Many years ago...
Before the realization that the web was "visible" to everyone who typed in the right search criteria...
There was a plethora of info of this sort that was readily available to anyone who took the time to "dig" hard enough...
I have seen everything from "pile" maps... to ferrous "hits" maps that were part of "collections" that were meant to be seen only by those who had access to a certain site...
Problem was... they did not know what they were doing... so security... or exclusivity was non existent for the pages containing the hard data...
This information has long since been removed or secured...
Ever search Corps of Eng charts ? heh...
Days and days worth of information.
Seems like for awile in the late 60's early into the seventies... Every few years they were making "maps" of ferrous hits.
This info I saw... is nowhere to be found online nowadyas... and has not been for years now.
Corp of Eng... has EXTENSIVE collection... Of what I felt was ... ENTIRE coastline of Florida.
Like they wanted to know "where" ANYTHING may be.
P.S. > I am talking Win 95 era.
P.S.S.> Lucky I did not totally fry my memory in my youth :P