OLCI reprojection

After I reprojected the OLCI data, the pixel size changed. Instead of 300m, it became a value larger than 300m, and the pixel size was different each time I reprojected

Please review FAQ before posting. It is bad form to make multiple posts on the the same topic, and also bad form to omit the details of your processing.

In general, mapping requires a geographic region, a map projection (e.g., given as a Coordinate Reference System (CRS), and the dimensions for the resulting mapped image in meters, pixels, etc. If you look at the ground footprints of level-2 OLCI data, you will see that each has a different shape. The ground footprint of a level-2 pixel expands towards the edge of a pass, so when you map a specific geographic region, the useful resolution will be lower when the region is near the edge of the pass. Binning algorithms were developed to deal with this problem, so one workflow to produce mapped images uses binning before mapping. There is a description of binning in the SNAP help.

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@gnwii actually said everything already.

The pixel size is not constant across track of the observation. It is bigger at the edges then at nadir.
This is because of the view angle. See
OLCI Instrument – Sentinel-3 OLCI Technical Guide – Sentinel Online - Sentinel Online (esa.int).
Also there it is said “approximately 300m”

I’ve deleted the other post.