# Layers, contributor values, and RAT metadata

BlueTopo source GeoTIFFs currently include elevation, vertical uncertainty, and contributor/source identifier bands. `bluertopo` exposes those as canonical layer names.

## Elevation only

    elevation <- bluertopo(
      aoi,
      layers = "elevation"
    )

## All canonical layers

    all_layers <- bluertopo(
      aoi,
      layers = "all",
      details = TRUE
    )

    names(all_layers$data)

The contributor band contains numeric identifiers. Interpret those identifiers with the NOAA RAT sidecar metadata and source documentation. `bluertopo` can download RAT sidecars with the original assets, but it does not currently normalize contributor tables into a package-level lookup.

NOAA’s BlueTopo product page is the package’s primary external reference: https://nauticalcharts.noaa.gov/data/bluetopo.html.

    assets <- bluertopo_download(
      aoi,
      path = file.path(tempdir(), "bluertopo-downloads"),
      rat = TRUE
    )

    subset(assets, asset_type == "rat")

## Vertical-reference metadata

`bluertopo(details = TRUE)` includes vertical-reference text discovered by `terra`/GDAL metadata inspection when available.

    result <- bluertopo(aoi, details = TRUE)
    result$provenance$vertical_reference_text