Packed data and time

A VRT points at the bytes in the source files. GDAL’s multidimensional VRT does not apply xarray’s CF decoding, so whatever xarray decoded on open has to be described again for GDAL. xvrt does this from the Dataset’s .encoding.

Scale, offset and nodata

Gridded products are often stored as int16 with scale_factor, add_offset and _FillValue. With xarray’s default mask_and_scale=True the in-memory values are floats and the packing has moved out of .attrs into .encoding:

import xarray as xr
from _fixtures import daily_netcdf

paths = daily_netcdf(n=2)
ds = xr.open_mfdataset(paths, combine="nested", concat_dim="time")
print(ds.sst.dtype, {k: ds.sst.encoding[k] for k in ("dtype", "scale_factor", "_FillValue")})

float64 {'dtype': dtype('int16'), 'scale_factor': np.float64(0.01), '_FillValue': np.int16(-32768)}

xvrt declares the storage dtype and lets GDAL carry the decoding:

from xvrt import write_mdim_vrt

out = write_mdim_vrt(ds, paths, paths[0].parent / "packed.vrt",
                     composition="concat", concat_dim="time")
print("\n".join(l for l in out.read_text().splitlines()
                if any(t in l for t in ("DataType>Int16", "Scale", "Offset", "NoData"))))

      <DataType>Int16</DataType>
      <NoDataValue>-32768.0</NoDataValue>
      <Offset>0.0</Offset>
      <Scale>0.01</Scale>

Declaring the decoded Float64 with no <Scale> instead would hand back raw counts as if they were physical values. scale_factor and add_offset are not repeated as <Attribute>s, so readers that decode from attributes do not apply them twice. With mask_and_scale=False the packing stays in .attrs and the result is the same.

Time

Decoded datetime64 (or cftime) coordinates are encoded back to numbers with xarray’s own CF encoder, using the original units and calendar from .encoding:

print(ds.time.encoding["units"])
print("\n".join(l for l in out.read_text().splitlines()[:12] if "time" in l or "Unit" in l))

days since 2026-01-01 12:00:00
    <Dimension name="time" size="2" indexingVariable="time" type="TEMPORAL" />
    <Array name="time">

This keeps an origin with a time of day (12:00:00 here) and non-standard calendars such as noleap. Without stored units the fallback is days since 1970-01-01.