The actual census is a huge, complex project and requires a lot of preparatory work.
By law the actual count will take place 2010 but you can't just go out the morning of the census and start counting people that walk by. You have to know where the people live beforehand and there is no single, up to date data base available with accurate and complete information. The data that is available from other sources is often partial, incomplete and in dissimilar formats.
Remember, people who must be counted live in a variety of places other than houses and apartments; dorms, church annexes, boats, dorms, tents, remote mountain cabins, forest camps, RV's and travel trailers, 18 wheelers, utility sheds, abandoned and condemned buildings, hobo jungles, etc. The only way to know if you have made the most accurate and complete count practical is to build your own data base, structure by structure, street by street, down every dirt road and forest path.
Once you have a data base of the locations where people live you are ready to perform an actual count at those locations.
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They've already finished with that step. They sent out 100,000 workers who visted every house, and entered the data into the Census dat base.
They have the GPS co-ordinates for every front door in the United States.