Benchmark elevation is not true summit elevation

Discuss geopolitical area highpoints, prominence, and similar lists.

Benchmark elevation is not true summit elevation

Postby Layne Bracy » Thu Sep 10, 2009 3:19 pm

Ken Jones just made an interesting post on cohp.org

In specific, his post notes that while the benchmark elevations of Colorado 14ers Eolus and Windom are at 14083 and 14082 feet respectively, neither BM is on the summit. When the USGS did their survey, they kept notes that the summits of the peaks were 1 and 5 feet above the BM's. This makes Eolus 14084 and Windom 14087. Thus, Windom is the actual La Plata County highpoint.

In general, one might wonder what the average error is when using BM elevation as a stand-in for summit elevation. Ken had to specifically request his info from the USGS, so presumably it would be a tedious task to correct the data for thousands of peaks.

While probably this is not a huge issue overall for the lists, some effects might include changes to highpoint lists or peaks getting a few more feet of elevation and prominence. For example, is "Iroquois", Colorado really ranked? Presumably the BM is not up on the precipitous boulder-top.
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Re: Benchmark elevation is not true summit elevation

Postby TWorth » Thu Sep 10, 2009 4:34 pm

Would be nice to have a list of cases where the BM is not at the highest point. Pretty much requires field observation.

A case similar to Eolus/Windom is Old Baldy/Greathouse Pk in Montana. Greathouse is listed at 8681, Old Baldy at 8678. When I visited Old Baldy in '02, I found the "Big Snowy" BM to be lower than the HP, by 3-5 feet. This puts Old Baldy at ~8682, which would make it higher than Greathouse and get its 4K+ of prominence and cohp status.

In CO, same thing w/ Andgee BM(8052) vs. UN 8057, on the Grand Hogback. The Andgee BM is lower than summit boulder by maybe 8'. Thus Andgee is really more like 8060', and would get the 1500+' prominence of UN 8057.

Probably many more cases out there, very few of which affect anyone's climbing goals/lists.
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Re: Benchmark elevation is not true summit elevation

Postby BrianR » Thu Sep 10, 2009 10:08 pm

Although it's not the same as fieldwork, public information on individual benchmarks can be obtained at:
http://www.ngs.noaa.gov/cgi-bin/datasheet.prl

I started doing this for high Colorado peaks back when the revised geoid was accepted which bumps Colorado peaks by about 3-8 feet depending on location (the datasheets have the old and new elevations for the benchmarks and give detailed information on whether the BM is the peak highpoint). However, it's very tedious to look up the benchmark datasheets and most peaks don't have benchmarks so I gave up.
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Re: Benchmark elevation is not true summit elevation

Postby BrianR » Thu Sep 10, 2009 10:24 pm

I should add that I recall that there were several small discrepancies between the datasheets and the normally reported 14er peak elevations even when looking at the old elevations before the geoid revision, let alone looking through the 13ers and lower peaks. Maybe other data was taken into consideration in deriving the "traditional" elevations of the 14ers.
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Re: Benchmark elevation is not true summit elevation

Postby Jeremy Hakes » Wed Apr 28, 2010 8:34 am

Same is true of Cheesman Mtn in JeffCo - benchmark is nowhere near the enormous boulder HP - it is a good 20' (at least) below it.
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