[Geocoding] How does Nominatim determine importance of places?

[Alex Weissman]

I originally posted this question to the OSM discussion forum, but someone
there suggested that I bring it to the geocoding listserve and ask it
here.  So, here goes:

I'm using Nominatim to reverse-geocode natural language location
descriptions for a research project. I spent some time looking through the
source code (in particular, website/search.php), but I can't seem to make
heads or tails of how the "importance" score is calculated.

>From what I can tell, there is some baseline calculation and then numerous
tweaks - one line, for example, says

$aResult['importance'] = $aResult['importance'] + ($iCountWords*0.1); //
0.1 is a completely arbitrary number but something in the range 0.1 to 0.5
would seem right

I also noticed in the documentation that Nominatim will use Wikipedia to
improve the ranking of results, but once again nothing specific beyond "the
importance value is calculated as log(totalcount)/log(max totalcount)." I
assume that "totalcount" is the number of internal links to an article
about a specific location in the result set, and "max totalcount" is the
maximum of that value across the entire result set. But this only tells me
the scoring contribution from Wikipedia, and not how the baseline score is
calculated.

My question is, what properties of the OSM data go into the calculation,
and then how is the importance score actually calculated? What special
tweaks and thresholds should I be aware of?

-Alex
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