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sea level determination?
How is sea level defined and measured? Is it constant everywhere?
What is the precision? I was thinking about the bad old days, before GPS... how did the cartographers determine the altitude of a mountain peak, relative to sea level? It you're in the Rockies, you need a pretty darned good telescope to survey out to the Pacific... -- Rich |
#2
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sea level determination?
RichD wrote: How is sea level defined and measured? Is it constant everywhere? What is the precision? I was thinking about the bad old days, before GPS... how did the cartographers determine the altitude of a mountain peak, relative to sea level? It you're in the Rockies, you need a pretty darned good telescope to survey out to the Pacific... As far as I can tell from a quick bit of Googling, such altitude measurements are based on "benchmarks", positions with accurately surveyed heights above sea level. http://www.geocaching.com/mark/ This begs the question of how the benchmark altitudes all over the country are determined to the necessary accuracy. - Randy |
#3
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sea level determination?
Dear RichD:
"RichD" wrote in message ps.com... How is sea level defined and measured? It is, for the most part, a statistical measure. An average. http://science.howstuffworks.com/question356.htm .... determination of sea level Is it constant everywhere? Sea level is an average. It is different distances from the Earth's center at different locations. For example, the Mississippi river head (where it starts) is ~50 feet closer to the center of the Earth than its mouth is (the Gulf of Mexico). What is the precision? Dead nuts, because it is a "geoid" established by a choice. But the "sea" moves all around the chosen "datumn". I was thinking about the bad old days, before GPS... how did the cartographers determine the altitude of a mountain peak, relative to sea level? You might get some history searching for: nad1929 OR nad29 OR nad-1929 OR nad-29 It you're in the Rockies, you need a pretty darned good telescope to survey out to the Pacific... http://education.arm.gov/studyhall/a...ion.php?id=589 .... altimeters have been used. David A. Smith |
#4
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sea level determination?
In article .com,
"Randy Poe" wrote: snip As far as I can tell from a quick bit of Googling, such altitude measurements are based on "benchmarks", positions with accurately surveyed heights above sea level. http://www.geocaching.com/mark/ This begs the question of how the benchmark altitudes all over the country are determined to the necessary accuracy. Before the days of electronics it was all done with optical instruments (theodolites &c.) and trigonometry. Presumably errors would accumulate as each new benchmark was positioned relative to the others, but the principles involved are pretty simple. -- Odysseus |
#5
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sea level determination?
"N:dlzc D:aol T:com (dlzc)" wrote in message ... Dear RichD: "RichD" wrote in message ps.com... How is sea level defined and measured? It is, for the most part, a statistical measure. An average. http://science.howstuffworks.com/question356.htm ... determination of sea level Is it constant everywhere? Sea level is an average. It is different distances from the Earth's center at different locations. For example, the Mississippi river head (where it starts) is ~50 feet closer to the center of the Earth than its mouth is (the Gulf of Mexico). You may want to check that; my understanding is that it's more like several* km than a mere 15 metres. *I have a memory that it's about 6km, but wouldn't argue the exact figure unless I'd double checked it. |
#6
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sea level determination?
rich Interesting question. I think the measurement starts at high tide?
I think Everest when surveying India raise the chain 4 feet off the ground. He must have done a lot of walking to get at the foot of Mt Everest. Well he did not have to go up the mountain to measure its height Bert |
#7
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sea level determination?
Dear OG:
"OG" wrote in message ... "N:dlzc D:aol T:com (dlzc)" wrote in message ... Dear RichD: "RichD" wrote in message ps.com... How is sea level defined and measured? It is, for the most part, a statistical measure. An average. http://science.howstuffworks.com/question356.htm ... determination of sea level Is it constant everywhere? Sea level is an average. It is different distances from the Earth's center at different locations. For example, the Mississippi river head (where it starts) is ~50 feet closer to the center of the Earth than its mouth is (the Gulf of Mexico). You may want to check that; my understanding is that it's more like several* km than a mere 15 metres. *I have a memory that it's about 6km, but wouldn't argue the exact figure unless I'd double checked it. OK. David A. Smith |
#8
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sea level determination?
