# 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