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

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

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

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

"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.

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

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

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

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

Dear RichD:

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?

In the sense that it gets farther away from the axis the Earth revolves
on, yes. Keep in mind that the Earth is rotating, and tends to "sling"
the water towards the equator...

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...

You have had some good answers to this one... you don't get the
average if you don't have the sea, and once you have the average we
tend to ignore the sea (well...).

And it begs the question, how do you determine the
distance to the center of the earth? Paging Jules Verne,
Dr. Verne...

There are ways to do this, without invoking The Master.

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 is a search term to use in Google. It refers to "North American
Datumn, 1929", and was a series of standard locations / elevations for
markers all over North America (or just USA?). Finding this might get
you some history, since you were asking about the bad old days.

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?

Ay-firmative. So this too will require time averaging, and attention
to the weather.

David A. Smith