Other than my own methodical searches, when I create these maps, the information comes from history, private parties, and others who have freely given me info to work with. I understand everyone's issues with Google Earth. This is why I am very careful, have others double check my work, and use other services. The main reason I use Google Earth, is because it is easier to send to others (formats: kmz, kml, pictures). I do not expect all of the marks and locations to be 100%, what I do expect is that if I plot enough information it will give a bigger picture as to what is really going on in an area.
Definitely test the heck out of the info, and if you find anything worthy of a find, please get in touch and let me know if a mark verified by some hands. I would like to network and create a team of tech savy people with boots on the ground people. This may be hard I'm sure, but I believe a team of people working towards similar goals can in fact accomplish quite a bit more than one person alone. With equal distribution of information and splits. People trust me with information and to do a good job, and that is what I do.
Let me know what you find!