Reddit Post by Czarrie
So I've been looking for some older portals near me that never really "fell into the game". Usually these lie on the outskirts of towns and out in the rural areas where few players tend to investigate. Most people have not submitted portals out here, but there are portals to be found -- the initial population of HMdb.org portals that Niantic brought into the game.
What is the HMdb.org database?
The Historical Marker Database is pretty much what it sounds like; it's an online collection of historical markers across the globe (primarily in the United States) from cemetery markers to roadside signs. At the time of their placement, these signs were designed to remind people of long-gone buildings and other areas of historical interest (battlefields, older businesses, etc) but the signs themselves have become a bit of historical interest as time rolls on. They can be located pretty much anywhere, even hundreds of miles from the nearest town in some special cases. HMdb.org rolls these up into a database with GPS coordinates, sign titles, and full or partial transcriptions.
What does this have to do with Ingress? If you've been playing for awhile, surely you've noticed that, outside of art displays and such, you tend to run across portals located at these markers, especially if you're a rural player. This is because Niantic auto-populated a large number of portals by using the aforementioned HMdb.org database. As a result, if it's on the HMdb.org and available to the public, it is almost assuredly an Ingress portal, as well.
So how can I use this information to my advantage? First, visit the GPX download page on the hmdb. This is a list of files you can download that have all of the aforementioned markers available for any specific area; in the United States, it's usually sorted in a traditional State->County manner. I recommend limiting the download to just a couple of counties in this case -- too many data points is going to be an issue for later, so we're trying to keep it under 1,000.
Next, visit this handy little page called GPSVisualizer. Ignore most of the options except for "Upload your GPS file here". Take whatever .gpx files you just downloaded and upload them here, followed by "Draw the Map!".
Congrats, you now have a nice, populated map of every HMdb.org marker in your area -- cross-referenced with the Ingress Intel map, you can use this to find extremely remote portals that essentially no one was aware of, very useful if you're trying to make some big, nice fields or set up a hard-to-reach farm. Also, extremely interesting from the stand-point of learning about your local area, but that's just a bonus :)
http://www.reddit.com/r/Ingress/comments/1kxisl/fun_with_the_hmdborg/