# Mixed UTM zones and terra collections

BlueTopo is published as source tiles. Adjacent tiles can differ in CRS, native cell size, origin, alignment, or delivery date. `bluertopo` treats those differences as source facts rather than hiding them with automatic resampling.

## Native output

    native <- bluertopo(
      aoi,
      resolution = "native",
      coverage = "fill",
      combine = "auto"
    )

When selected rasters share a compatible grid, the result is a single `terra::SpatRaster`. When grids differ, the result is a `terra::SpatRasterCollection` with stable collection names derived from grid metadata.

    class(native)

## Requesting a collection

Use `combine = "collection"` when downstream code should always receive a collection, even for a single compatible grid.

    collection <- bluertopo(
      aoi,
      resolution = "native",
      combine = "collection"
    )

## Requesting one output grid

Use `combine = "single"` together with `output_crs` and `output_resolution` when your analysis requires one grid. This is explicit resampling and is reported by a warning.

    single <- bluertopo(
      aoi,
      resolution = "native",
      output_crs = "EPSG:26918",
      output_resolution = 10,
      combine = "single",
      resampling = c(
        elevation = "bilinear",
        uncertainty = "bilinear",
        contributor = "near"
      )
    )

Geographic target resolutions are rejected in this release because degree-based cell sizes are ambiguous for bathymetric analysis. Use a projected CRS for explicit output grids.