How do they figure out elevation?

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I figure today they can use GPS to pinpoint the elevation above mean sea level of any location on earth. But how did they do it years ago? The 'mile high city' was called that way before GPS. Even has a marker on the Capitol steps at the exact spot.

So, how'd they figure it out? Anyone know?
 
Old Geek said:
Survey and define a series of reference points from a known origin.
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Tens of thousands of horizontal triangles measuring the fraction of a degree rise every time? Sounds like there would be too much error introduced over time for that to be accurate.
 
Heck I don't even understand how GPS does it. Without a pressure sensor how can you determine height. Saturday I was trying to figure out how my iPad knows how high it is.
 
Personal_Landsurveyor_3787_Wip5-760x508.jpg
 
Keep in mind GPS is just a measurement tool. It still references latitude, longitude and vertical reference based on established survey datums. I think NAD 83 is used as the vertical datum for GPS in most cases.
 
I would imagine that the idea of "density altitude" existed prior to aviation. Would not a surveyor with a thermometer, barometer, and a table of figures be able to figure out altitude? I guess that begs the question - who made the table? Perhaps that was made using survey techniques at a seaside cliff?

Pure speculation on my part, but after seeing books or things like tide tables and trig functions, that makes the most sense to me.
 
SixPapaCharlie said:
Heck I don't even understand how GPS does it. Without a pressure sensor how can you determine height. Saturday I was trying to figure out how my iPad knows how high it is.
Click to expand...

Remember that GPS isn't so much measuring your height as it is finding your exact position in a three-dimensional space. Your device figures out the difference in time signals from a few satellites and triangulates exactly where you must be. From that perspective "height" is really no different that latitude or longitude. Just one of three axes in space.

Since we have 3D maps of the entire Earth's surface we can determine the AGL height if we have GPS location and a sufficiently-high resolution terrain map.
 
But in the sky it isn't using the height of the terrain in my XY location.
Or did I misunderstand you?
 
image004.jpg


Add in a 4th sphere (aka the earth), and you reduce the number of potential points from 2 to one.
 
Captain said:
Tens of thousands of horizontal triangles measuring the fraction of a degree rise every time? Sounds like there would be too much error introduced over time for that to be accurate.
Click to expand...

Nope. It can be done more accurately than GPS. Uncorrected GPS altitude is several hundred feet low in North America. That correction comes from surveys.

These days, it can be done from orbit, but not by GPS.

Error propagation is your friend. It's worth understanding that before speculating. Geometry can be extremely precise, and GPS is using the same geometry, except it assumes an ellipsoid. That's where it gets those wrong altitudes from. Local surveys do not need to assume the whole planet is an ellipsoid. It really isn't.

Heck, Eratosthenes got the circumference of the whole planet right 2200 years ago based on measurements around 500 miles apart. Don't underestimate what a good geometer can do.
 
dukeblue219 said:
Remember that GPS isn't so much measuring your height as it is finding your exact position in a three-dimensional space. Your device figures out the difference in time signals from a few satellites and triangulates exactly where you must be. From that perspective "height" is really no different that latitude or longitude. Just one of three axes in space.

Since we have 3D maps of the entire Earth's surface we can determine the AGL height if we have GPS location and a sufficiently-high resolution terrain map.
Click to expand...


Nope. Your height in 3D space is known. Sea level isn't, by GPS. It's not constant height, and it's not the ellipsoid uncorrected GPS presumes. This is a significant error that local surveys do not suffer.

Most local altitudes in the US are defined using the "state plane" coordinates, not a global ellipsoid.

Sometimes people forget that angles can be measured excruciatingly precisely. Degrees are enormous. Radio astronomers routinely measure thousandths of an arc second, and the rest of us would do that if the *$#@ atmosphere didn't limit us. Roughly, that means we can resolve a translation of 100 millionth of the distance to the target. So, if I want to see a displacement of 1 cm (dime-sized), I could resolve that on a straight line 1000 km away. Not bad, eh?
 
MAKG1 said:
Nope. It can be done more accurately than GPS. Uncorrected GPS altitude is several hundred feet low in North America. That correction comes from surveys.
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My dad bought my son a handheld GPS for his graduation ten years ago. We took it to the airport where we knew the exact altitude, and it read about four feet high when he was holding it. Set it on the ground, it would read exactly the altitude. Lift it a couple of feet, it would indicate two feet higher. It was deadly.

