From fpb01@health.state.ny.us Thu Jul 5 12:00:46 2001 Received: from mxu4.u.washington.edu (mxu4.u.washington.edu [140.142.33.8]) by lists.u.washington.edu (8.11.2+UW01.01/8.11.2+UW01.04) with ESMTP id f65J0a049198 for ; Thu, 5 Jul 2001 12:00:36 -0700 Received: from gate1.health.state.ny.us (gate.health.state.ny.us [192.135.176.62]) by mxu4.u.washington.edu (8.11.2+UW01.01/8.11.2+UW01.04) with SMTP id f65J0Zs26026 for ; Thu, 5 Jul 2001 12:00:35 -0700 Received: by gate1.health.state.ny.us id PAA22780 (InterLock SMTP Gateway 4.2 for waphgis@u.washington.edu); Thu, 5 Jul 2001 15:00:29 -0400 Message-Id: <200107051900.PAA22780@gate1.health.state.ny.us> Subject: Re: Zip Codes, ZCTA and areal interpolation. To: waphgis@u.washington.edu From: fpb01@health.state.ny.us Date: Thu, 5 Jul 2001 14:59:44 -0400 X-Mimetrack: Serialize by Router on MailGate/Server/DOH(Release 5.07a |May 14, 2001) at 07/05/2001 03:00:31 PM Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii I believe that conversion from Zip Codes to ZCTAs is, unfortunately, more than just an areal interpolation problem. The two entities are not the same thing - they differ along the edges. On a road that forms the boundary of a zip code, both sides of the road may be in the same Zip Code. This is reasonable and facilitates the delivery of the mail. But the two sides of the road will be in different census blocks and hence different ZCTAs (since ZCTAs are built from census blocks). In heavily populated areas, the distinction is almost negligible, but not in rural areas. In fact, the census produced a graph which shows that zip code-ZCTA agreement is almost perfect in zip codes with more than 15,000 addresses, but under 80% where there were fewer than 250 addresses. In terms of health mapping, if a nursing home or hospital or other special type of address falls on a zip boundary, its entire caseload could be assigned to the wrong ZCTA, resulting in substantial error even in a heavily populated area. Frank Boscoe NYS Department of Health .