Regarding the Sea Ice Index, Version 3 data set accessible through the National Snow and Ice Data Center (NSIDC), the daily and monthly GeoTIFF files for the period 01 January 2021 to present are in a slightly different projection. The GeoTIFFs for 01 January 2021 to present are in the WGS 84 / NSIDC Sea Ice Polar Stereographic North or South projections (EPSG: 3413 and EPSG:3976) which use the WGS 1984 ellipsoid as the datum. The GeoTIFFs for 26 October 1978 to 31 December 2020 are in the NSIDC Sea Ice Polar Stereographic North or South projections (EPSG:3411 and EPSG:3412), which use a datum based on the Hughes 1980 ellipsoid. The differences between the two projection pairs are minimal - the diagonal distance across a WGS 84 / NSIDC Sea Ice Polar Stereographic 25 km grid cell differs about 1 m from the original NSIDC Sea Ice Polar Stereographic grid cell.
To ensure a consistent projection across the time series, we will reprocess the GeoTIFFs from 01 January 2021 to present so they are in the NSIDC Sea Ice Polar Stereographic North or South projections (EPSG:3411 and EPSG:3412). An additional user notice will be sent out once these reprocessed files are available, likely by the end of April.
Data set DOI: https://doi.org/10.7265/N5K072F8