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.