Gannett Peak Additions and Corrections
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|The National Geodetic Survey has the elevation at 13,809'. 13,804' was an older measurement.|
|Posted Aug 14, 2017 8:26 pm|
|The National Geodetic Survey link you provided is providing old data. Plus, using the data link you provided, the marker from which that data is based was atop "A 5.0 FT. TALL ROCK CAIRN ON THE HIGHEST POINT OF THE MOUNTAIN."|
The rock cairn was manmade, not a natural highpoint.
13809' - 5' cairn = 13804' (which is the accepted HP elevation by most resources)
|Posted Aug 16, 2017 9:59 pm|
|Ammon Hatch||Re: Elevation?|
|Very good points, and your explanation is perfectly reasonable. I did make that connection, but I was unsure if the measurement was the top of the cairn or the base of the cairn (it's non-specific). It seems weird for them to supersede the old value with a new one simply because a cairn was built. I wonder if there is newer data or an official clarification anywhere on the matter.|
I'm no expert on the matter, but including the height of the station mark seems like bad surveying.
I know it's weird to get stuck on 5 feet, I'm just a pedant and want to find the right answer more often than it's worth it.
|Posted Aug 22, 2017 5:16 pm|
|By the way, I have seen the USGS do this on other peaks, too. Kind of corny way of measuring, very true.|
|Posted Sep 9, 2017 5:26 pm|
|It is all about the change in the reference point (aka a “datum”) for measuring the height of any point on the earth. NAVD 88 replaced the National Geodetic Vertical Datum of 1929 (NGVD 29), previously known as the Sea Level Datum of 1929. As maps get updated, they adopt the latest datum numbers. The elevation difference between points in a local area will show negligible (5 ft.?) change from one datum to the other. NGVD 29 used a simple model of gravity based on latitude to calculate the geoid (earth shape model) and did not take into account other variations. Thus, the elevation difference for points across the country does change between datums. Same thing happens with horizontal datums – NAD27, NAD83, WGS84. Places get moved when they really don’t move at all. On the other hand, things like plate tectonics, earth quakes, glacial rebound, ground subsistence, etc. actually do move things on occasion. BTW official elevations are measured to natural points. Anything man made like cairns, bridges, excavations, fills, etc. are not counted.|
|Posted Nov 13, 2017 4:32 pm|