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The tallest mountain in America shrunk

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Re: The tallest mountain in America shrunk

Postby Derby Ale » Thu Sep 12, 2013 5:00 pm

I hesitate to dignify that one with a response! :lol:

... BUT ... did anyone in your group have a GPS unit turned on at the time???

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Re: The tallest mountain in America shrunk

Postby MountainHiker » Thu Sep 12, 2013 5:00 pm

“The 20,320 height had stood since 1952, when the mountain was measured using a technology called photogrammetry, Treadwell's announcement said.”

This fits for the date of the USGS topo map Mt Mc Kinley A-3, scale 1:63,360. I used to work with ortho-photos, which are produced using photogrammetric techniques. We would usually expect accuracies better than this. But, this project would have had a lot of realities working against accurate results.

The aerial photography would have been high altitude and small scale compared to the 1:24,000 topos in the lower 48. The 1:24,000 scale maps already used high altitude small scale photos compared to most urban planning photography. For this situation wider angle lenses (3.5” instead of 6”) might have been used to capture more area within the ceiling of the airplane.

The mountain is near the edge of the map. I’m more familiar with the 1:24,000 topos – with those, edge of map also meant edge of the photograph. If this was the case, the geometry that allows photogrammetry to work would have more potential for slop. This would have been especially true with a 3.5” lens.

The resulting resolution of the photo wouldn’t have been as good so the stereovision the operator depended on would have been compromised. The stereoplotter operator probably would have found glacier to be challenging (result less accurate) compared to non-glaciated terrain. With a stereo pair of aerial photos there are sun angle realities resulting in contrast differences and glare. Water and snow are not your friend with aerial photos.

An aerial photo project needs ground control. While there would have been some solid control points, but the density would not have been what it is in the lower 48. So the resulting block adjustment used to set up each stereo model probably was not as tight. And within a stereo model, the accuracy of the set up can get sloppier if points are hard to measure.

So while elevations far more accurate than this were possible in the 50s, I can see why this one wasn’t.

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Re: The tallest mountain in America shrunk

Postby Scott P » Thu Sep 12, 2013 5:44 pm

Denali has been long suspected to be slightly lower than the official 1952 surveyed figure of 20,320 feet.

In 1989, for example, the much published satellite calculations gave a figure on 20,306 feet and were considered to be more accurate than Washburn's figure. (The old figure before Washburn's was 20,300 feet; though I don't know what year that was surveyed. Sometime before 1910 for sure since the books on the first ascent use this figure).

It wasn't that Washburn and team were bad surveyors; quite the contrary. It's that Denali has such a huge bulk and stands so much higher than its neighbors that it distorts gravity (as do other mountains). Surveyors know this and have for a long time, but with such a big and solitary mountain, it is possible (and probable) that it could have skewed the results more than expected or more than was usual for a mountain of its elevation.

Satellites can provide more accurate calculations, but just because someone comes up with a new figure that doesn't quite match the old one, it may or not become the official elevation.

The 1989 measurement was 24 years ago and still hasn't taken precedence over the 1952 figure. I wouldn't expect the 2013 figure to start showing up on all the maps soon and I would also suspect that down the road a few years someone else is going to come up with another figure they claim to be more accurate. This kind of thing actually happens every few years (or perhaps a decade or two in some cases) with mountains such as Denali and Everest.
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Re: The tallest mountain in America shrunk

Postby moneymike » Thu Sep 12, 2013 7:30 pm

MountainHiker wrote:
geojed wrote:QOTD:
"It's still high, it's still hard, it's still cold," climber Nick Parker told the Anchorage Daily News. "As long as it's higher than Texas, I don't care."
:lol: =D>

And still hundreds of feet ahead of Canada. That's where North America's second-tallest peak, Mount Logan at 19,551 feet, sits.

Take that Canada!

I shouldn't really tell you this .... but when Canadians move to the US, we are instructed to take rocks from the tallest US mountains, then return to Canada and add them to Canadian mountains. Eventually Logan will be higher! :twisted:


Hey! Don't give away our secret!

Maybe it was just the cold that made Denali shrink. That often happens to me.

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