Dear David A. Smith,
N:dlzc D:aol T:com (dlzc) wrote: How is sea level defined and measured? It is, for the most part, a statistical measure. An average. http://science.howstuffworks.com/question356.htm ... determination of sea level Is it constant everywhere? Sea level is an average. It is different distances from the Earth's center at different locations. For example, the Mississippi river head (where it starts) is ~50 feet closer to the center of the Earth than its mouth is (the Gulf of Mexico). The Mississippi flows uphill? What is the precision? Dead nuts, because it is a "geoid" established by a choice. But the "sea" moves all around the chosen "datumn". So, sea level is, by definition, a sphere around the center of the earth? Which means, not constant for gravity, or atmospheric pressure, or other things which we would intuitively expect constant. In fact, you don't even need the sea... And it begs the question, how do you determine the distance to the center of the earth? Paging Jules Verne, Dr. Verne... I was thinking about the bad old days, before GPS... how did the cartographers determine the altitude of a mountain peak, relative to sea level? You might get some history searching for: nad1929 OR nad29 OR nad-1929 OR nad-29 huh? It you're in the Rockies, you need a pretty darned good telescope to survey out to the Pacific... http://education.arm.gov/studyhall/a...ion.php?id=589 ... altimeters have been used. But they depend on air pressure, no? -- Rich |
#9
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sea level determination?
RichD wrote: Dear David A. Smith, N:dlzc D:aol T:com (dlzc) wrote: How is sea level defined and measured? It is, for the most part, a statistical measure. An average. http://science.howstuffworks.com/question356.htm ... determination of sea level Is it constant everywhere? Sea level is an average. It is different distances from the Earth's center at different locations. For example, the Mississippi river head (where it starts) is ~50 feet closer to the center of the Earth than its mouth is (the Gulf of Mexico). The Mississippi flows uphill? No it doesn't. "Uphill" means in the direction opposite to the gravitational field -- ie. to a region of higher potential. The ocean is at a lower potential than the head, so it flows into the ocean, not the head, otherwise conservation of energy would be violated, and this cannot happen. What is the precision? Dead nuts, because it is a "geoid" established by a choice. But the "sea" moves all around the chosen "datumn". So, sea level is, by definition, a sphere around the center of the earth? Which means, not constant for gravity, or atmospheric pressure, or other things which we would intuitively expect constant. In fact, you don't even need the sea... And it begs the question, how do you determine the distance to the center of the earth? Paging Jules Verne, Dr. Verne... I was thinking about the bad old days, before GPS... how did the cartographers determine the altitude of a mountain peak, relative to sea level? You might get some history searching for: nad1929 OR nad29 OR nad-1929 OR nad-29 huh? It you're in the Rockies, you need a pretty darned good telescope to survey out to the Pacific... http://education.arm.gov/studyhall/a...ion.php?id=589 ... altimeters have been used. But they depend on air pressure, no? Of course. You measure the pressure at sea level, then at the top of the mountain, and compare it. -- Rich |
#10
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sea level determination?
RichD wrote: Dear David A. Smith, N:dlzc D:aol T:com (dlzc) wrote: How is sea level defined and measured? It is, for the most part, a statistical measure. An average. http://science.howstuffworks.com/question356.htm ... determination of sea level Is it constant everywhere? Sea level is an average. It is different distances from the Earth's center at different locations. For example, the Mississippi river head (where it starts) is ~50 feet closer to the center of the Earth than its mouth is (the Gulf of Mexico). The Mississippi flows uphill? No, the sea "level" is not a sphere. What is the precision? Dead nuts, because it is a "geoid" established by a choice. But the "sea" moves all around the chosen "datumn". So, sea level is, by definition, a sphere around the center of the earth? No, sea level is by definition a "geoid", an ellipsoid which is slightly smaller pole-to-pole than across the equator. Different geoids have been adopted over time. Which means, not constant for gravity, or atmospheric pressure, or other things which we would intuitively expect constant. In fact, you don't even need the sea... The geoid is meant to approximate an equipotential surface, the shape of the sea on the rotating earth, if there weren't such things as tides, gravitational anomalies and storms. And it begs the question, how do you determine the distance to the center of the earth? Paging Jules Verne, Dr. Verne... No idea, but I think the fact that the polar radius of the earth is different from the equatorial radius has been known since at least the early 20th century. I was thinking about the bad old days, before GPS... how did the cartographers determine the altitude of a mountain peak, relative to sea level? You might get some history searching for: nad1929 OR nad29 OR nad-1929 OR nad-29 huh? If you search for those terms, you find it was some sort of reference surface adopted as a US standard in 1929. http://www.ngs.noaa.gov/faq.shtml#WhatVD29VD88 - Randy |
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