So my wife and I have been visiting all the California Historical Landmarks. Photos on theadventuresoflizandgary.com.
A while back I found this great website (http://www.landmarkquest.com) run by David Schmitt. He did a ton of work loading all the locations into Google Maps.
I have a Garmin GPS and learned I could load custom points of interest (POI). I downloaded all the Google KML files from David’s maps, wrote a simple program to parse the XML and create a POI file, which in turn is a simple Comma Separated Values (CSV) file.
I plugged my GPS into my computer with a Micro-USB cable, installed the Garmin drivers, and used Garmin POI Loader (http://www.garmin.com/us/maps/poiloader) to load all the landmarks into the GPS unit.
This has been a huge help for us to find our landmarks. We can easily search by name or number to find the Historical Landmark we want to visit next.
Download POI File: ca-landmark-poi.csv
I asked David if I could post this, since it was based upon his hard work and he agreed. He also wanted to point out that the accuracy of the coordinates cannot be guaranteed. Use this only as a guide to get you in the general vicinity.
Enjoy!