Why The Difference?

[+Spoo]

This week, I recovered PF0290 .

 

Monumented in 1933, It was stamped with an elevation of 1465.279 Feet.

 

It WAS found in 1936. It was NOT FOUND in 1975 By the Maine Department of Transportation but I managed to located it.

 

The current NGS data sheet shows that during NAVD 88, its elevation was listed at 1464.83 feet.

 

How does this occur? It is my first finding of a data sheet being revised in terms of its position. Could NAVD 88 be more accurate than the original 1933 survey?

[+Renegade Knight]

Sounds like a different datum.

 

Each datum models the world differently. In theory imrpoved. That means vertical election of the same point can change even though it's in the same spot.

 

Also the world is not static. There are places in Californa that have subsided (due to withdrawal of groundwater) by several meters. Other changes can be expectd elsewhere.

 

Your difference isn't that much.

Edited August 13, 2005 by Renegade Knight

[holograph]

Look at the NGVD 1929 datum height, since that's the datum they would have used at the time. It's still 0.04 feet different from the stamping, not a lot, and probably the difference is due to the subsequent adjustments after the disk was stamped.

 

Since the station elevation was not remeasured, all the differences between the current NAVD 88 height and the stamped height have to be due to datum shifts and adjustments, not because of actual station movement. Of course, the station may have moved, but no one will know that until someone does another high accuracy survey of it.

[+PFF]

I'm glad SPOO brought this up. I find the same elevation differences from time to time.

 

It looks like EZ0606 was restamped, or a new disk was placed where EZ0605 was mounted.

 

I've never been sure how to follow the elevation records. Frankly, I never paid much attention since .04/feet does not affect anything I do in life. Still, one gets curious about these things......

 

404.51 EZ0605 DATA SHEET ELEVATION

404.49 EZ0606 DATA SHEET ELEVATION

405.153 ELEVATION STAMPED ON DISK

 

-Paul-

[+Spoo]

RK and holo:

 

Thank you both. Yes, I now see the datum difference, and I myself had realized the elevation difference was small. 5.388 inches if my math is correct.

 

I guess I was interested in why someone in a different time that had NOT visited the spot would list a different elevation then someone who had carefully measured it to 3 decimals places in their time.

 

I assume that the different datums will therefore give that much difference?

 

(LOL........datum = sea level at a certain time of day and temperature and pressure and location and .......???????)

Edited August 13, 2005 by Spoo

[evenfall]

NGVD29 was the datum in use when the original survey was done. The National Geodetic Vertical Datum of 1929 was a vertical datum based on leveling done which was averaged to Mean Sea Level, tied to local mean sea level at 26 tide stations along the east and west coasts of the United States, Canada and the Gulf of Mexico.

 

In 1978, NGS began a program to combine leveling surveys into a single least squares adjustment to provide improved heights for over 700,000 vertical control points throughout the United States. This adjustment was completed in June, 1991 and has been designated the North American Vertical Datum of 1988 (NAVD88). NGS selected the GRS80 Ellipsoid at a single station, the primary tidal benchmark at Father Point/Rimouski, Quebec, Canada, as the minimum-constraint datum point for NAVD88.

 

NGS no longer Supports NGVD29, but some areas of the country still use it.

 

If you play with the VERTCON program: http://www.ngs.noaa.gov/TOOLS/Vertcon/vertcon.html You will see the datum Shift between the NGVD29 and the NAVD88 Datum all over the country. There are places where the difference, or Datum Shift Varies by Feet, not Inches.

 

The Problem that NGVD29 had was that Mean Sea Level was never stable enough, the Average taken from all the Tide Gauges over the course of a year, or many years was subject to how much water was in the sea at any particular time, Constantly changing tides, and the Geoid models which were observing that gravity affects the oceans and isn't a static force. NAVD 88 Locks Vertical references to a more stable reference, and renders all the measurements across the continent in a more stable framework.

 

Rob

Edited August 13, 2005 by evenfall

[+Spoo]

evanfall:

 

You bring up yet another good point. My reading (as written by Historians, NOT surveyors) shows that the Pacific and Atlantic Oceans stand at different heights.

 

A quick review of my books lists the following differences:

 

At the Panama Canal: 1 foot difference

At the coasts of California and New York: 6 foot of difference

At Tierra Del Fuego: 10 foot of difference

 

Where and when IS Sea Lavel?

[holograph]

It pays to remember that every survey is a relative survey, always as a distance or direction from something else.

 

The surveyors who stamped 1465.279 on the disk did so because they had measured the difference between its elevation and the elevation of some other control points that had already been determined (which in turn had been measured from other control points, and which ultimately tied back to the tidal stations as stated by evenfall).

 

When the datum was revised, it was because there were problems when the networks were connected and compared across wide areas, so the adjustment process smoothed out and distributed the differences as evenly as possible.

