Floods Leave 186000 Homeless, Brazil

In the USA, the federal government's HUD agency has required federally funded organizations to use a computer tracking system for the homeless and their statistics, called HMIS (Homeless Management Information System). There has been some opposition to this kind of tracking by privacy advocacy groups, such as EPIC. However, HUD considers its reporting techniques to be reasonably accurate for homeless in shelters and programs in its Annual Homeless Assessment Report to Congress.
Actually determining and counting the number of homeless is very difficult in general due to their lifestyle habits. There are so-called "hidden homeless" out of sight of the normal population and perhaps staying on private property.