This past weekend, I was standing on the summit of Mt Washington, looking at the benchmark bolted to the rock at the very top of New England. On this saucer-shaped piece of metal is inscribed (amongst other things) 6288.176 feet above sea level. Two questions popped into my mind when I saw this:
I know “sea level” is kind of an abstract concept, but how on earth do they measure to 1/1000 of a foot on top of this rock pile? I could just about keep my balance on the rocks surrounding the summit, let alone plop a transit down and take dead-on-accurate measurements. Are they sure its not 6288.178’ high?
Where abouts on this benchmark is the actual 6288.176’ measured to? The mark looks like a saucer with a raised nub in the center, with a dimple inside that nub. Is the level the outer edge of the saucer, the bottom of the nub, the top of the nub, or inside the dimple? If we’re measuring to 0.012" accuracy, there must be a standard measuring point for these things. Also, with the scratches and gouges in the benchmark (its pretty badly beaten up), how would they take a measurement from it again?
While the USGS is in charge of benchmarks now, it was actually the National Geodetic Survey that established most of them.
If you visit this link http://www.ngs.noaa.gov/faq.shtml
you can find the e-mail address of the scientist who is in charge of determining the heights of objects.
I would assume that the height of Mt. Washington, like that of any other geographical feature can only be determined by a doing series of readings over a period of time.
The height listed on the benchmark is probably what ever the geodetic datum for Mt. Washington is. The actual height listed is probably below the marker.
"Mean Sea Level’ is now derived from satellite radar (including laser). Depending on when the benchmark was placed there, it probably HAS shifted. That’s one of the things the geologists want to know. I doubt they used a transit for the thousandth-of-an-inch measurement, but they might well have used an average of several GPS readings with the antenna fixed at a known height above the dimple. Or they might have used a laser to or from a satellite.
From the bottom of the dimple. Even if the monument has tilted, an object could still be calibrated from that point. They probably wouldn’t measure FROM it again, they would probably measure TO it again.
Now that I think about it though, I would rather hear this from a USGS cartographer.
You know, it would be nice to have a board function that allowed you to get notifications of when a topic has been updated when you are interested in the topic but don’t have anything to contribute.
GPS measurements can be that accurate, I think. Even before the government removed the spoofing, geophysicists were measuring changes in thousand kilometer baselines in the South Pacific down to the centimeter per year. How precise, I dunno.
The thought of GPS and/or laser measurements had gone through my head, but that idea was quashed when I read the whole benchmark:
So if this mark was originally planted here in 1925, then updated in 1980, I’d think that would rule out present day high-tech measuring devices. Then again, I’m not 100% sure when GPS was first used and/or if the NGS had a laser transit back then (1980, not 1925).
I think I can shead a little light on this subject as a cartographer and geographer. The USGS bechmarks are extreamly accurate and are used by professonal surveyers as a reference point. Every so often, sorry I don’t know how often off the top of my head, they go back and revise and move the marks. You don’t need GPS or lasers, I’ve never heard of using lasers from a satilite to measure stuff anyway, to get good accuracy. a good surveyer can use a transit and get a measurement to within .001". The Mason-Dixon line was recently remeasured using the GPS and found to be only 4mm off or something close to that.
GPS was first developed in the mid 70s and has finally gotten all 28, only 24 are used, satilites in orbit within the last few years. If I remember and can find my books I’ll look it up as to what exactly is the actual part of the benchmark that is used. I do remember when I was working at NOAA they had a benchmark out front that they would time to time work off of. intersting stuff.
Having been a part-time surveyor summers in college, I can say that they’d set the transit up down low, and get the rod man(otherwise called shit-job guy) to climb up on the top of the thing & hold the stick up so that they can sight on it.
USGS benchmarks are done from teh dimple- the rods are pointed at the bottom, so the dimple gets it centered nicely- another thing I learned as a surveying lackey, along with how to drink beer on the job, kill snakes with the surveying rod, and what a fucked-up mess metes & bounds surveying in Texas is.