Also, because ZIP Codes are not true geographic entities (they are collections of addresses that share a common numeric identifier) it is not possible to determine the extent of territory they cover with any precision. Therefore, the Census Bureau does not provide area measurement values for them, even though the 1990 STF 3B ZIP Code data tabulations are based on whole census blocks.
The accuracy of any area measurement data is limited by the inaccuracy inherent in (1) the location and shape of the various boundary features in the TIGER data base, (2) rounding affecting the last digit in all operations that compute and/or sum the area measurement values of individual polygons, and (3) conversion from square meters to other measures. Furthermore, it is preferable to sum the areas of polygons, or census blocks and/or tabulated block groups, rather than to perform an independent measurement of an entire large area, because independent area measurement values tend to become inaccurate for very large geographic entities. Although the effects of these limitations tend to be insignificant, especially since they affect measurements represented in very small units (square meters), users of the data should be aware that these inaccuracies exist.
As updated USGS files and other more accurate map bases replace current information in the TIGER data base, and as the GBF/DIME-File features there are replaced by new USGS or local records, the water and land content of the TIGER data base will become more accurate. Basing the TIGER records for Alaska on a data base similar to the one used for the coterminous 48 States will improve the land and water definition considerably, and can be expected to change the area measurement figures for portions of this State significantly. The result will be further changes to the area measurement figures of the United States and its component geographic entities when the 2000 census figures are calculated—changes that may have nothing to do with any changes to the areal configuration of the United States or those entities.
