2/8 Normalize satellite timestamps across units - #39
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Convert per-pixel acquisition timestamps to elapsed days with NumPy timedelta arithmetic rather than assuming nanosecond precision. This preserves correct LOWESS timing for microsecond-resolution MODIS timestamps, preventing distinct observations from collapsing onto day zero; add a regression test covering the affected normalization path.
Timothy-W-Hilton
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Normalize satellite timestamps across units
Summary
Convert per-pixel satellite acquisition times to elapsed days using NumPy
timedelta64arithmetic rather than assuming that datetime values use nanosecond units.Why
vprm_preprocessor.sort_and_merge_by_timestamp()previously divided elapsed datetime values by1e9and then by the number of seconds per day. That is correct only fordatetime64[ns]. MODIS inputs can carrydatetime64[us]timestamps; under the old calculation, distinct acquisition dates then collapsed toward day zero. LOWESS consequently received incorrect time coordinates and could return missing EVI/LSWI values.Change
Divide the elapsed NumPy timedelta directly by
np.timedelta64(1, "D"). This is independent of the underlying datetime precision and produces elapsed days for nanosecond-, microsecond-, and other supported NumPy datetime units.Validation
[0.0, 8.0], rather than collapsing both observations to zero.1 passed(one existing deprecation warning from xarray'sdropmethod).