TDM-EST-TAZ-Age-Percent Methodology

Goal: Estimate age‑group shares (0–17, 18–64, 65+) for each USTM v4 TAZ using 2020 Census blocks, 2023 ACS age structure, and spatial splits, with sensible fallbacks and QC.

1. Inputs & setup

   • Geometry: 2020 Census blocks for Utah; USTM v4 TAZ polygons (must include CO_TAZID); USTM v3 Medium Districts (DISTMED) for temporary district mapping.

   • Census data:

     • 2020 Decennial PL at block: P1_001N (total pop), P3_001N (18+ pop).
     • 2023 ACS 5‑year B01001 at block group and tract to derive 0–17, 18–64, 65+ counts and %65+ of 18+.

   • CRS: Reproject as needed; use a projected UTM CRS for accurate areas/centroids.

2. Assign Medium Districts to TAZs

   • Compute TAZ centroids in a projected CRS.
   • Spatially join centroids to v3 Medium District polygons to attach DISTMED to each TAZ.
   • Join DISTMED back to the original TAZ polygons.

3. Create spatial splits

   • Block→TAZ splits: Overlay blocks with TAZs; compute intersection areas.
   • Filter out slivers: drop intersected pieces < 1% of the original block area.
   • For each block, recompute the remaining total area and normalize each piece to get blk_area_ratio that sums to 1 across its kept pieces.
   • Parse GEOIDs into state, county, tract, blk, blkgrp.
   • Tract→TAZ splits: Aggregate intersected areas to tract–TAZ, compute tract_area_ratio by normalizing within each tract.

4. Fetch and prepare demographic inputs

   • 2020 PL at block for all Utah counties: total population and 18+ population.

   • 2023 ACS B01001:

     • Block group level: compute Pop_0to17, Pop_18to64, Pop_65plus; derive acs_blkgrp_pct65PlusOf18Plus = Pop_65plus / (Pop_65plus + Pop_18to64).
     • Tract level: same aggregation to get acs_tract_pct65PlusOf18Plus.

5. Estimate age detail on split geographies

   • Block‑split estimates (preferred source):

     • Allocate total and 18+ block population to each block–TAZ piece using blk_area_ratio.
     • Apply the block‑group %65+ of 18+ to the allocated 18+ to estimate 65+; derive 18–64 as residual of 18+; derive 0–17 as total − 18+.
     • Enforce a single county label per TAZ by replacing piece‑level county with the mode county within each CO_TAZID.

   • Tract‑split estimates (fallback):

     • Allocate tract‑level total and 18+ populations to tract–TAZ using tract_area_ratio.
     • Apply the tract‑level %65+ of 18+ to estimate 65+; derive 18–64 and 0–17 as above.
     • Apply the same county‑mode enforcement per CO_TAZID.

6. Aggregate to reporting geographies & set source precedence

   • By TAZ (block source): Sum block‑split estimates to CO_TAZID × county; keep TAZs with total estimated persons ≥ 50; tag SOURCE = "block".
   • By TAZ (tract fallback): For any CO_TAZID not present in the block result, sum tract‑split estimates; tag SOURCE = "tract".
   • By Medium District fallback: Additionally aggregate block‑split estimates by DISTMED × county, then map back to TAZs via (DISTMED,CO_FIPS) for any remaining CO_TAZID missing from both block and tract; tag SOURCE = "distmed".
   • Concatenate all sources and sort by CO_TAZID.

7. Convert to percentages & quality checks

   • Compute PCT_0TO17, PCT_18TO64, PCT_65P as shares of (0–17 + 18–64 + 65+).
   • Round to two decimals with a final small adjustment so the row sum equals exactly 1.00 (PCT_SUM check).
   • Verify no duplicate CO_TAZID remain after source precedence.
   • Keep CO_FIPS (county FIPS), SOURCE, and ;CO_TAZID (Excel‑friendly leading semicolon).

8. Output

   • Write results/Lookup - BYTAZAgePct - AllCo.csv with columns:

     • ;CO_TAZID, PCT_SUM, PCT_0TO17, PCT_18TO64, PCT_65P, SOURCE, CO_FIPS.

Notes & assumptions

• Sliver threshold = 1% of the original block area (applied before normalizing proportions).
• Population thresholds: exclude TAZ rows with fewer than 50 estimated persons (per source path) to reduce instability.
• ACS year = 2023 to reflect recent age structure; 2020 used for cross‑checks where noted.
• Medium District mapping is temporary (v3 districts) until districts are assigned directly to the new TAZs.