Planet Four Talk

Shadows

  • AstroCam by AstroCam

    Hi all;

    I've been wondering what methods you guys use to distinguish shadows from blotches. I've been getting some pictures of particularly rugged landscape with a lot of dark patches, but the problem is that some of these are clearly blotches and some are clearly shadows. It's the ones that I'm not sure about that I'm wary of, and I'd like to hear how you deal with the problem.

    I've noticed that some blotches are dark black, and some are simply a darker shade of the background colour. For the most part, I've marked everything within reason, ignoring patches which are clearly shadows, but I figured it would be best to mark as much as possible.

    Advice, thoughts, and anecdotes/examples welcome!

    • Cameron

    Posted

  • Portyankina by Portyankina scientist

    Hi Cameron!

    We do not have a really good way to distinguish between shadows and dust patches. Light conditions are tricky in polar areas and surface is very uneven. So if you can somehow distinguish, do your best, if not - you can also mark them as blotches. We hope to get enough statistics with many people doing it.

    Posted

  • michaelaye by michaelaye scientist

    And then, if we are really really interested in an area, we can ask for a so called stereo observation mode, which means to come back later and look at things with a different angle to recreate a 3D dataset that we then would use to calculate shadow areas. But all that is very hard to do in polar areas for the spring time, which is when all these activities are happening, because for stereography, one needs to have things constant, so that the fitting routines later find where to anchor/coregister the data together. As one can see, there's so much going on at the poles around these times, that it's basically impossible to do. :S

    Posted

  • Dafijis by Dafijis

    I mainly rely on whether there are blotches/shadows near a ridge/mountain that could possibly cause such a large shadow/blotch. If there is, I leave it as there is a bit of ambiguity. But if there isn't, i just highlight and classify it. sometimes we get things that look too muck like shadows and don't fit in with any blotch classification. So, really we just have:

    1. does it look like a blotch? If no, it is a shadow. If yes continue.
    2. is there anything nearby which could've caused such a shadow? If yes, it is ambiguous, leave it. If no, It is a blotch

    Hope this helped show how little we have for defining, but also helps for situations that can be identified.

    Posted

  • michaelaye by michaelaye scientist

    i don't understand what is your criteria to go from step 1 to step 2? What would for you make something more look like a shadow than a blotch? Also, think of the geometries involved: How should a shadow be roughly circular in shape, when the sun always comes from a low angle (as it happens for polar areas)?

    Posted

  • mheretakih by mheretakih

    I look at the texture of the image. The most problematic images so far are those with spiders like features. What I look at is whether the dark spot follows the contour of the feature or does it radiate beyond that feature and it ignores basic sun angles, using surrounding features when possible. I've noticed with one image with most boulders that had dark streaks projecting from where the boulder sat so it resembled a shadow. I then found a few boulders that lacked those streaks and was able to determine that the shadow was on the opposite side (the shadows better matched the curve of the boulders) and using that information determined that the streaks were indeed fans and not shadows.

    Posted

  • michaelaye by michaelaye scientist

    Very deterministic and thorough, mheretakih! Our idea for the boulders have 2-things involved, having seen their involvement with fans for years now (there will be a paper soon): 1. they have a thermal impact on the ice, something like a hot needle, making a smaller hole (but earlier in spring) 2. the boulders are for some un-understood reason, mostly on the inclined areas of the Inca City region. Inclinations means they get the earliest spring sun after the polar winter, but not very intense. That all fits very well with thin, not so massive fans. The very long stretch of the fans is explained by strong thermal winds that set in on those inclinations when the whole thermal environment is undergoing change due to the end of polar winter.

    Posted

  • mheretakih by mheretakih

    Thank you! I am glad to put my remote sensing experience to some good use. The interaction of the boulders on the fans sounds very fascinating. I would love to read the article when it is published. Will there be an announcement through the blog? Thank you for the explanation on the two possible boulder fan interactions and their relation to seasonal change! I did notice in a second boulder/fan image that there were two fans originating from a single boulder and that the second fan was much smaller and was rotated 90 degrees from the first fan. This same region also had a single red boulder, which stood out as the ground was white and the remaining boulders were also white.

    Posted