Rastertools shaded snow algorithm is missing some shadow especially on mountain ridges
While comparing results of LIS blue in #21, I observed some shadow detection problem with the new rastetools algorithm used in 1.11.0 version.
The overall observation is that:
- In 1.10.2 version, saga gis algorithm (or the threshold we use) tends to overestimated shaded zones, and on casted shadow, there seems to be some little shift on the area detected in shadow.
- In 1.11.0 version using rastertools, the shaded areas are much more conservative: casted shadow are really precise, but on slopes and especially mountain ridges, some shaded areas are missed, leading to strange patterns when viewing the results.
The following images illustrate these results and problems near Goulier area in France on the S2 image taken on 19 Nov 2019.
The hillshade mask is green is issued from saga-gis, wherease the one in blue on the top is from rastertools:
On the left, rastertools mask is missing shadow near the mountain ridges. On the top, we can see that saga-gis mask detects also slopes with lower illumination. On the right, rastertools mask is better on casted shadow.
It is hard to argue on some areas which one of the mask is the best, a choice between over estimation and conservative one. But the problem of the ridges of rastertools mask is clearly visible on the final result (LIS 1.11 in blue, LIS 1.10 in green):