By Paul Cote For Guy Stuart March 21, 2003
Overview: This document describes a procedure for digesting and assembling census TIGER files for a regional study examining the types of boundaries that divide demographic areas in a region.
This set of procedures is intended to take raw tiger files, to convert them to ArcInfo coverages, do various manipulations on these, join them with a table of demographic variables. The ouput is an ArcInfo coverage database that and then put the result into a ArcGIS geodatabase, with a bunch of standard layers so that they can be easily explored in ArcMap.
The directory on the CD, named asmple is an example of a directory that would be created for any geographic refion that would be studied. This directory contains the raw material, the scripts and the outputs related to the procedure as applied to Suffolk and Norfolk counties, Massachusetts.
The files and directories in sample:
If you use ArcGIS to open the file arcmap\arcgis.mxd you will be able to look at the geometric results, and the associated tabular attributes.
The layers in arcmap.mxd are different views of the polygon and arc feature classes that resulted from the CALVIN procedure. These are linked with the census feature class lookup table, or the table of block demographic attributes. Roll over the names in the left column of the following table to see illustrations.
|Water Polygons||Elementary and Unified School Districts||Voting Districts||Polygon Landmarks||Key Geographic Locations||Percent White||Arc (Line) Features|
The topological structure of TIGER files and ArcInfo coverages stores the relationship between each line segment and the polygons that fall on the left and right side. Which side is the left, and which is the right is arbitrary. If we zoom in on the map and use the info tool, we can look at the attributes of the polygons and lines. For example, the cfcc attribute for each line designates the type of line, which may be a physical or invisible attribute. The attribute for segdif contains the difference in the segregation for each side of the line. The attribute, length, tells us the length in meters for each of the lines. This table should give us enough information to answer questions such as: What is the total length of arcs that are voting district boundaries? What is the total length of arcs that are voting district boundaries AND divide blocks having a difference in segregation over a certain amount?
|Arcs by Census Feature Class Code||Arcs that are voting district boundaries||Arcs colored by the differencr in Segregation on either side||An arc showing its left and right attributes.|
The complete attribute tables for each layer in ArcMap may be accessed by right-clicking on the layer name and selectin Open Attribute Table. Once opened, the attribute table may be exported to a dbf file by selecting Export from the options menu at the bottom of the table.
Here is a data dictionary for the arc attribute table.
To run this program, follow these steps:
arc: w c:\chicago
arc: &r calvin.amlIf you have run calvin before, there may be pre-digested TIGER file coverages existing. Calvin will ask if you want to remove these. If the last run of calvin was successful, then you should keep them. This will save a bunch of time. Specifying 'all' at this prompt will remove them all.