My first exposure to a work dedicated to county boundary changes in the United States was William Dollarhide's classic Map Guide to the U.S. Federal Censuses, 1790-1920.
[cover as seen on Amazon.com's page]
Created primarily to use in conjunction with each of the federal censuses during that time period, this book is great for anyone wishing to visualize the growth of the United States and the individual counties. Knowing where your ancestors lived helps you locate additional information about them, and oftentimes you can locate records about a place in 2 different counties, either because one county was shaved off a larger "parent" county, or because of their past or present proximity to each other.
The maps in this book show at a glance why your ancestors could stay in the same place over time and yet change counties. It also shows how it would be impossible for your 4th great grandfather to have been born in Sac County, Iowa in 1790, as that county (and state) hadn't been created yet. Well, they may have been born at the exact present location of Sac County, Iowa if they were Native American or possibly French, but then it would have been called something else.
Is the date range from 1790-1920 too limited for your research?
The MAPofUS Historical Atlases and Maps of U.S. and States is an online source of maps that includes not only the US maps from 1790-1920, but also includes British colonial boundaries in America. Each state has "rotating animated maps" that can go back to the 1600's in the eastern states like Virginia. There is also a selection of David Rumsey maps for each state.
© 2014 Copyright, Christine Manczuk, All Rights Reserved.