AFAIK in London the data are extracted from the archives of the local councils that planted the trees in the first place and were responsible for maintenance af

AFAIK in London the data are extracted from the archives of the local councils that planted the trees in the first place and were responsible for maintenance afterwards. I can imagine a best-case scenario is that they already have maps of the borough where they mark all the trees, and it's a matter of digitializing those maps and combining them. It still takes time to combine all these data, presumably, but there were some useful studies on the ecologicical effects of different types of trees that came out of these data.