If you are dealing directly with the original scan and you know the DPI (dots per inch) they were scanned at, you can work it through from there.  Can be fussy indeed, but once you do a few you'll likely be able to recognize trends.
Some image viewers have properties displays that will do the pixel calculations for you.  If you are just in windows, you should be able to right-click on the file and look at the properties there (details tab)
If you are dealing with a PDF, you can sometimes get similar information from inside your pdf viewer.

Another option you may have, depending on the map, is they will sometimes say 1cm=5km or the like.  (My guess is you don't have this, but it would nice if you just missed it and could now save a lot of time!)

If they were born digital, it's a different can of worms.  At that point there may not be a "print size" per se.  Some digital products are rated in detail recorded at a particular ground-distance-interval.  I think.  I'm a bit shaky on that aspect.  (I mostly deal with paper and scans.)

If you are dealing with a picture someone posted on the web and have no resources to determine it's digital handling or history, then you may have a different issue.  If the image quality has been degraded, it may no longer be accurate to whatever the scale was originally was due to data loss.  At that point you may want to punt...

  *   Maybe call out observed resolution and a best guess of quality of information?
  *   Possibly determine a pixels per area covered ratio if you can get a bounding box?
     *   Like, if it's 10 degrees by 10 degrees, and the distance from one edge of the mapped are to the other is 1000 pixels, that does give you a number to work with that could have value?

Another course of action may be to ask the people who will be using the maps what information will be most useful to them.

While I know this was short on 'proper' answers, I hope it gives you a few ideas to pursue.  I wish you the best of luck and a Bucket of Perseverance!

-Kate Cramer
East View Geospatial
Geodata Project Lead

From: Maps-L: Map Librarians, etc. <[log in to unmask]> On Behalf Of Soller, David R
Sent: Tuesday, March 28, 2023 3:23 PM
To: [log in to unmask]
Subject: Determining map scale, online?

Hi -

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.

Dave Soller
National Geologic Map Database
U.S. Geological Survey