As much as I hate to say it, the most accurate GPS in a watch is the new Pixel Watch from Google, and its accuracy comes from uploading the GPS signal to Google to do building-aware post processing using Google’s database of worldwide buildings to solve the multi-path issue, plus DGNSS. Simply having dual frequency isn’t enough when you run on the streets of New York City. With Google’s solution, it can even detect which side of the sidewalk you are on while running.

I’m just not aware of any method that can do this without uploading your data to the cloud.