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Grasscatcher

BM inside Colo Natl Mon

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There is a USGS BM disk near the lower trailhead of Serpents Trail which is just inside the East entrance to the park. It is shown on Topo maps and the elevation is shown as 5058.

 

On the BM is stamped "CC 12" and 5058 ft above sea level. (Set in 1934)

 

I'm familiar with the difference in accuracy of "Scaled" vs "Adjusted" coordinates and elevations, and am just curious as to how accurate that elevation should be considered as being.

 

Is there anywhere that a datasheet could be viewed on a BM like that?

Near the upper trailhead for that same trail, there is a "Witness Corner" with the elev given as 5886.I'm looking for the "official" data on that one too.

 

Thanks in advance for any info.

 

Grasscatcher

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There is a USGS BM disk near the lower trailhead of Serpents Trail which is just inside the East entrance to the park. It is shown on Topo maps and the elevation is shown as 5058.

 

On the BM is stamped "CC 12" and 5058 ft above sea level. (Set in 1934)

 

I'm familiar with the difference in accuracy of "Scaled" vs "Adjusted" coordinates and elevations, and am just curious as to how accurate that elevation should be considered as being.

 

Is there anywhere that a datasheet could be viewed on a BM like that?

Near the upper trailhead for that same trail, there is a "Witness Corner" with the elev given as 5886.I'm looking for the "official" data on that one too.

 

Thanks in advance for any info.

 

Grasscatcher

 

Got coordinates?

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NAD83 = WGS 84

 

No conversion, they are the same.

 

How do you want them sent?

 

For exapmle, the cache you logged a few days ago was at:

 

N 38° 02.022 W 107° 19.382

 

That is the format that I use.

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I can't find no data on them either, but did find a similar BM CC 11 listed on the Waymarking site.

 

As for elevation accuracy, I wonder myself about the accuracy from 70+ years ago and modern GPS units.

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Pinned that Lat/Long on Google Earth [6 August 2011] and came up with 5069 ft. I think that this would be aa close as a recreational grade GPSr. kayakbird

Edited by kayakbird

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The stamped elevation was probably within a fraction of a foot on the old NGVD29 datum, if the monument hasn't been moved. Since it's Geological Survey and not Coast & Geodetic Survey (predecessor to NGS) it might be a little looser than those on NGS data sheets. The stamped elevation could be different by several feet on the current NAVD88 datum - I haven't checked for the difference in that area.

 

GPS of any grade does not measure elevations on either of those datums. It measures height above the mathematical ellipsoid model of the earth. That is typically several to many meters different from either of the elevations. Your handheld receiver probably has an approximate model of the difference (the geoid height) to give you a reading somewhat closer to the datums than the raw height measurement. Elevation isn't GPS's strong suit anyway, as you may notice by watching your reading wander around. Even surveyor grade GPS and the best geoid model isn't quite the same as NAVD88, but NGS is working on an improved gravity model that will let them replace NAVD88 within a few years.

 

NAD83 is NOT exactly the same as WGS84, although the difference less than the typical repeatability of a recreational GPS so they treat them as the same. When WGS84 was defined, it and NAD83 were intended to be the same, and were to the accuracy that could be attained at the time. Now we know they are 3 or 4 feet different in the US, and growing further apart by 1.5 to 2 cm per year.

 

Once you get to measuring things this accurately, it gets to be a headache to keep track of what you're measuring.

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Finally a kind of interesting on for this mostly lurking vulture. Went to the National Geodetic Survey Data Explorer, Grand Junction area and found a few in the 1926 'ALPHA' 11 string mostly with 'POSTED' elevations that are about 2 1/2 ft higher than the stamping. KM0066 Q 11 is adjusted and reported good and GECACHed in 2002:

 

'KM0066 HISTORY - 20021201 GOOD INDIV'

 

A rather interesting 'What the heck is really the elevation ---' is:

 

JM0030 DESIGNATION - 4643.497 RESET

 

JM0030* NAVD 88 ORTHO HEIGHT - 1416.134 (meters) 4646.10 (feet) ADJUSTED

 

JM0030_STAMPING: 4642.760

 

JM0030 HISTORY - Date Condition Report By

JM0030 HISTORY - 1959 MONUMENTED USGS

JM0030 HISTORY - 1984 GOOD NGS

JM0030 HISTORY - 20081223 GOOD CODOT

JM0030 HISTORY - 20090530 GOOD GEOCAC

 

JM0030'------------------------------------- NOTE--THE ORIGINAL BENCH MARK

JM0030'4643.497 USGS WAS ACCIDENTALLY PULLED UP DURING EXCAVATION FOR THE

JM0030'GAGING STATION. THE UNDERGROUND PART OF THE 3 1/2-INCH PIPE WAS

JM0030'NEARLY RUSTED AWAY. THE USGS STANDARD CAP HAD BEEN BATTERED SOMETIME

JM0030'IN THE PAST. BM 4643.497 USGS WAS RESET AT THIS TIME IN THE SAME

JM0030'LOCATION AT THE SAME ELEVATION, 4642.760 FT, WHICH WAS STAMPED ON IT.

 

Did the reset crew bring in the new name elevation, or did they hold the old location? Take your pick of the three! The MESA CBL KM0228-0231 is just SE of Frutia if you want to check your GPSr.

 

kayakbird

 

NorthWes photo

 

61ccab21-0d0e-4f5b-b097-f42d8bb02615.jpg

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