ZIP Code Tabulation Area Delineation for 2010

1. ZIP Code Tabulation Area Delineation for 2010 • Initial ZCTA line insertion (before the 2010 benchmark) • Assigning 2010 blocks to 2010 ZCTAs (after initial Final Tab Benchmark, before SF-1 delivery)

2. Initial ZCTA Line Insertion Steps for the Initial ZCTA Line Insertion 1. Download ACS09 tabulation block and edge shapefiles 2. Extract most current address data 3. Run ArcGIS-based ZCTA line insertion software.

3. Initial ZCTA Line Insertion Software S1. Identify potential split blocks that do not meet the majority ZIP Code percentage threshold Get the percentage of MAF Addresses for each ZIP Code within each block Block 1 Boundary and address points ZIP Code Percentages for Block 1 Any block where the most frequently occurring ZIP Code is less than the minimum threshold of 80% becomes a potential split block. Potential split blocks may be split by the insertion of ZCTA split lines.

4. Initial ZCTA Line Insertion Software An Example of Block and Address Configuration Where a Block Split Should Occur The most frequently occurring ZIP Code in this block is associated with only 60% of the addresses.

5. Initial ZCTA Line Insertion Software S2. Create Thiessen Polygons for all addresses that are within potential split blocks Any point within a Thiessen/Voronoi Polygon is closer to the seed point than it is to any other seed point.

6. Initial ZCTA Line Insertion Software S3. Dissolve the Thiessen Polygons by ZIP Code

7. Initial ZCTA Line Insertion Software S4. Transform the dissolved Thiessen Polygons into lines This is now a line that can be used to split the block

8. Initial ZCTA Line Insertion Software S5. Remove branching potential ZCTA lines that are not critical There are lines that may have been created that are unnecessary and can be removed without affecting any addresses. Since there are no points within the polygon formed by line a, line b, and the block boundary, either line a or line b can be removed Line b has been removed. The resulting ZCTA line still maintains the same points on each side of the line. The same would be true if line a were eliminated rather than line b. a a b

9. Initial ZCTA Line Insertion Software S6. Check the left and right polygons of each potential ZCTA line and verify that the following criteria are met: a). There are at least 5 addresses* on each side of the line. b). The most frequently occurring ZIP Code on at least one side of the line meets the minimum threshold of 80 percent of the total number of addresses* on that side on the line. Fails: a). More than 5 addresses on each side b). Only 57% and 76% for most frequently occurring ZIP Code on respective sides Qualifies: a). More than 5 addresses on each side b). More than 80% max zip on each side Fails: a). Less than 5 addresses on north side b). No test required since test a failed *Group Quarters (GQ) addresses receive a weight of 5 so that a GQ with a unique ZIP Code may be held as a ZCTA Boundary

10. Initial ZCTA Line Insertion Potential ZCTA Boundary Preexisting MTDB Line Software S7. Identify preexisting MTDB edges that can be used as a part of a ZCTA boundary • There will be situations where a portion of a ZCTA line can be approximated by an edge that already exists in the MTDB. • There is no reason to add more lines into the MTDB when they are not necessary • Consider the example below:

11. Initial ZCTA Line Insertion Potential ZCTA Boundary Potential ZCTA Boundary Preexisting MTDB Line Preexisting MTDB Line Software Since this gray shaded area does not contain any addresses, the portion of the ZCTA line that shares a boundary with the shaded polygon can be deleted. The 2010 Block Boundary Flag (CBBFLG) also needs to be set to 3 so the existing edge gets held as a block boundary. Area of Intersection of MTDB line, ZCTA line and block boundary The result is that a current block will be split formed using a preexisting MTDB line and a portion of the original ZCTA line

12. Initial ZCTA Line Insertion Software Example of Holding Preexisting MTDB Features as ZCTA Boundaries MAF address points for Zip Code 1 MAF address points for Zip Code 2 ZCTA boundary to be inserted into MTDB ZCTA boundary that will not be added Preexisting hydro (will be held)

13. Initial ZCTA Line Insertion Software Example of Adding a Split to a Block

14. Final ZCTA Assignment Software A1. Assign each 2010 tabulation block to the ZCTA associated with the most frequently occurring ZIP Code in the block. A2. Verify all addresses are assigned to a ZCTA. A3. Verify each resulting ZCTA has includes at least 40 residential addresses. A4. Review all noncontiguous ZCTAs. A5. Fill in enclave blocks that are completely surrounded by a single ZCTA. Examples are on the following slide: A6. Review all enclaves between ZCTAs. A7. After loading ZCTAs into the MAF/TIGER Database, create files of the percentage of addresses with a specific ZIP Code for each ZCTA.

15. Final ZCTA Assignment Software There are no addresses in this block, but it is an enclave that is completely surrounded by the same ZCTA, so it gets assigned to the neighboring ZCTA. This block has addresses, but they are split 50/50. It is an enclave that is completely surrounded by the same ZCTA, so it gets assigned to the neighboring ZCTA.

16. Final ZCTA Assignment Software A6. Identify any unassigned blocks that have addresses and assign them to the ZCTA that has the longest shared boundary. This step is necessary to assign blocks that have addresses, do not have a most frequently occurring ZIP Code (i.e. there is a tie for the most frequently occurring), and are not enclaves. This block does not have a most frequently occurring ZIP Code, but it has addresses. Because 75% of its boundary is contiguous with the green ZCTA, it will be assigned to the green ZCTA.