easysnowdata.processing.normalized_difference#

easysnowdata.processing.normalized_difference(a: DataArray, b: DataArray, *, valid_range: tuple[float, float] | None = (-1.0, 1.0)) → DataArray[source]#

(a - b) / (a + b) as float, NaN where the index is undefined.

A normalized difference of two non-negative quantities is in [-1, 1] by construction. Surface reflectance is not always non-negative: atmospheric correction over dark targets returns slightly negative values, and where one band is a small negative and the other a small positive the denominator approaches zero and the ratio explodes. Measured on HLS S30 over Mount Rainier, 2023-08-01/06, 120 m: green reaches -0.0135 and swir16 -0.0196, and 29 of 66 045 pixels come out beyond [-1, 1] — one of them at -19.

Those pixels are not a darker or brighter surface, they are a division by almost nothing, so they are masked rather than returned: the function already did this for an exactly-zero denominator, and this is the same guard widened to the case that actually occurs. Pass valid_range=None for the raw ratio.

Parameters:

  • a – The two bands, in the order that makes the index positive for the feature of interest.

  • b – The two bands, in the order that makes the index positive for the feature of interest.

  • valid_range – Results outside this range are set to NaN. None disables it.