Pixel Area, L3b ISIN grid

[erehm]

Has someone computed the corrected pixel area per row (or bin) for SeaWiFS and MODIS 4.64 km and 9.28 km ISIN grids? 

I'm looking for the result that takes into the variation of pixel size due integerization of the number of pixels per row, including the special pie-shaped sectors at the poles?  For Arctic work, the pixel area changes per row are not insignificant.

/eric

[OB.DAAC - SeanBailey]

Eric,
It is true that the ISIN grid is not strictly equal area, as the interigization process necessarily introduces a small error, which is indeed larger at extreme laittudes. Appendix A in Volume 32 of the SeaWiFS Pre-Launch Technical Memoranda series covers this in some detail.  The pertinent paragraph is extracted below:

"Because the x-length of the tiles is adjusted to ensure an integer number at each row, the equal area characteristics of this binning scheme are not rigorously preserved.  However, variations in the tile size are negligible throughout most of the globe and only become relevant at very high latitudes, where there are fewer tiles per row, and any adjustments are more noticeable.  As the number of tiles increases with distance from the poles, the difference between the tile sizes rapidly becomes practically unnoticeable.  To provide an idea of the magnitude of the fluctuations in tile size, the worst possible case occurs when half a tile remains uncovered after filling a zonal row with an integer number of tiles.  Once a row has 100 bins (approximately 16 rows, or 148 km from the poles), the worst possible difference between the actual tile x-length and the standard x-length is of the order of 0.5%, i.e., half a tile's length is redistributed among about 100 tiles.  For a tile of about 9 km a side, this represents a difference in the x-length of about 45 m.  Through a similar calculation, a row with 50 bins (about 80 km away from the poles) has a 1% variation with respect to the standard bin size." 

While the above does not directly answer your question, it does provide a magnitude for the error in size.   Now the answer to what you seek, i.e. determining the x-length  and the calculation for the triangular wedges at the first and last row - the paragraphs above and below the one I've extracted provide to the equations :grin:

Regards,
Sean

[erehm]

Thanks Sean.  Yes, I saw that text in Vol 32.  I hoped someone had already done the calculations and could share them with me. :eek: