re: #61 researchok

See this for starters

This too, along similar lines.

There are other references I can dig up..

I’d have to dig around myself, but it seems to me that the USDA changed their methodology, which was the subject of much gnashing of teeth at the time. Don’t recall the time frame or if was another mapping system - there are several. If I have time, I’ll see if I can find the references again, although when I looked into this it was at the library, before there was much on the Internet.