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Elevation of unnamed peaks

Colorado 13er questions, conditions, and other info should be posted here. Also includes topics related to 13ers.com. 13er Trip Reports
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Elevation of unnamed peaks

Postby jdorje » Tue Sep 04, 2012 11:51 pm

How accurate are the surveys (listed elevations) on unnamed peaks? These were all done in like 1925, right?

I climbed Pt 13153 today, and when I was on the summit my GPS read 13342 above sea level (a course EGM2008 adjustment - the GPS/WGS84 elevation was something like 80 feet lower), with regular readings in the 13290-13350 range as I sat there. This same GPS may be off by up to 100 feet on occasion, but 200 feet seems excessive (I've never seen it off by more than 100' when taking readings at a known location).

Looking across at 13546' and the 13546-Adams saddle at about 12900', my elevation visually appeared to be closer to Pt 13546. And as I hiked up, I noticed a point when I was even with the 12900' saddle and it seemed to take quite a bit longer to finish the ascent from there.
-Jason Dorje Short

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Re: Elevation of unnamed peaks

Postby RyanSchilling » Wed Sep 05, 2012 8:37 am

There are certainly mistakes -- take the erroneous closed contour on the Willow Lakes quad where a would-be ranked peak is actually soft-ranked (http://listsofjohn.com/PeakStats/Climbers.php?Id=872).

But I've found that without hand-leveling, it's easy for the eye to be deceived. Were you using one?

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Re: Elevation of unnamed peaks

Postby jdorje » Wed Sep 05, 2012 8:41 am

reinhold wrote:There are certainly mistakes -- take the erroneous closed contour on the Willow Lakes quad where a would-be ranked peak is actually soft-ranked (http://listsofjohn.com/PeakStats/Climbers.php?Id=872).

But I've found that without hand-leveling, it's easy for the eye to be deceived. Were you using one?


Alas, no. I should have done so.
-Jason Dorje Short

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Re: Elevation of unnamed peaks

Postby Rarefied » Wed Sep 05, 2012 11:22 am

Part of this dovetails right back to the discussion on the Homie thread regarding difficulties obtaining accurate elevations in the field from GPS-type devices (including SPOT). From the Garmin FAQ site:

"How accurate is the GPS elevation reading?

06/05/2012

GPS heights are based on an ellipsoid (a mathematical representation of the earth's shape), while USGS map elevations are based on a vertical datum tied to the geoid (or what is commonly called mean sea level). Basically, they are two different systems, although they have a relationship that has been modeled.

The main source of error has to do with the arrangement of the satellite configurations during fix determinations. The earth blocks out satellites needed to get a good quality vertical measurement. Once the vertical datum is taken into account, the accuracy permitted by geometry considerations remains less than that of horizontal positions. It is not uncommon for satellite heights to be off from map elevations by +/- 400 ft. Use these values with caution when navigating."
(Note the +/- 400'.)

As for visual assessment (aka "eyeballing"), the unaided eye is easily deceived as reinhold said. One such experience I had was a summit that had two small "snowbank-sized" mounds on it separated by maybe 75'. (I don't recall the peak but do know it wasn't a 14er.) As I arrived atop the first mound, the second one clearly looked to be a few feet higher. But once I went over to it, the first one then appeared to be the higher of the two! I even went back & forth another time but each time it sure seemed like I was on the lower one! As I recall even my barometric altimeter (non-GPS) gave conflicting values. In the end, I concluded/guessed that the two mounds were at about the same elevation despite how they appeared.

Finally, could the elevation on the topo be off? I don't really have a firsthand feel for such discrepancies but I have read threads here about how more recent, modern measurements have yielded slightly different elevations for some of the 14ers from those long held to be "official". But as I recall, those have been on the order of just a few feet and not hundreds of feet.

R

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Re: Elevation of unnamed peaks

Postby jdorje » Wed Sep 05, 2012 5:17 pm

LoJ has an inlined image of the topo:

http://www.listsofjohn.com/PeakStats/Climbers.php?Id=652

Unlike the century-old survey that may have determined the elevation to be 13153', the USGS topographic data is relatively recent and comes from comparison of aerial photographs. It can be thrown off by steep terrain (generally, when the terrain is steeper than the elevation of the photographs). That is likely what happened in the above wrong-contour example, but doesn't seem likely here.

I think I have to conclude you're right and that my GPS was over 200 feet off.
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Re: Elevation of unnamed peaks

Postby edhaman » Thu Sep 06, 2012 9:32 am

A while ago I saw a list by Gerry Roach of the highest 100 peaks in Colorado, with "old" and "new" elevations listed. For example, the old elevation for Elbert is 14,433 and the new elevation is 14,440. There was no explanation for the changes, but I had assumed they were the result of new, more accurate, information from the GPS system. From all of your posts here, it sounds like GPS determinations are no more accurate than the old surveys. Am I correct in this conclusion?

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Re: Elevation of unnamed peaks

Postby Rarefied » Thu Sep 06, 2012 12:13 pm

edhaman wrote:A while ago I saw a list by Gerry Roach of the highest 100 peaks in Colorado, with "old" and "new" elevations listed. For example, the old elevation for Elbert is 14,433 and the new elevation is 14,440. There was no explanation for the changes, but I had assumed they were the result of new, more accurate, information from the GPS system. From all of your posts here, it sounds like GPS determinations are no more accurate than the old surveys. Am I correct in this conclusion?


If the unit is like what many of us carry (i.e., designed for the casual hiker/climber), then its nominal +/- 400' accuracy (see previous post) would almost certainly put it at a disadvantage when compared to the old surveys.

However, high-end professional systems could provide precision that the old surveyors could only have dreamed of. (Yes, I suspect this is how the slighty revised elevations were derived.) See this trip report for an idea along that line:

http://14ers.com/php14ers/tripreport.php?trip=4708&parmpeak=320&cpgm=tripmain&ski=Include

R

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Re: Elevation of unnamed peaks

Postby rijaca » Thu Sep 06, 2012 12:58 pm

Rarefied wrote:
edhaman wrote:A while ago I saw a list by Gerry Roach of the highest 100 peaks in Colorado, with "old" and "new" elevations listed. For example, the old elevation for Elbert is 14,433 and the new elevation is 14,440. There was no explanation for the changes, but I had assumed they were the result of new, more accurate, information from the GPS system. From all of your posts here, it sounds like GPS determinations are no more accurate than the old surveys. Am I correct in this conclusion?


If the unit is like what many of us carry (i.e., designed for the casual hiker/climber), then its nominal +/- 400' accuracy (see previous post) would almost certainly put it at a disadvantage when compared to the old surveys.

However, high-end professional systems could provide precision that the old surveyors could only have dreamed of. (Yes, I suspect this is how the slighty revised elevations were derived.) See this trip report for an idea along that line:

http://14ers.com/php14ers/tripreport.php?trip=4708&parmpeak=320&cpgm=tripmain&ski=Include

R


The new elevations were derived from a change in the way the shape of the earth is modeled, not from new surveying systems.
"Spent a little time on the mountain
Spent a little time on the hill"

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Re: Elevation of unnamed peaks

Postby edhaman » Thu Sep 06, 2012 1:38 pm

So are the "new" elevations correct, or at least more accurate? If so, why is almost nobody using them?

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Re: Elevation of unnamed peaks

Postby Rarefied » Thu Sep 06, 2012 2:26 pm

rijaca wrote:The new elevations were derived from a change in the way the shape of the earth is modeled, not from new surveying systems.


Ahh, interesting ... good to learn. Thanks.

R

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