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I have two raster layers: One with 56 m resolution and the other has 30 m resolution. Usually, I just use the 'Combine' command in ArcGIS to combine them, getting a single value for each unique combination of values of the 2 rasters. ArcGIS internally controls the resampling, and the resulting raster is 56 m in resolution.

Alternatively, one can split the 56m raster into two (getting two 28m rasters as result), resample the 28m rasters into 30 m rasters and then combine with the other 30 m raster.

Which way is better in terms of being more accurate? Any citations to this extent would be super helpful.

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The maximum accuracy you can get is that of the lowest cell resolution (or largest cell size), and splitting your cells won't increase the accuracy. That's why ArcGIS uses by default the largest cell size when combining rasters. Combining rasters using the lowest cell size is misleading, it lets you think your accuracy is higher than what it actually is.

You can control the way rasters will by combined by resampling the raster with the highest resolution (lowest cell size) first. Use the cell size and alignment of the coarser raster and choose the resampling method that is suitable for your data (you won't choose the same method for continuous data like elevation and for category data like soil types, for example).

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