A circle with a 690 meters radius is about half a square mile. The top 50 most densely populated cities according to Wikipedia all have over 20,000 people per half square mile, and that includes lots of European cities and even one in the United States:
On the other hand, it says that 690 meters is the average. I wonder if the accuracy is higher or lower in more densely populated areas. My uneducated guess is that it would be higher and that you'd actually have a more accurate location for densely populated cities.
