The map above shows a portion of an 1889 map of Utah published by Rand McNally and Company overlaid on a current map. This map was created using the David Rumsey Map Collection tool called the Georeferencer. See http://www.davidrumsey.com/view/georeferencer.
The process of overlaying historic maps on current maps or satellite views of the earth has been rapidly developing with the advent of GPS coordinate systems and computers with enough memory and processing speed to create such maps almost instantly. Why would a genealogist want to overlay an old map on a current one? The answer to that question really involves a consideration of the amount of the involvement of any individual genealogical researcher in complex research questions. Map overlays are sophisticated tools for resolving some difficult questions usually concerning identifying an elusive ancestor. However, it is important that all genealogical researchers know about this process so that if and when they are faced with a situation that can be resolved by resort to map overlays, they will realize that this is even an option.
The events in our lives and our ancestors' lives are all associated with specific geographic locations on the earth. Any records that might exist about those events are also either specifically or generally associated with those same locations. One of the challenges of genealogical research is identifying the location of those events with enough particularity to help in finding the pertinent records and further, help in differentiating our ancestors with common names from all of the others with the same or similar names.
There are presently several ways to view maps through overlays and some of these ways include timelines that allow you see the changes in the maps over time. The first of these is the Unites States Geological Survey's National Map. See http://nationalmap.gov/. This website is a fabulous resource for finding and exploring both the present and past in maps.
One of the many valuable resources on this particular webpage is the Historic Topographic Maps. This part of the website lets the user view a set of historic maps with a slider that overlays the maps for a particular place and see the changes over time. This program is called TopoView. See http://ngmdb.usgs.gov/maps/TopoView/.
You choose the location you wish to view from a large map of the United States.
You zoom in to see the individual maps that are available for that area.
In this case, there are multiple maps available. You can view the individual maps or any set of maps in various views. Detailed information about how the program works is shown in this video.
Overlaying the maps is done with the USGS Historical Topographic Map Explorer. See http://historicalmaps.arcgis.com/usgs/
You can then use the maps along the bottom individually or in sets to see them in layers that can be adjusted by the sliders under each map. The maps are interactive and you need to explore the controls to see all the functions.
Using the Newberry Atlas of Historic County Boundaries, (See http://publications.newberry.org/ahcbp/) you can view the boundary changes of every county in the United States over time overlaid on Google Earth. You need to have downloaded Google Earth and then go to the Newberry Atlas of Historic County Boundaries website and choose a state to view.
I clicked on Pennsylvania.
At the bottom of the page, I further selected the link to "Download the KMZ File for use with Google Earth." The KMZ file is a special file containing the historical boundary information from the Atlas that will be able to be viewed on Google Earth. The KMZ files usually downloads as a Zip file. Click on the Zip file to open it and then drag the icon for the file onto your icon for Google Earth or you can open the file from the menu in Google Earth.
Here is a screenshot of how the file appears in Google Earth.
You may have to unclick several of the viewing options in Google Earth to get a clearer view. There is a slider in the upper left hand corner of the screen and you can use that to see the boundary changes. You can zoom in on the image to see the changes down to the house level in Google Earth. You can use this to view exactly where your ancestor's land was located in reference to all the historic boundary changes.
Next, here is one example of using a map overlay program to overlay an historic map over a current satellite view. There are several programs that can do this including a limited link in Google Earth. You can search for "overlay historic maps google" to find a long list of options.
Above I have an example of a map overlay using the David Rumsey Historical Map Collection's Georeferencer. See http://www.davidrumsey.com/view/georeferencer. The program itself contains detailed step-by-step instructions about the process.
You can see the results in the first image above.
Another website for overlaying maps is the New York Public Library's Map Warper. See http://maps.nypl.org/warper/maps/27613
Again, you will find detailed instructions on the website for overlaying the maps on the "rectified" view. Here is a copy of a map as shown on the rectified view. You will have to sign up for an account and login to view the maps. This process may take some time depending on your computer and the speed of your connection to the internet. You can then vary the opacity of the image with a slider. Most of these programs must be experienced to fully understand how they work.
This entire process is not just a toy, it provides valuable historical information that can be used by genealogists to located their ancestors' events with enough specificity to find additional records about their lives.