I need to determine the map scale for hundreds of scanned geologic maps that have only a bar scale (i.e., no fractional scale is provided).  I have very limited access to the USGS Library to inspect the printed map, so I was hoping for guidance -- is it possible to accurately determine the scale from an online view of the map?  If not, how do you address the issue of map scale in your citation records?

I've made map scale guesstimates for a few maps in the past, by viewing the map online at what I think is the actual size of the printed map.  But this current batch is too important to proceed without experienced guidance from the Library community.

Thanks.