Slope Angle Data

sambisu's Avatar


17 Jan, 2019 08:52 PM

There was a recent avalanche fatality in Colorado ( that was at least in part attributed inaccurate slope angle data. Or at least the people utilizing the data were not fully aware of its limitations. The report from CAIC says that CalTopo uses the USGS National Elevation Dataset (NED) to derive slope angles. I know that parts of Colorado have higher resolution lidar surveys at 1 m accuracy (see attached image). Is this higher resolution data used in the areas available? Would be good to know for planning trips into avalanche terrain within that highlighted area.

  1. Support Staff 1 Posted by matt on 19 Jan, 2019 10:15 PM

    matt's Avatar

    For various reasons, I am not in a position to incorporate piecemeal elevation data from disparate sources into a single coherent layer. It's a tremendous amount of work, but I also want to note that I can't just plunk in a new dataset as there's also a significant amount of vetting required - for example, while the data may work great above treeline, a lot of LIDAR datasets pickup on trees and produce very noisy results below treeline (Google's Terrain layer being an example in many locations).

    I don't think it's really accurate to say that the accident was attributed in part to inaccurate slope data. Not understanding the limitations / micro-interpretation of a dataset beyond its actual resolution, yes. Higher resolution data certainly wouldn't hurt but it also wouldn't solve the problem entirely, e.g. cornices forming along ridges and dramatically changing the surface slope angle vs summertime.

    I am considering ways to to make the existing slope angle shading layer's limitations more visually apparent.

  2. 2 Posted by sambisu on 22 Jan, 2019 01:59 PM

    sambisu's Avatar

    I was not trying to imply in any way that this website bears any responsibility for this accident (if that was how my comment was received). Users should be aware of the limitations of these tools. From personal experience though, it is tempting to try to use this type of data to predict the avalanche danger on very specific slopes and terrain features.

    I do like the idea of making the limitations of the slope angle layer more apparent though. This can still be a very useful tool for planning trips into avalanche terrain, as users we just need to be smart about how we use it. The more we know about its limitations, the better.


  3. Support Staff 3 Posted by matt on 22 Jan, 2019 06:00 PM

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    I didn't interpret it that way, I just see a difference between being inaccurate and just being low resolution and this is a case of the latter not the former. Although there are places where the NED is probably flat-out wrong as well, I think addressing those two issues requires different approaches.

  4. 4 Posted by Tom Cal on 26 Jan, 2019 04:06 PM

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    Also see "Avalanche Slope Analysis",

    It might be useful to add a link to this near the slope angle shading controls, and "color key".

  5. System closed this discussion on 02 Oct, 2019 04:24 PM.

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