Add 3DEP_1M data source for native 1-meter DEMs

[scottstanie]

Summary

• Adds --data-source 3DEP_1M for accessing native 1-meter resolution lidar DEMs from the USGS 3DEP program
• Queries the TNM Access API to discover which tiles cover the requested bbox, then fetches them as Cloud Optimized GeoTIFFs from S3 via /vsicurl/
• Uses a two-step GDAL warp: first reproject from source UTM (various zones) to EPSG:4326, then convert NAVD88 heights to WGS84 ellipsoidal

Why a separate source from 3DEP?

The existing 3DEP source uses the ImageServer exportImage endpoint, which dynamically resamples all 3DEP data at the requested resolution (hardcoded to ~30m via DEFAULT_RES). While the ImageServer can serve 1m data at fine pixel sizes, it has practical limitations:

• 2000-pixel export limit per chunk (would need thousands of requests for large areas at 1m)
• Silently falls back to coarser data where 1m coverage doesn't exist
• Dynamic resampling vs. native source tiles

The 3DEP_1M approach:

• Explicitly discovers 1m tiles and fails clearly when coverage doesn't exist
• Direct COG access from S3 (efficient partial reads, no chunk limits)
• Outputs at native ~1m resolution

Key fix during review

The original implementation set srcSRS = "EPSG:4269+5703" (NAD83 geographic + NAVD88), but 1m tiles are in NAD83/UTM (meters). This would cause GDAL to misinterpret UTM meter coordinates as geographic degrees. Fixed with a two-step warp:

1. Reproject UTM → EPSG:4326 (let GDAL read source CRS from file metadata)
2. Convert NAVD88 → WGS84 ellipsoidal (now that coords are geographic, compound CRS is correct)

Test plan

• Unit tests for TNM API tile discovery (mock-based, 3 tests)
• All 16 existing tests pass
• Integration test with real download: sardem --bbox -118.4 33.7 -118.3 33.8 --data-source 3DEP_1M -o test_3dep_1m.tif

🤖 Generated with Claude Code