The LandScan Global Population Databases, developed at the Department of Energy's Oak Ridge National Laboratory, represent resolution global population distribution databases available. This is a geographically based, population distribution model.
LandScan is a global population database that shows geographical distribution of population at one-kilometer resolution over an average 24 hour period.
The LandScan algorithm uses spatial data and imagery analysis technologies and the most up-to-date census data within an administrative boundary. Accurate administrative boundaries are an integral part of the LandScan population distribution modeling process. These population distribution models are tailored to match the data conditions and geographical nature of each individual country and region.
Most national censuses count populations by measuring where people sleep (or reside) rather than where they work or travel. LandScan integrates daytime movements and collective travel habits into a single measure to produce a better representation of where people are located during an average day.
With improvements added this year, high-resolution imagery sources were used extensively for validation to refine urban areas, land cover data, and thousands of smaller villages and populated places.
Description adapted from http://www.eastview.com/online/landscan , which provides additional information