What MassGIS provides
As federal digital data products, Census data including TIGER files and matrices are available for purchase directly from the Census Bureau in Washington, D.C. Data are also available to the public at 41 Federal and Census Depository Libraries in Massachusetts, including many university libraries and the Boston Public Library.
MassGIS has extracted and reprocessed data from the original TIGER files for use in its Geographic Information System. The reprocessed Census datalayers have been converted into the Massachusetts State Plane Coordinate system; to minimize processing requirements, the data have been extracted into three datalayers:
- Census 1990 Blocks - Block boundary polygons with a few demographic data items appended from a variety of Census Bureau publications.
- Census 1990 Block Groups - Block Group boundary polygons with many demographic data fields from the Census Bureau's STF-1a and STF-3 publications.
- Census 1990 TIGER Linework - complete set of TIGER linework - roads, trains, rivers, boundaries, etc.
Considerations when using TIGER data
The development of a nationwide, standard 1:100,000 scale geographic data set for the 1990 Census has been hailed as the "backbone" of a federal geographic data infrastructure. The TIGER files are a unique resource, containing a wealth of geographic data attributes unavailable in earlier data sets such as the 1:100,000 scale Digital Line Graphs published by the U.S. Geological Survey. The link between the TIGER files and Census Bureau data--and potentially with data to be published by other federal agencies--makes TIGER data an attractive option for GIS users. Furthermore, the relatively low cost of Census Bureau data and its availability at depository libraries makes TIGER data easily accessible.
As with all sources of GIS data, TIGER data is not suitable for use at scales larger than that at which it was compiled. In the case of TIGER data this scale is 1:100,000--a regional scale which is not recommended for use on the larger scale of a Massachusetts town. MassGIS has found the accuracy of TIGER linework to be inconsistent, especially in metropolitan areas where a variety of source maps were used to compile the TIGER files. Another concern for potential users of this data is the size of the TIGER files. As issued by the Census Bureau, county TIGER files are very large and may strain the processing capacities of microcomputers; the smaller town coverages produced by MassGIS may reduce this problem.
TIGER linework frequently does not match the MassGIS "base map" coverages, so care should be exercised when using other MassGIS datalayers together with the Census datalayer for spatial analysis. For this reason, the individual town coverages may contain small polygons with Census codes relating to neighboring towns.
Due to the large volume of data, not every town in the Commonwealth has been checked systematically. The TIGER files contain many errors that were created by the Census Bureau during the production process--for example, legitimate arcs that are smaller than 0.1 feet in length. While these arcs have little meaning in a cartographic database, they are part of the TIGER data structure; without them, the relationship between graphics and attributes is degraded or destroyed. In order to maintain this relationship, MassGIS does not intend to edit or make corrections to the TIGER linework.