Can't remember the brand.

Dan
 
Dan Thomas said:
My dad bought my son a handheld GPS for his graduation ten years ago. We took it to the airport where we knew the exact altitude, and it read about four feet high when he was holding it. Set it on the ground, it would read exactly the altitude. Lift it a couple of feet, it would indicate two feet higher. It was deadly.

Can't remember the brand.

Dan
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Yes, that's corrected GPS. The correction had to come from somewhere. The earth is not an ellipsoid on the scale of a few feet.
 
What will really blow your mind is how there is the Piri Reis Map of 1513 of Antartica that has its coastline completely and accurately mapped out. Interestingly Antartica and the coast has been covered in ice for millions of years.
 
thehulk06 said:
What will really blow your mind is how there is the Piri Reis Map of 1513 of Antartica that has its coastline completely and accurately mapped out. Interestingly Antartica and the coast has been covered in ice for millions of years.
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Got a source for that?

Mine say that that particular map had enormous errors in the southern hemisphere, especially in South America, and looking at the places people are claiming are "Antarctica" seems to indicate excessive consumption of four leaved herbs.
 
MAKG1 said:
Got a source for that?

Mine say that that particular map had enormous errors in the southern hemisphere, especially in South America, and looking at the places people are claiming are "Antarctica" seems to indicate excessive consumption of four leaved herbs.
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Of course people will downplay its accuracy. How the hell else can they explain it? Its pretty crazy though.Its a map that is depicted as if its seen from space..... anyways, theres lots of stuff on it as its pretty well known. But here's something else for you to read.http://theunexplainedmysteries.com/piri.htmlIm just saying, an old map with accuracy beyond its time..... gets you thinking
 
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thehulk06 said:
Of course people will downplay its accuracy. How the hell else can they explain it? Its pretty crazy though.Its a map that is depicted as if its seen from space..... anyways, theres lots of stuff on it as its pretty well known. But here's something else for you to read.http://theunexplainedmysteries.com/piri.htmlIm just saying, an old map with accuracy beyond its time..... gets you thinking
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"Unexplained mysteries," eh?

Did you know "gullible" wasn't in the dictionary?

Look at the map yourself. Antarctica is not there. Period. Neither are several other well known features like Cape Horn -- since it wasn't discovered until several years after the map was made.

No big mystery. Just someone trying to make a few bucks by fooling the gullible. If you believe all the BS out there, the universe ended in 2012… and dozens of other times in my lifetime.

Oh, and get the link right.
 
Captain said:
Tens of thousands of horizontal triangles measuring the fraction of a degree rise every time? Sounds like there would be too much error introduced over time for that to be accurate.
Click to expand...

Wait! You want the answer to be accurate?

My Dad used to tell me that a man who owns one watch knew what time it is. A man who owns two watches never knows for sure what time it is. :confused:

The first survey crew to hit Denver set the standard....

-Skip
 
Ron Levy said:
Personal_Landsurveyor_3787_Wip5-760x508.jpg
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That picture sums it up. Surveyor and rodman. I worked as a rodman at the Tx Highway Dept for several summers in college. We set elevation stakes every 100 ft. 5 stakes per station (centerline, outside left lane, outside right lane, left shoulder, right shoulder). IIRC measurements were to the 1/100th of a foot, or .12 in. We set the stakes to within +/- 2/100s. On a good day we could could do about a mile. At the end of the day we'd backtrack to where we started. It was typical to be under 5/100s error at the end of the day.
 
Captain said:
I figure today they can use GPS to pinpoint the elevation above mean sea level of any location on earth. But how did they do it years ago? The 'mile high city' was called that way before GPS. Even has a marker on the Capitol steps at the exact spot.

So, how'd they figure it out? Anyone know?
Click to expand...

They didn't always get it right -- there are 3 "mile high" markers on the steps of the state Capitol. The original marker when the capitol was built, a second marker that was placed decades later when someone figured out the original was not correct. Now a 3rd marker that was added when it was determined the second one was still wrong.
 
Skip Miller said:
Wait! You want the answer to be accurate?

