The methods behind the development of the Historical Urban Population, v1 (3700 BC - AD 2000) can be found in the following open access journal article:
Reba, M. L., F. Reitsma, and K. C. Seto. 2016. Spatializing 6,000 Years of Global Urbanization from 3700 BC to AD 2000. Scientific Data 3: 160034. https://doi.org/10.1038/sdata.2016.34.