I've worked professionally with geometrics engineers to correctly display maps and place markers in all the right places. Your description matches my experience

I've worked professionally with geometrics engineers to correctly display maps and place markers in all the right places. Your description matches my experience. I will say, though. I was surprised at how much you can do with a flat Earth model. It takes a few km to accumulate significant error. For some calculations, it's a valid model... but you do need that accurate reference point to start from.