OpenStreetMap
Score! I ran across another small municipal airport that had not been mapped, a few miles south of Fortuna, CA. This is apparently the Rohnerville Airport.
There is apparently a convention of labeling these small general aviation airports as aeroway=aerodrome, and reserving aeroway=airport for larger airports that handle commercial passenger aviation. I have also updated the Healdsburg Municipal Airport to follow this convention.
Discussion
Comment from Gustav F on %დ %თ %წ at %ს:%წ
I am not sure if this is a convention. See the description of aerodrome here:
osm.wiki/Mapping/Features/Airport
Comment from Icycle on %დ %თ %წ at %ს:%წ
I guessed that this was the convention based on the tagging of a couple of local airports, as well as the Mapnik rendering of these aeroways. The ones tagged as aeroway=airport are rendered more prominently than those tagged as aeroway=aerodrome. This page suggests that there is a legitimate distinction to be recognized here, and that the emerging airport/aerodrome convention seems like a reasonable way to represent it:
osm.wiki/Talk:Mapping/Features/Airport