $500 to $1500.
It allows the output to be set to pure decimal degrees which is really nice for engineer types like me, or it can be set to a number of combinations of degrees, minutes, and seconds.
I have mine set to just decimal degrees because that's easier to parse when typed into a script or calculator for doing calculations.
The theodolite has been carefully designed to keep errors as small as possible. When leveling, you use the level on the rotating part, then when you think you got it level, swivel the head 180 degrees and see if it's still level. If not, you adjust your level so that it's true, then you check again.
With care, you can level to within perhaps 5-10 arcseconds on mine, but probably much better on more expensive ones.
And they have a built in software routine to calibrate the vertical scale. You sight a distant target, mark that as point one, then flip the telescope over then turn around the head and sight again and mark that as point 2, then it applies (and saves) the calibration so that guarantees that a reading of 0 degrees will be a true level.
I also made a tutorial showing how to use it and how to check the missing height of a mountain:
(And how to check the instrument to make sure it's working correctly against a water level.)
Source: https://www.youtube.com/watch?v=TPkRNVA70Sc
And here's an unrelated photo set that shows Victora BC from across 20 miles of water:
https://i.imgur.com/UGLqSe3.jpg