From peter@caliper.com Mon Jul 9 14:45:54 2001 Received: from mxu3.u.washington.edu (mxu3.u.washington.edu [140.142.33.7]) by lists.u.washington.edu (8.11.2+UW01.01/8.11.2+UW01.04) with ESMTP id f69Ljp055996 for ; Mon, 9 Jul 2001 14:45:51 -0700 Received: from iris.caliper.com (iris.caliper.com [198.22.17.3]) by mxu3.u.washington.edu (8.11.2+UW01.01/8.11.2+UW01.04) with ESMTP id f69LjoX11483 for ; Mon, 9 Jul 2001 14:45:51 -0700 Received: from HEMLOCK.caliper.com (hemlock.caliper.com [198.22.17.104]) by iris.caliper.com (8.9.3/8.9.3) with ESMTP id RAA03776 for ; Mon, 9 Jul 2001 17:44:47 -0400 (EDT) Message-Id: <4.3.1.2.20010709172844.015e0668@iris.caliper.com> X-Sender: peter@iris.caliper.com X-Mailer: QUALCOMM Windows Eudora Version 4.3.1 Date: Mon, 09 Jul 2001 17:44:15 -0400 To: waphgis@u.washington.edu From: "Peter H. Van Demark" Subject: RE: Zip Codes, ZCTA and areal interpolation In-Reply-To: Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii"; format=flowed All: There seem to be some misunderstandings of ZIP Codes and ZCTAs: 1. ZCTAs are new for Census 2000. They have never existed before. 2. ZIP Codes are not areas, they are collections of mailing addresses. Any attempt to put a boundaries around collections of mailing addresses will be flawed. There are two main strategies: - Commercial ZIP Codes generally bound the mailing addresses based on grouping blocks that have the greater length of their block faces (or all of their block faces) in a particular ZIP Code. - ZCTAs bound the mailing addresses based on grouping blocks that have the greater number (or all) of the households in a particular ZIP Code. The Census Bureau can do that because they have (confidential) information on each household, from the census forms. 3. Except for dealing with areas that don't already have ZIP Codes (water, for example), ZIP Codes and ZCTAs will have the same numbers. 4. If you want to do longitudinal analysis based on stable postal geography in the U.S., forget it: the USPS is interested in delivering the mail, not in providing a framework for research and analysis. The ZCTAs are just the Census Bureau's attempt to bound these unstable collections of mailing addresses. Census tracts are a little better, but even they change every ten years. Peter ---------------------------------------------------------------- Peter Van Demark Director of GIS Products and Training Phone: 617-527-4700 Caliper Corporation Fax: 617-527-5113 1172 Beacon Street E-mail: peter@caliper.com Newton MA 02461-9926 Web site: http://www.caliper.com .