[Z15]

A vertical datum defines a system for

elevation comparisons. It is used in

floodplain management, waterway

navigation management, roadway design,

agricultural management and surveying in

general.

NGVD29 was established as the standard in

1929. Sea levels naturally changed in

decades since, but the designation itself was

based on certain faulty assumptions.

NAVD88 was established in 1988. It is more

accurate because it references a single

point. NGVD29 and NAVD88 measurements

differ as much as three feet in some areas.

In St. Augustine alone, the difference is

12.6 inches.

 

Something to read

Edited August 13, 2005 by Z15

[evenfall]

Spoo,

 

This is probably a question best asked in the NGS Forum, but since you have asked...

 

http://forums.Groundspeak.com/GC/index.php?showforum=40 For Future Reference. Highly technical questions are probably better over there.

 

Anyway...

 

Both the Historians and Surveyors are correct. The Historians researched Survey data for their information. There is nowhere that the sea is level. If you could see the Oceans in a three dimensional way, they would look a lot like the Geoid models do, only with tidal and wind wave action added. Mean sea level at any given Tide Gauge is taken as an average. In the case of NGVD29, the averages of 26 Gauges were then averaged as well, and thought there was some heavy variance between all of them, they had to have one solid number to base all the elevation leveling on land.

 

So to answer you as most specifically as I can, Sea is level only accurate in relation to where the observations are taken, and those observations are averaged. You have a Sea Level near you on the East coast which will differ from what I would observe here on the West Coast. Both would be correct as it can be, for it's observed locale. Sea level is considered a "Mean" Average, but it is only true in a Local way, not in a Global one. The National Ocean Service of which NGS is a part, maintains data on Sea Level here:

 

http://co-ops.nos.noaa.gov/sltrends/sltrends.shtml

 

Today, we use a reference ellipsoid, and frequently updated models of the geoid, to derive elevations. These elevations which are the real ground are referred to as orthometric heights. This is a GPS method, and is formulaically given as h= N=H Where h= the ellipsoidal height, which is the reference frame the Satellites consider the average surface of the Earth without form, N= the Geoid Height were scientists have determined the gravitational surface of the Earth to be, and H= the Orthometric Heights, which are the actual physical elevations.

 

During the 80's as I said in my previous post, NGS undertook a massive project and re-leveled the entire United States, Optically, so as to derive the Basis for NAVD 88, and using the methods which derive it. This leveling, when taken with the ellipsoid and geoid references formulaically and least squares adjustments give us what we observe today.

 

Something interesting to note about leveling. You may have seen Differential Leveling diagrams here in the past. I think both Z15 and Geo have posted pictures. The diagrams show the "Path" which was taken to derive the elevations. It is important to be able to retrace the steps because leveling elevations has a "path dependent" outcome. Different paths taken will derive slightly different outcomes, so the paths must be observed when setting up the networks. This way the paths can be re-leveled a number of time the same way, and all the observations can be compared with a least squares adjustment so as to make them highly accurate themselves, and amongst other elevations in the Level Lines, Circuits and Base lines.

 

Rob

[+PFF]

Rob wrote:

During the 80's as I said in my previous post, NGS undertook a massive project and re-leveled the entire United States

 

Refresh my memory. At that time, was it leaning to the Left or to the Right? (Sorry, Rob. it was too good a shot to pass up--grin.)

 

Seriously, your info put me at ease concerning what we would do if global warming raises "sea level" significantly! In the case of railroads (where we find so many elevation markers), the primary concern seemed to be how much the elevation changed between Point A and Point B. Your example makes it clear that the existing systems meet this requirement.

 

In like manner, history scholars tell us that the birth of Christ differs by several years from the boundary between B.C. and A.D. Nevertheless, I am confident that I know my age!

 

-Paul-

[evenfall]

Well, It's all relative, really Paul... But I was pretty sure the whole thing was leaning right until 92...

 

That's the year after NAVD 88 began to take effect!

 

Rob

[ArtMan]

I was pretty sure the whole thing was leaning right until 92... 

Then, of course, what political scientists call the see-saw compensation effect began to take hold, with the result that in 2000 a new rightward tilt began without, as far as I am aware, any new datum to blame.

 

-ArtMan-

[Z15]

Seriously, your info put me at ease concerning what we would do if global warming raises "sea level" significantly!

 

You will be gone for centuries before this.

 

People along the Great Lakes are worried about the low water levels.

Edited August 14, 2005 by Z15

[ArtMan]

Z15,

 

Well you might not notice it in Michigan, where even the lowest elevations are hundreds of feet above sea level. But residents of low-lying areas around the world — particluarly poor residents of poor countries like Bangladesh and most Pacific island nations — will be facing catastrophic consequences of global warming in the coming decades.

 

Most Americans may remain high and dry, but global warming is certain to cause signifcant global political, social and economic upheaval. We won't be unaffected. Especially with the head-in-the-sand attitude of certain leaders in Washington.

