U.S. Compilation: Commentary

The U.S. Historical Counties dataset holds the polygons, metadata, and attribute data for every different configuration of every county or county equivalent in the area that became the fifty United States, dated to the day, from 4 March 1629 through 31 December 2000. The data included in the interactive map excludes Alaska and Hawaii. However, these states are included in the datasets available for download.

The data included in the interactive map was also generalized (i.e., simplified) for better website performance. This generalization is apparent when zoomed-in to scales greater than 5 miles = 1 inch. U.S. Historical Counties datasets at various levels of generalization (including ungeneralized) are available for download.

Differences from individual state datasets

The U.S. Historical Counties dataset was derived from the individual state datasets published by the Atlas of Historical County Boundaries project. The individual state datasets were first combined, and then a number of adjustments were made to produce the full United States dataset.

Duplicate copies of counties were removed. These generally were early versions of counties that crossed several modern state borders, and were therefore included in several state datasets.
Proposed counties were removed, as were counties from the extralegal Territory of Jefferson. Polygons representing overlap areas between counties or disputed areas were also removed.
County boundary changes occurring after 31 December 2000 were eliminated from the dataset. This removed Broomfield, Colorado, which was created on 15 November 2001.
Areas under the jurisdiction of foreign governments after the colonial period were excluded. These included Spanish areas in Texas and Florida, British areas in Florida, Mexican municipalities established in Texas, as well as several counties established in New Mexico before that area became part of the United States.
Partial counties within separate state datasets were combined as necessary to form complete versions of various counties or non-county areas, such as the Unorganized Federal Territory and the Northwest Territory.
In some cases new polygons were added to the dataset, generally for early non-county areas such as the Louisiana Purchase and Department of Alaska which were not included in the individual state datasets.
Additional county polygons were added to the United States dataset when there was a jurisdictional change without a county boundary change; for example, when an area changed from Wyoming Territory to the state of Wyoming.

State exceptions

In the case of South Carolina there were three overlapping layers of county-level jurisdiction; counties (10 May 1682 - 28 Jul 1769, and 12 Mar 1785 - 31 Dec 2000), parishes (30 Nov 1706 - 26 Sep 1865), and judicial districts (29 Jul 1769 - 15 Apr 1868).

Louisiana also had two overlapping layers of jurisdiction; counties (10 Apr 1805 - 24 Jan 1846), and parishes (14 Apr 1807 - 31 Dec 2000).

Polygons for all of these overlapping jurisdictions are included in the dataset, with the layer specified by the CNTY_TYPE field value in the attribute table.

In addition, all changes for Alaska are by either the end of the particular census year, or by the official date of the census (when known). Alaska boroughs and census areas are treated as county equivalents in the dataset.

U.S. Historical States and Territories

The U.S. Historical States and Territories dataset contains polygons representing the various historical configurations of state, territorial, and equivalent boundaries in the United States, from 3 September 1783 to 31 December 2000. This dataset was generated from the U.S. Historical Counties dataset, which is turn was derived from the individual state datasets created by the Atlas of Historical County Boundaries project.

This starting date of 3 September 1783 was chosen because it is the date of the Treaty of Paris, which established the boundaries of the United States after the War of the American Revolution. Prior to that date, the U.S. Historical Counties dataset focuses on county boundaries, and does not include all of the extensive and conflicting territorial claims of the original 13 colonies. The U.S. Historical Counties dataset therefore cannot be used to create polygons for the full extent of colonial boundaries and claims.

Except for the creation of West Virginia, the U.S. Historical States and Territories dataset does not include any jurisdictional changes related to the Confederate States of America and the secession of southern states.