If I have a website with a number of registered users, and each user has their geolocation recorded, is there a formula to guestimate the total number of people living in a particular area, e.g. within $1km^2$?
I've just found out about the German Tank Problem and how remarkably accurate it is with sparse data, but obviously people don't have serial numbers. I'm wondering if there's anything similar, potentially using other commonly collected data such as birthdays, date of account creation, how frequently the site is used, etc.