While most Tiger Polygons are simply connected, this is sometimes false.
Example: Hudson County in New Jersey (tgr34017.zip) has the polygon CENID-POLYID C4562-2886 which contains CENID-POLYID C4562-3333 (``Reservoir No. 2''). Similarly, CENID-POLYID C4562-2970 contains C4562-4157.
NOTE: In the TIGER files, polygons are uniquely identified using two fields: CENID (Census File Identification Code), and POLYID (Polygon Identification Code). CENID is a 5 character alphanumeric field, and POLYID is a numeric field of at most 10 digits (if less than 10, it is right- justified and padded with blank spaces).
While most counties are simply connected, they can violate this in one of two ways. Thus, New York County (a.k.a. Manhattan, tgr36061.zip) is not connected, because it owns Statue of Liberty Island and part of (!) Ellis Island. The water surrounding these two outposts of New York County belongs to Hudson County (tgr34017.zip) of New Jersey.
Hudson County, from what we just said, is therefore not simply connected, though it is connected.
DETAILS: Ellis Island today is partly landfill. The original Ellis Island (polygon C4714-3852) belongs to New York, and this is completely surrounded by landfill belonging to New Jersey (polygon C4562-4314).
Actually, the structure of this landfil is quite complex and appears to be the intersection of two polygons (perhaps surveys performed at different periods, but simply overlaid into the data set). As a result there are actually many tiny polygons (triangles and quadrilateral) abutting the large polygon C4562-4312. This is what you would expect when you overlay two polygons that roughly cover the same area. These tiny polygons are C4562-4464, -4367, -4457, -3191, -443, -1081, -2463, and -397.
The following is a screen shot of Ellis Island surrounded by the landfill (roughly rectangular shape) in the Morris Can Basin.
Ellis Island and Statue of Liberty Island (polygon C4714-3169 in New York) together form the Statue of Liberty National Park. The waters surrounding both islands is the Morris Can Basin (polygon C4562-2623), which is in New Jersey. Thus we have a triple nesting of original Ellis Island inside a landfill inside Morris Can Basin.
While most states are subdivided into counties, Virginia has many townships that are not counties. These townships are treated as counties from the viewpoint of TIGER data organization. Moreover, the areas of the townships may be fully contained in a real county. There is even an example where such a contained township, in turn, contains a piece of the surrounding county.
DETAILS: Prince William County (tgr51153.zip) has two holes that correspond to the town of Manassas (tgr51683.zip) and Manassas Park (tgr51685.zip) (they are almost contiguous). In turn, Manassas has a polygon (C6870-387) that contains polygons C6782-3351 and C6782-3352 belonging to Prince William County.
The preceding example of Prince William County and Manassas has one implication for displays: if you want to display the counties using the painter's algorithm, you cannot.
Tiger Lines are supposed to be globally consistent; that is, the endpoints and detail points of Tiger lines that are in two counties (county border lines) should agree. (They may have different LINEIDs.) However, when working with multiple counties, it is important to be sure that both counties are from the same release! Most of the TIGER files indicate the release in the VERSION field in characters 2-5 of each record. This field is in the form MMYY, for the month and year that the data was extracted from the TIGER data base.
In order to be sure that all our data is from the same release, even though we may download different parts at different times, we have decided to use the "Redistricting Census 2000 TIGER/Line Files". This is the "official" TIGER data that was sent to the states to use for redistricting purposes, and it was extracted from the TIGER database in October of 2000 (MMYY = 1000). It is not the most recent release, but it is a "static" release, that they leave on their website without changing, so different states and counties can be downloaded at different times without mixing counties from different releases.
Unfortunately, we have found two mistakes in the Redistricting 2000 release. (1) On the border between Warren County, Ohio, and Clermont County, Ohio, the tiger line 118457848 in Clermont matches up with two lines in Warren: 159041118 and 159041119. The line 118457848 should be split into two lines to match the two lines in Warren. (2) On the border between Clay County, Missouri, and Jackson County, Missouri, the tiger line 89768904 in Clay matches up with three lines in Jackson: 91515084, 91515090, and 91515091. The line 89768904 should be split into three lines to match the three lines in Jackson. We have informed the Census Bureau of these problems, and they will be adding them to their errata page. The errors may also appear in more recent releases.