My Dad used to tell me that a man who owns one watch knew what time it is. A man who owns two watches never knows for sure what time it is. :confused:

The first survey crew to hit Denver set the standard....

-Skip
Click to expand...

ColoPilot said:
They didn't always get it right -- there are 3 "mile high" markers on the steps of the state Capitol. The original marker when the capitol was built, a second marker that was placed decades later when someone figured out the original was not correct. Now a 3rd marker that was added when it was determined the second one was still wrong.
Click to expand...

Ah, so the short answer is 'they can't'.

I like the quote about a man with one and two watches. Don't be mad if I lift that one for myself...
 
Not to forget that elevation and altitude are two different things, entirely.


Sent from my iPad using Tapatalk
 
Ron Levy said:
Personal_Landsurveyor_3787_Wip5-760x508.jpg
Click to expand...

I earned the funds for my first 10 hours of flying lessons working as a rodman for my first flight instructor, who was a professional surveyor. I took surveying at Georgia Tech in the late 80's, and steel tapes, rods and old-time theodolites were still the order of the day.

While the advent of GPS markedly changed surveying, it has also created new challenges, as I frequently face problems in my work with inconsistent datums being used, and incomplete documentation of exactly which local datum was used to correct the GPS data.

Something to remember is that the Earth isn't staying still, so benchmarks move. One good earthquake can cause pretty significant local shifts too. The plates are moving in a way completely disconnected from the GPS orbits, so the GPS references need periodic corrections. This is a different set of problems than were faced when only ground-based methods were used.
 
Van Johnston said:
I worked as a rodman at the Tx Highway Dept for several summers in college.
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Jeff Oslick said:
I earned the funds for my first 10 hours of flying lessons working as a rodman for my first flight instructor, who was a professional surveyor. I took surveying at Georgia Tech in the late 80's, and steel tapes, rods and old-time theodolites were still the order of the day.
Click to expand...

You both earned money at a profession that three of the four people in the picture below also worked at.

Mountrushmore.jpg
 
Captain said:
Ah, so the short answer is 'they can't'.

I like the quote about a man with one and two watches. Don't be mad if I lift that one for myself...
Click to expand...

They can, but "elevation" is a nebulous concept- mllw in the pacific doesn't equal mlw in the Atlantic. Also, plates are forming, people do earthwork, and earthquakes do things like shifting the entire island of honshu an average if 8 feet in minutes. GPS can give a very acurate answer, but you have to wait 4 weeks for the super precise correction data.

If you want a more detailed answer, consult a geomatics textbook.
 
If you're using the ellipsoid as your sole geometry you're not accurate either. I made my fortune in photogrammetry. It's more involved than that, but when it comes down to it, it is still math. A transit and chain is pretty darn accurate if you know what you are doing, but they have gone the way of the dodo. They use laser rangefinders and differential GPS (which is very accurate over a short area even if there are bigger issues with datums). They don't even use stakes all the time. I was watching them build the new freeway on ramp here and they put a differential GPS on the excavator and the guy was placing the roadbed to the precise height and position based on that.
 
jhausch said:
I would imagine that the idea of "density altitude" existed prior to aviation. Would not a surveyor with a thermometer, barometer, and a table of figures be able to figure out altitude? I guess that begs the question - who made the table? Perhaps that was made using survey techniques at a seaside cliff?

Pure speculation on my part, but after seeing books or things like tide tables and trig functions, that makes the most sense to me.
Click to expand...

Nope.

To use a barometric device to determine elevation / altitude you first need to correct for the local "sea level" barometric pressure which changes as "highs" and "lows" roll through independent of the temperature - that is done by measuring the pressure at a known elevation and applying the standard correction for the known elevation. So, basically, if you know your elevation, you can measure the pressure, and if you don't wait too long you can measure the pressure again and determine that you haven't moved since the last measurement.
 
Or you can go to the building super and say "I'll give you this nifty barometer if you tell me how tall the building is."
 
Captain said:
Ah, so the short answer is 'they can't'.

I like the quote about a man with one and two watches. Don't be mad if I lift that one for myself...
Click to expand...

Yea, they were off by two steps. The original marker was on the 15th step, the current marker is on the 13th step.
 
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