Return the basic structural metadata of a raster source understood by GDAL. Subdatasets may be specified by number, starting at 1. See vapour_sds_names for more.
vapour_raster_info(x, ..., sds = NULL, min_max = FALSE)data source string (i.e. file name or URL or database connection string)
currently unused
a subdataset number, if necessary
logical, control computing min and max values in source ('FALSE' by default)
Some sources provide multiple data sets, where a dataset is described by a 2- (or more) dimensional grid whose structure is described by the metadata described above. Note that subdataset is a different concept to band or dimension. Sources that may have multiple data sets are HDF4/HDF5 and NetCDF, and they are loosely analogous to the concept of layer in GDAL vector data. Variables are usually seen as distinct data but in GDAL and related 2D-interpretations this concept is leveraged as a 3rd dimension (and higher). In a GeoTIFF a third dimension might be implicit across bands, i.e. to express time varying data and so each band is not properly a variable. Similarly in NetCDF, the data may be any dimensional but there's only an implicit link for other variables that exist in that same dimensional space. When using GDAL you are always traversing this confusing realm.
If subdatasets are present but not specified the first is queried. The choice
of subdataset is analogous to the way that the raster package behaves, and
uses the argument varname. Variables in NetCDF correspond to subdatasets,
but a single data set might have multiple variables in different bands or in
dimensions, so this guide does not hold across various systems.
From https://gdal.org/user/raster_data_model.html.
The affine transform consists of six coefficients returned by
GDALDataset::GetGeoTransform() which map pixel/line coordinates into
georeferenced space using the following relationship:
Xgeo = GT(0) + Xpixel*GT(1) + Yline*GT(2)
Ygeo = GT(3) + Xpixel*GT(4) + Yline*GT(5)
They are
the x position of the lower left corner of the lower left pixel
the scale of the x-axis, the width of the pixel in x-units
y component of the pixel width
the y position of the upper left corner of the upper left pixel
x component of the pixel height
the scale of the y-axis, the height of the pixel in negative y-units
Please note that these coefficients are equivalent to the contents of a world file but that the order is not the same and the world file uses cell centre convention rather than edge. https://en.wikipedia.org/wiki/World_file
Usually the skew components are zero, and so only four coefficients are relevant and correspond to the offset and scale used to position the raster - in combination with the number of rows and columns of data they provide the spatial extent and the pixel size in each direction. Very rarely a an actual affine raster will be use with this rotation specified within the transform coefficients.
Calculation of 'minmax' can take a significant amount of time, so it's not done by default. Use 'minmax = TRUE' to do it. (It does perform well, but may be prohibitive for very large or remote sources.)
If there are no overviews this element will simply be a single-element vector of value 0. If there are overviews, the first value will give the number of overviews and their dimensions will be listed as pairs of x,y values.
The structural metadata are
the affine transform
dimensions x-y, columns*rows
numeric values of the computed min and max from the first band (optional)
dimensions x-y of internal tiling scheme
text version of map projection parameter string
number of bands in the dataset
not implemented
not implemented
the number and size of any available overviews
On access vapour functions will report on the existence of subdatasets while defaulting to the first subdataset found.
vapour_sds_info
# NOT RUN {
f <- system.file("extdata", "sst.tif", package = "vapour")
vapour_raster_info(f)
# }
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