Lazy raster

A lazyraster is a metadata-only shell around a raster file source. Only metadata is read and and used for extent and resolution logic. A lazyraster may be cropped lazily and if plotted or converted to in-memory raster only a reasonable level-of-detail is actually used.

lazyraster(gdalsource, band = 1, sds = NULL, ...)

Arguments

gdalsource	a file name or other source string
band	which band to use, defaults to 1
sds	which subdataset to use, set to 1 if in doubt (see vapour::vapour_sds_names)
...	ignored for now

Value

a lazyraster object, a simple shell around a GDAL raster source

Details

See crop() for cropping - it works the same but returns a lazyraster, and as_raster() for converting to an in-memory raster.

Warning

If the inferred Y extents appear to be reversed (ymax > ymin) then they are reversed, with a warning. This occurs for any GDAL data source that does not have a geotransform and so is required for use with raster. This might not be the right interpretation, geotransforms are very general and it might mean the data is meant to be oriented that way. (I don't know why GDAL sets a positive Y pixel height as the default, it's a bit of a pain - should the data be flipped, or should Y be interpreted negative - no way to know!).

Examples

sstfile <- system.file("extdata/sst.tif", package = "vapour")
lazyraster(sstfile)
#> class            : LazyRaster
#> dimensions       : 286, 143 (nrow, ncol)
#> resolution       : 0.07000000, 0.07000389 (x, y)
#> extent           : 140.00000, 150.01000, -60.01833, -39.99722 (xmin, xmax, ymin, ymax)
#> crs              : <placeholder>
#> values           : NA, NA (min, max - range from entire extent)
#> window extent    : <whole extent>
#> window index     : <->
#> window dimension : (full) (ncol, nrow)
## convert to raster (in memory, but not all of the source)
as_raster(lazyraster(sstfile))
#> class      : RasterLayer 
#> dimensions : 504, 504, 254016  (nrow, ncol, ncell)
#> resolution : 0.01986111, 0.03972443  (x, y)
#> extent     : 140, 150.01, -60.01833, -39.99722  (xmin, xmax, ymin, ymax)
#> crs        : NA 
#> source     : memory
#> names      : layer 
#> values     : -3.4e+38, 289.859  (min, max)
#> 
## crop and stay as lazyraster
crop(lazyraster(sstfile), raster::extent(142, 143, -50, -45))
#> class            : LazyRaster
#> dimensions       : 286, 143 (nrow, ncol)
#> resolution       : 0.07000000, 0.07000389 (x, y)
#> extent           : 140.00000, 150.01000, -60.01833, -39.99722 (xmin, xmax, ymin, ymax)
#> crs              : <placeholder>
#> values           : NA, NA (min, max - range from entire extent)
#> window extent    : 141.96000, 143.01000, -50.00778, -44.96750
#> window index     : 27, 42, 71, 143
#> window dimension : 16, 73 (ncol, nrow)
## crop and convert to raster
as_raster(crop(lazyraster(sstfile), raster::extent(142, 143, -50, -45)))
#> class      : RasterLayer 
#> dimensions : 504, 504, 254016  (nrow, ncol, ncell)
#> resolution : 0.002083333, 0.01000056  (x, y)
#> extent     : 141.96, 143.01, -50.00778, -44.9675  (xmin, xmax, ymin, ymax)
#> crs        : NA 
#> source     : memory
#> names      : layer 
#> values     : 280.418, 284.397  (min, max)
#>