Based on an idea from a current user (thanks Steve!), there’s now an option to overlay present-day county lines on top of historical county lines.
This gives you the ability to see at a glance if you’re researching in an area and timeframe where the county lines have shifted between the historical date you’re viewing and the present-day. To use this feature, just check the “Also show present-day county lines” checkbox in the lower left of the map on the Historical U.S. Counties on Google Maps tool.
When you check that box, you’ll see thin red lines (present-day county lines) overlaid on top of the thicker brown lines (historical county lines). You’ll also see the “Present-day County” name listed in small red text above the map (next to the latitude/longitude).
As an example, let’s say we’re researching the Nashville area as of 15 Jul 1830. (Type Nashville in the “Search places” box above the map and type 7/15/1830 in the “As of date” box above the map).
Then, check the “Also show present day county lines” checkbox, and you’ll get a map that looks like this:
On the above map, I’ve highlighted in yellow where the present-day county information is shown, based on the exact location of the red dot on the map. This tells us that this location was Davidson County, TN in 1830 and today, it still is Davidson County.
But, see the faint red lines in the above map? Those are the present-day county lines. So we can clearly see that the western side of Davidson County in 1830 is another county today, as is a small section in the southwest corner of 1830 Davidson County. To find out what present-day county is in those two areas, just click the map. You’ll get this:
The above map shows us that the western part of 1830 Davidson County where I clicked (the red dot) is now Cheatham County.
This alerts us that if we are researching an ancestor who lived in Ashland City in 1830 (see the northwest part of 1830 Davidson County), those records may be housed in Davidson County’s courthouse (barring the court house didn’t burn down!). Importantly, it tells us that if we’re doing an 1830 Census search on Ancestry or FamilySearch (for example) for those ancestors that lived near Ashland City in 1830, we should make sure and filter the place based on Davidson County since that is where Ashland City was located in 1830 (even though it’s in Cheatham County today). I wrote an in depth how-to on this topic here: How you may be sabotaging your search for ancestors…and how to fix it!
What about that little southwestern part of 1830 Davidson County? What county is that area part of today? Well, you’ll have to go to the Historical U.S. Counties on Google Maps tool and find out!