Actual Enumeration is a Constitutional requirement established by the Supreme Court which counts the exact number of people in a district during a census, rather than sampling or estimation. Districts in a state are required to contain roughly the same number of people, so that each person's vote will be approximately equal. When a census occurs, every state "redistricts" in order to maintain an equal number of citizens between districts.
Article I Section II of the Constitution reads: "The actual Enumeration shall be made within three years after the first meeting of the Congress of the United States, and within every subsequent term of ten years, in such manner as they shall by law direct."
The Supreme Court found a requirement for actual enumeration in <case>
Since actual enumeration is established in Article I of the U.S. Constitution, it seems to imply that census enumeration is the responsibility of the Legislative branch. Thus, President Obama's attempt to have the Executive Branch oversee his planned census is unconstitutional.