True elevation of Cottonwood BM? (Hot Sulphur Springs quad)

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True elevation of Cottonwood BM? (Hot Sulphur Springs quad)

Postby BrianR » Fri Jul 08, 2011 3:07 pm

http://www.listsofjohn.com/PeakStats/Climbers.php?Id=3082

I noticed that the highest contour for Cottonwood BM is 10,000 feet on both the new "MyTopo" version of the topo map and the old "USGS" Terraserver version (40ft contours on this quad). However, the spot elevation is given as 10,119. I checked the NGS site for the benchmark datasheet (http://www.ngs.noaa.gov/cgi-bin/ds_radius.prl) and it also gives 10,119 as the old elevation (10,124 with the new geoid), so one would be inclined to think that this elevation is correct and there are 2 missing contours. On the other hand, the "Terrain" view from Google, which presumably uses DEM data, indicates that the summit area is quite flat within the 10,000 contour. Not a big deal, but if the "Terrain" and contours are correct, a 100ft reduction in altitude would make this peak an "error range" ranked peak instead of a guaranteed ranked peak. Does anybody who has been there have any comments/data on the observed altitude or flatness?

(There's a spot elevation on an unranked peak near where I live in AZ that appears to be exactly 100ft in error, but I would be rather surprised for this to happen at a benchmark.)
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Re: True elevation of Cottonwood BM? (Hot Sulphur Springs qu

Postby John Kirk » Fri Jul 08, 2011 5:28 pm

That's interesting - didn't notice that... however, the summit area is not very flat (the map indicates a quarter-mile wide highest contour). This is definitely not the case. Here's a photo from the south about 100 yards away from the summit (notice on the right the ground at the same level as I am taking the picture from drops off about 100ft).
IMG_2085.jpg
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Re: True elevation of Cottonwood BM? (Hot Sulphur Springs qu

Postby BrianR » Fri Jul 08, 2011 6:08 pm

Your picture definitely implies significant relief within the contour, enough to reconcile the 10,000-foot contour and 10,119 for the summit, with two missing contours. I thought to check the Colorado map I use with my GPS (http://www.gpsfiledepot.com/maps/view/197/), which for sure uses the latest 1/3-arcsecond data (~10 meter linear resolution) from the National Elevation Database to generate contours, and it also shows the 10,000-foot contour as being large with no higher contours, just like Google Terrain does. Must be an issue with NED...
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Re: True elevation of Cottonwood BM? (Hot Sulphur Springs qu

Postby TWorth » Sat Jul 09, 2011 10:38 am

Good observation Brian, when I visited I found the summit to be sort of rounded, just like in the photo, so I'd agree its a case of missing contours.

The thing with NED even at 1/3 arc sec is that its still based off the 7.5' quads, so if the quad is in error the digital data will be also. Vestal Peak with its packed contours near the summit comes to mind.

From what I've heard the high res Lidar digital data is based on remote sensing projects not related to the original quad data so any quad errors should be corrected, but Lidar is not available for most mountainous areas.
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Re: True elevation of Cottonwood BM? (Hot Sulphur Springs qu

Postby BrianR » Wed Jul 20, 2011 4:37 pm

I happened to be in the neighborhood and decided to check this out for myself today. I had my Etrex Legend HCx in hand walking up the short summit road and when I crossed the 10,000-foot contour, I was reading about 10,020. On the summit, I had 11 satellites, 9-foot reported accuracy (WAAS was turned off), and got 10,153 at chin level twice a few minutes apart, so 10,148 on the ground. I measured 9812 at the start of the summit road after the GPS was sitting on the dashboard for the drive up (didn't check the reported accuracy). Finally, I recorded a 320-foot difference between the start of the road and the summit on my altimeter watch, which with the mild weather translates to more like 330-340 of actual gain. So...if one accepts that the GPS was reading systematically high, then everything checks out as far as the missing contours.

Apropos the DEM issue, I guess I had it in mind that they also used well-calibrated point elevations, e.g., benchmarks, when putting values into the lat/lon grid, but that's obviously not the case.
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