Economist Angus Maddison wrote a number of a books concerning the development of the world economy. He developed a detailed database of GDP by country by year.

The most useful part of his work for designers can be found in this spreadsheet. Here is spreadsheet for 1939, where you can set the total world income.

The GDP information is useful guideline for determining area income values. You will often need to interpolate data, as most pre-WW2 data is decennial at best. Before 1820, only the following years are covered 1,1000,1500,1600,1700. Be aware that data before 1870 often requires heroic assumptions and the process of converting GDP figures is inherently subject to dispute. Also note the Maddison is generally using modern boundaries.

Bolt & van Zanden  have updated the Maddison Database  to include figures up to 2010 and added a 730 AD datapoint.

If you want more coverage of the period before 1500, the only real source is hard copy, The Atlas of World Population History. Before 1500, most populations were so close to subsistence, that GDP can be considered to be proportional to population. Note that more recent research has shown that the populations of Pre-Columbian America were considerably higher than the estimates used in this atlas.

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