 

You might find some interesting facts on the website of the Union of Concerned Scientists, an activist but sober and sensible mainstream group.

 

-ArtMan-

[+2oldfarts (the rockhounders)]

I saw an article several months back in a trade magazine (Geospatial something or other. Old age is great.). Anyway, they had satellite photos of the polar caps, taken several years apart. The ice caps have shrunk about 30%. By theory the ocean level should have risen 75 to 100 feet. It appears they need to rethink their theory about coastal flooding.

 

John

[ArtMan]

The melting of free-floating ocean ice, such as much arctic ice, doesn't affect ocean levels. Just like a melting ice cube in your drink doesn't change the level in your glass. Glaciers and other land-based ice melting and running into the oceans will raise sea levels.

 

-ArtMan-

(a science journalist in my day job)

[Z15]

I am not not interested in the liberal arguements that we are the fault of everything in the world...

Edited August 15, 2005 by Z15

[evenfall]

Well, What I think we can bank on with certainty is that what we think we know about any of this is ongoing and subject to change. Survey data is not alone, in that it left a record of what was thought best at a time in the past, which we must still somehow integrate into our modern models of thinking. That is in part what makes this subject such a wrangle to get one's mind around.

 

Many things are like this, and I cannot say what I would go with on global warming or cooling. We haven't been around long enough to know all of the come and go. If Ying and Yang are true, as my avatar might suggest, then the several Ice Ages the world was known to have had were also countered with an opposite age, and who knows if man could really influence it at all. The best we can do is be good stewards of today and set ourselves up for successful tomorrows.

 

At least we have the Vertical Datum Shift Phenomena under control. It's all about perspectives anyway.

 

Rob

[ArtMan]

I am not not interested in the liberal arguements that we are the fault of everything in the world..

Actually, Mike, I don't believe I said anyone was at fault. I was trying to have a scientific discussion, not a political one. And the science, while containing a certain amount of ambiguity as science almost always does, strongly suggests that the planet is heating up and we are going to have to live with its consequences.

 

Anyway, this is probably not the right forum for this conversation.

 

-ArtMan-

[Z15]

Thats why I said that, the tide was turning from the subject of thead....

 

On another subject, I can't get rid of my signature no matter what I do. I have edited it many times and yet it still there. @#$%^&* computers, see if it works this time..

 

What, it finally worked, all be darned. That should make one person happy, "AKA as" was the sore point. LOL

Edited August 15, 2005 by Z15

[+Harry Dolphin]

And, so, with a scientific backround, I'd like to bring up the question of Precision versus Accuracy.

Precision is always great to have, but it means nothing without accuracy. (This is an argument that I have ceased to argue at work. No one has any idea what I'm talking about!) If I measure the dimensions of a box to the nearest inch, the cubic feet of that box is only accurate to the nearest cubic foot, if that.

How accurate were the surveyors of 1934? What instruments did they use to give them accuracy to the a thousandth of a foot?!? Yesterday, I found LY0421. I'm rather proud of this one, since it has not been reported since 1934. I note that the attitude has been determined to a thousandth of a foot. 838.492 feet above sea level. That'd be 5.90 inches.

Horizontal is scaled. Fine. In understand that. My GPS said it was off 102'. Not bad for scaled and my GPS. I found it rather easily. Surprising, since no one has reported it in 71 years. I wandered around the thirty foot margin north of the rail until I found it. Only took me a half hour.

Question: Does ayone really expect me to believe that surveyors in 1934 could measure altitude accurately to a hundredth of an inch?!? Can anyone today measure altitude accurately to a hundredth of an inch? Would anyone like to buy a bridge that I have for sale?

Precision is beautiful, but without accuracy it is meaningless.

Edited August 15, 2005 by Harry Dolphin

[+2oldfarts (the rockhounders)]

Harry,

 

To use a shooting analogy - precision is putting 5 shots into one ragged hole. Accuracy is putting the same 5 shots on the correct target.

 

John

[evenfall]

Question:  Does ayone really expect me to believe that surveyors in 1934 could measure altitude accurately to a hundredth of an inch?!?  Can anyone today measure altitude accurately to a hundredth of an inch?

I'd Keep that bridge. :-D

 

The accuracy you are observing is from the methodology of multiple observations of the same station, and a Least Squares Adjustment. Statistically accurate in a network to the precision you observe. Yes the accuracy in observation is very high. Geodetic observations were and continue to be rigorous. The highest of standards were followed and repeated. Even then.

 

The NGS website has several good papers about their methodologies:

 

http://66.102.7.104/unclesam?q=cache:m2rPB...t+Squares&hl=en (An html version of a pdf document)

 

There are others, Feel free to google if you're interested.

 

Rob

Edited August 15, 2005 by evenfall