Spurious valley/cleft on Mt Bradley (Ruth Gorge)

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psathyrella

13 May, 2021 12:00 AM

A friend and I were recently on Mt Bradley, and as far as we can figure it seems like the topo shows a ~1400 ft valley/cleft just west of the summit that, well, it just isn't there. As you walk west from the summit, there's a series of two valleys/whoop-de-doo things, but they are at most 100-200 ft deep. So to be clear, from the summit you can simply walk down/up snow slopes (over the points marked "A" and "B" in the images) down to the backside glacier.

Do you have any idea where this artifact could come from?

The same spurious 1400-ft cleft is seen in Gaia, fwiw, but is not on the (NGS I think?) paper topo; i.e. the paper topo (which is reproduced in the super topo climbing guide) is correct.

I've included a screenshot of the topo, as well as a view looking W from the Bradley summit, and a shot from the top of "747 peak", just S of 747 pass (so looking at the N side of the W "ridge" of Bradley).

  1. 1 Posted by Tom on 13 May, 2021 01:00 AM

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    psathyrella, Is this the ~URL for the map snippet you shared above? https://caltopo.com/map.html#ll=62.92249,-150.7319&z=15&b=mbt

  2. 2 Posted by psathyrella on 13 May, 2021 01:13 AM

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    Yep, that's it.

  3. Support Staff 3 Posted by Meghan on 14 May, 2021 03:00 PM

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    The underlying elevation data for MapBuilder Topo (which is the base layer in the screenshot you shared) for the particular area you shared (Mt Bradley in the Ruth Gorge, AK) comes from the USGS' National Elevation Dataset. This data has 1/3 arc second or lower resolution. Because of the resolution of the data, terrain features (such as clefts or snow whoop-de-doos) may be missing. It is also always possible that there can be errors in the source data as well, which is why you should ground truth everything, regardless of what base layer or map you are using.

    Of note, where available we incorporate LIDAR data from the USGS 3DEP program which has up to 1-m resolution. Currently we don't have coverage in that particular area (you can see an interactive map of LIDAR coverage here ) but we periodically update our coverage as the 3DEP program expands their coverage.

    Hope this helps shed a little light!

  4. 4 Posted by psathyrella on 14 May, 2021 03:52 PM

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    Thanks for the reply, that all sounds reasonable. But I guess that's why I was confused -- the issue isn't that it's missing the small 100-200 ft features, I agree that that's expected. The issue is that it's adding a 1400ft-deep feature that isn't there, which is harder for me to understand. If it were just lower-than-I-was-expecting resolution, I would think the mountain would just have a roughly rounded top, like in the USGS paper topo, but instead it's added in this massive cleft.

    So maybe the question is what could cause a discrepancy between the National Elevation Dataset and the paper topo?

  5. 5 Posted by Tom on 14 May, 2021 04:45 PM

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    psathyrella, FYI.
    From USGS: "There is webform to submit such things to the production division of USGS at https://answers.usgs.gov/ ."

    ---
    Source: Email to the "National Map Liaison AK", found at https://www.usgs.gov/core-science-systems/ngp/user-engagement-office/connect

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