I will be doing area calculations for 100+ countries using country polygons taken from GADM, a database of global administrative areas. I am using ArcGIS. The GADM data is in WGS84 and is unprojected. I am doing some tests to gauge how area results differ whether I use customized projections vs. continental projections vs. national grids.
There does not appear to be a difference in calculated area whether I use Customized Albers Equal Area Conic Projection (I create country specific parallels and meridian) or Continental Albers Equal Area Conic Projection (built-in projections in ArcGIS). I thought a customized projection would lead to more accurate measurements but apparently the only difference is cosmetic - how the country is oriented on layout. In my limited test sample, all customized vs. continental results are the same except for Ghana. What might explain that? (Column F, Ghana)
I then compared the Albers resuls to results obtained using a country's national grid. There are very very minor differences vs. National Grids, always less than 1% except Brazil, where the difference is 1.12%.
What explains these minor difference and the Brazil difference in particular? Should the areas be the same?
Given that the observed differences are so small, I am inclined to do all calculations in Continental Albers Equal Area Conic rather than create customized parallels and meridians, or seek out national grids for all countries.
My core question is the whether Continental Albers Equal Area Conic Projections will always produce the same approximate areas as national grids (error <1%, column E)? Is Brazil the exception? Or is this a large country problem more generally? I did a few tests and do not want to test each and every country. I thought I could seek out the wisdom of this StackExchange.
I have scoured the Internet and can't find exact answers to these questions.