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The last two are the coordinate of the center of the upper-left pixel. For most satellites the *outer* edges of the pixels are always the same between resolutions, but this means that the center point changes depending on the resolution. So a 2km grid would be 1000 meters from the outer edges (a.k.a. the extents) of the grid and the 500m grid would have its top-left pixel center 250m from the edges. So the 2km grid I gave you was calculated from real data in Satpy where I had access to the outer extents and then shifted the upper-left corner 1000 meters in both directions (x_left_extent + 1000, y_top_extent - 1000). I reversed those calculations today to get the outer extents again, then shifted it 250 meters (x_left_extent + 250, y_top_extent - 250) to get the coordinate in the P2G 2.3 grid definition file.

Hope that makes some sense.

That makes perfect sense. Thanks for the information.