Arctic mosaic and colors revisited


Continuing from the previous post here as promised more examples for application of pixel statistics methods to Landsat and Sentinel-2 data and how this can help to produce more accurate colors.

I have mentioned before that based on the spectral characteristics for accurate natural colors Landsat offers a significantly better basis than current low resolution systems like MODIS and Sentinel-3 and that from Landsat 7 and EO-1 via Landsat 8 to Sentinel-2 there is a notable trend to less suitability for accurate color reproduction.

Visible light spectral bands of common open data earth observation satellite sensors

This assessment which is based on the spectral characteristics is something that is hard to demonstrate practically based on individual images because the differences in viewing conditions are usually large compared to the differences in colors due to different spectral characteristics.

I have now produced a larger area image mosaic based on pixel statistics methods (which i discussed in the previous post) and by comparing with the MODIS based Green Marble mosaic i can point out the effects of different spectral characteristics much better. The image uses data from Landsat 7, Landsat 8 and Sentinel-2 for the land surfaces and the Green Marble as background for water areas. The data basis is not that broad so there are also significant color differences due to incomplete convergence. But you can still pretty prominently see the color differences compared to the MODIS mosaic.

Arctic mosaic based on Landsat and Sentinel-2 data

Green Marble for comparison

The most striking difference is that the MODIS based Green Marble rarely features true gray tones. Most areas that are gray in the Landsat/Sentinel-2 mosaic show up in red and brown colors in the Green Marble. This is a result of the more narrow spectral bands in the MODIS instrument, in particular the green band. Gray colors mean reflection is more or less equal in all three spectral bands of the human eye but this does not necessarily mean it is completely uniform across the visible range. If it is not a narrow spectral band will usually result in a non-neutral color being registered for a surface that would appear to be of neutral color in direct view by the human eye. The opposite is possible as well but practically much less likely.

Moscow based on Landsat pixel statistics

Moscow based on Green Marble

Verkhoyansk Mountains based on Landsat pixel statistics

Verkhoyansk Mountains based on Green Marble

This Arctic mosaic shown by the way is to my knowledge the first complete natural color mosaic of the Arctic in a better-than-MODIS resolution. I don’t want to specify an actual resolution because of the limitations of pixel statistics method described in the previous post. It was processed in a 30m grid (based on the multispectral Landsat resolution). Obviously my regional mosaics like those of Greenland and Scandinavia offer a significantly better resolution but are also more costly to produce.

Greenland sample form the Artic mosaic based on Landsat and Sentinel-2 data

Same area based on the Landsat mosaic of Greenland

Same area based on Green Marble

I also have full coverage of Europe but so far not beyond. If you are interested in other areas let me know.

Europe mosaic based on Landsat and Sentinel-2 data

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