What authority does the U.S. Census Bureau have for sending anyone to my front door to mark it with GPS coordinates?

Am I the only one suspicious with this action, the fact, by law you can not prevent the census reader from taking GPS readings at your door step – seems odd to me. What next when they take the nest census, sense our property for gunpowder and home-grown marijuana, etc. Why has the Obama Administration chosen to take over the oversight of the Commerce departments Census duties. Is it really as innocent as making sure Elmer can easily find the house ten years from know. What is one legitimate reason for having a GPS coordinates on every residense in this country.