docs: contour tile rebuild design — Terrarium DEM alignment fix

2026-05-20 14:44:39 +02:00 · 2026-04-27 21:17:20 +00:00 · 2026-04-27 21:17:20 +00:00 · 3434ba4cdd
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+# Contour Tile Rebuild — Terrarium DEM Alignment Fix
+
+## Problem
+
+Contour lines are misaligned from hillshade terrain. Root cause:
+- Contours: built from SRTM HGT (EPSG:4326, ~30m resolution)
+- Hillshade: built from Mapzen Terrarium (EPSG:3857, ~10m at z12)
+
+Different DEMs = different elevation values at same coordinates = visible offset.
+
+## Solution
+
+Regenerate contours from the same Terrarium DEM tiles used for hillshade.
+The `hillshade-na.pmtiles` file contains 93GB of Terrarium-encoded RGB PNGs.
+Decode elevation from those tiles, generate contours. Same pixels = guaranteed alignment.
+
+## Infrastructure
+
+| Resource | Location | Specs |
+|----------|----------|-------|
+| Compute | cortex (192.168.1.150) | i9-10900X, 16 cores, 22GB RAM |
+| Storage | pi-nas NFS `/mnt/pi-nas` | 16TB free on /export/data |
+| Source DEM | hillshade-na.pmtiles | 93GB, z0-12, Terrarium encoding |
+| Tools | cortex | GDAL 3.8.4, tippecanoe, pmtiles |
+
+## Terrarium Encoding
+
+```
+elevation_meters = (R * 256 + G + B / 256) - 32768
+```
+
+Source tiles are EPSG:3857 (Web Mercator), 256x256 PNG, z0-12.
+
+## Output Specification
+
+Must match existing `contours-na.pmtiles` structure for frontend compatibility:
+
+```json
+{
+  "layer": "contours",
+  "attributes": {
+    "elevation_ft": "Number",
+    "elevation_m": "Number", 
+    "tier": "String"  // "minor" | "intermediate" | "index"
+  },
+  "zoom": "4-14",
+  "format": "MVT"
+}
+```
+
+Tier mapping:
+- 40ft interval → tier: "minor"
+- 200ft interval → tier: "intermediate"
+- 1000ft interval → tier: "index"
+
+## Regions
+
+Process in chunks to fit RAM:
+
+| Region | Bounds (W,S,E,N) |
+|--------|------------------|
+| alaska | -168,51,-130,72 |
+| canada-west | -141,48,-90,72 |
+| canada-east | -90,42,-52,72 |
+| conus-west | -125,31,-104,49 |
+| conus-central | -104,25,-84,49 |
+| conus-east | -84,24,-66,48 |
+| mexico | -118,14,-86,33 |
+| caribbean | -86,14,-59,28 |
+
+## Pipeline Phases
+
+### Phase 1: Extract DEM from Terrarium PMTiles
+
+1. Read z12 tiles from hillshade-na.pmtiles
+2. Decode Terrarium RGB → elevation (meters)
+3. Mosaic into regional GeoTIFFs (EPSG:3857)
+4. Reproject to EPSG:4326 for gdal_contour
+
+### Phase 2: Generate Contours
+
+For each region:
+```bash
+# Convert meters to feet
+gdal_calc.py -A region.tif --outfile=region_ft.tif --calc="A*3.28084"
+
+# Generate contours at all intervals
+gdal_contour -a elevation -i 40 -f FlatGeoBuf region_ft.tif region_40ft.fgb
+gdal_contour -a elevation -i 200 -f FlatGeoBuf region_ft.tif region_200ft.fgb
+gdal_contour -a elevation -i 1000 -f FlatGeoBuf region_ft.tif region_1000ft.fgb
+```
+
+### Phase 3: Build Vector Tiles
+
+1. Add tier attribute and merge intervals into single FGB per region
+2. Generate mbtiles with tippecanoe:
+   ```bash
+   tippecanoe -o region.mbtiles -l contours -Z4 -z14      --no-feature-limit --no-tile-size-limit      --simplification=10 region_merged.fgb
+   ```
+3. Join all regions: `tile-join -o contours-na.mbtiles *.mbtiles`
+4. Convert: `pmtiles convert contours-na.mbtiles contours-na.pmtiles`
+
+### Phase 4: Deploy
+
+1. Copy to VM 130: `/mnt/nav/tiles/contours-na.pmtiles`
+2. Verify with `pmtiles show`
+3. Browser test at navi.echo6.co
+
+## Workspace
+
+```
+/mnt/pi-nas/nav/contour-rebuild/
+├── dem/           # Regional GeoTIFFs (keep for future rebuilds)
+├── contours/      # Intermediate FlatGeoBuf files (delete after)
+├── tiles/         # Output mbtiles and pmtiles
+├── logs/          # Timestamped logs per phase
+├── scripts/       # Pipeline scripts
+└── backup/        # Original contours-na.pmtiles backup
+```
+
+## Verification
+
+After deploy, manual browser test:
+1. Hard refresh navi.echo6.co
+2. Toggle contours on
+3. Zoom to Twin Falls / Snake River Canyon
+4. Contour lines should align precisely with hillshade canyon walls
+
+## Constraints
+
+- All operations as zvx (pi-nas root_squash)
+- Do not modify hillshade-na.pmtiles (read only)
+- Do not touch frontend code
+- Keep intermediate GeoTIFFs for future interval changes
+- Delete .fgb and .mbtiles after final pmtiles verified
+
+## Created
+
+2026-04-27 — Alignment fix rebuild