After the larger regional Musaicum satellite image mosaics of Europe and West Asia this further extension of coverage of my Musaicum satellite image mosaic series probably does not look very significant. But it represents a milestone in several aspects i am going to explain here.

From past work on local mosaics of the area – Iceland, Jan Mayen and the Faroe Islands i knew that these are the hardest part of Europe as far as producing a quality image is concerned. The short summer season combined with high cloud incidence with very diverse cloud types in summer make this a challenge. That was partly the reason why i did not include these areas in the EU-Plus coverage in 2023 – because that would have meant spending an extraordinary amount of time to make it work in the desired quality back then.

Now, with an additional year worth of data available and the improvements meanwhile made to the assembly process i decided to give it a try. And the results are remarkably good.

Stress testing the process

I discussed the difference between classical mosaic production techniques and pixel statistics methods before. With pixel statistics methods you can rely on the simple rule that if you use more data the quality of your results will improve (all other things being equal of course) – if that is not the case the method used is flawed. The rate of convergence, i.e. how much the results improve from using more data, varies, but the very principle that it does is universal.

With classical mosaicing techniques this is not generally the same. Adding more source data will only improve the results when that data is better than the data you already have. Otherwise it is equally possible that the quality of the results will decrease. Avoiding that is one of the primary goals in developing mosaicing techniques.

The North Atlantic Islands are a good test case to evaluate that because you need a large number of source images to ensure that every part of the area is covered with decent quality data. Even if you have a largely cloud free summer image in some area (which is rare in many parts) that might feature fresh snow in the mountains for example (which is common in Iceland even in summer).

The results are quite decent in light of this – the cloud incidence in the mosaic is not measurably higher than in continental Europe and West Asia and the uniformity is good as well. And there is substantially less seasonal snow than in my older manually produced Landsat/Sentinel-2 mosaic.

Without shading and shadows

The other major improvement is about the additional layers generated in the Musaicum production process. I had introduced the shading compensated version of the mosaic already with the Europe image. This offers – as discussed there – substantially better results than the L2A data available from ESA. The process to produce this was adapted from methods previously used in local mosaics – discussed in 2019. But the handling of shadows left a lot to be desired so i spent some time in 2024 on improving these methods. This was already tested at scale in the production of the West Asia mosaic but i did not have the time back then to evaluate the results in more detail.

Again the North Atlantic Islands were a good test case for this because at high latitudes shadows in satellite images are more of a problem than at lower latitudes. The results can be seen here.

There is some degree of under-compensation of shading (which – at least for visual applications – is much preferable to over-compensation) and shadows are not always reliably detected (in particular shadows on wet snow are notoriously difficult to discern from water surfaces). But apart from that i am quite satisfied with the results here as well.

The other additional layer – the fractional vegetation data set – received an upgrade as well. This improves continuity across the tiles the data is processed in.

Both the shading compensated version of the image and the vegetation data set are not considered experimental any more and are now available for licensing like the regular mosaic. I am also going to make both of these available retroactively for the West Asia mosaic as time permits.

If you are interested in the North Atlantic Islands image please have a look at the product page for more details and samples. If you are interested in additional Musaicum coverage in areas so far not available please get in touch.