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DaveD

Nad 83 - Wgs 84

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Additionally, when you ask "how far apart", you inherently

ask "how far apart, using a meter defined in a specific

way".  Unfortunately, NAD83 and WGS84 have a scale factor

difference, meaning that a "meter" in NAD83 isn't the same

as a "meter" in WGS84.   Do you want the answer in NAD83

meters or WGS84 meters?  (This makes a difference

at the mililmeter level!)

 

BDT,

 

As you may remember the Geodesist was upfront about the problems the Meter has with Scaling Factors. You know the terminology; "all things being equal" Well the difference between the datum is not equal, but in order to compare it, you have to look at it through the one's frame of reference or the other's.

 

Say hypothetically that if we look at NAD83 as seen through WGS84. WGS84 will determine it's meter is perfect and correct, but looking at NAD83 from here it looks like their Meter is longer than a Meter. Conversely let's look at WGS84 as seen through NAD83. NAD83 will determine it's meter is perfect and correct, but looking at WGS84 from here looks like their meter is shorter than a meter. Either way, or looking through the eyes of either method will cause a scaling to occur. You have to make them "be" the same, same in reference to the datum acting as the control Datum, in order to properly compare them. It is just something we have to realize happens and accept it.

 

The most important thing we need to remember and take away from this is that we do not compare them. They are real close to one another and we accept that, but we just leave it at that. We just use the datum determined to be most appropriate to what we are accomplishing. Again in the United States and Canada, we stick with NAD83. We don't even think about WGS84. WGS84 provides zero survey data to anyone that pertains specifically to North America in any High accuracy format. They do not give us any known points to adjust, per se. To the DOD, the owner of that Datum, non DOD users do not have a need to know. All the DOD WGS84 Datum Owners let us use is their reference ellipsoid and geoid models. It is up to any end user to accept that and develop their own work from there, as many countries have, or as you know, in North America, we use something else. NAD83 Is the Horizontal Datum of choice for any civil work of any kind in North America. NAD27, in it's various iterations, is number two.

 

Again, It is perfectly fine to reread this topic's thread and revisit what has already been said. It is a lot to take in.

 

Rob

Edited by evenfall

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Each time the experts add their comments, it opens up something new to read about. The business about "different meters" started me reading about datum scales and why they are different.

 

It comes down to error and uncertainty in measurement again, not error or disagreement in the length of a meter. As has been stated, when the NAD83 datum is "realized", it means that the positions and velocities of the CORS stations are determined to the best of everyone's ability, and then those positions are published and all other NAD83 positions are determined with respect to those stations.

 

However, "the best of everyone's ability" is constantly improving, and the different datums use different techniques and perform adjustments at different times. In particular, it is a challenge to determine the position of stations with respect to the center of mass of the earth, since no one can directly measure that position.

 

The techniques to find the center of mass of the earth and the heights of a critical few stations are based on observations of celestial objects and satellites, and those techniques are susceptible to subtle relativistic and atmospheric effects. Various theories are used to estimate those effects, but the theories have evolved and the estimates have changed.

 

Every time those estimates change, if means that all the positions that were previously measured need to be revised. Unfortunately, the keepers of the datums don't ever agree on when and by how much those changes are done. When NAD83 determined the heights of its CORS stations relative to the center of the earth using what it considered to be the best techinques, it found slightly different heights than what would have been found if IETR/WGS had used its techniques.

 

For instance, if every time WGS84 would measure and find the height of a station to be 6378000 meters, NAD83 would measure and find it to be 6378000.06 meters (a disagreement of 6 cm) then when you convert from WGS84 to NAD83 you would have to multiply by about 1.00000001 -- a factor of ten parts per billion. In fact, the NAD83 and WGS84 datums disagree by less -- about 0.6 parts per billion. That's not much, but its enough to make a difference of some millimeters when measuring positions at the surface of the earth.

 

Since the station is really at some actual height, but each datum will say it is at a different height, you can say informally that WGS84 meters are "larger" than NAD83 meters, even though both datums are using the same meter. Their disagreements are in their measurements, not in their meters, and to convert from one datum to another you have to multiply by a scale factor to account for the systematic differences in the measurements that were used to establish, or "realize" the datums.

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Excellent points, holograph. Referring to a distance difference between these two separately adjusted groups of stations in terms of an effective meter length difference is surprising to me. If the difference between WGS 84 and NAD 83 is statistically random in different locations, this 'meter ratio' would be different and sometimes even opposite.

 

To see distances of about a meter (sometimes) between WGS 84 and NAD 83 for the same set of coordinates is also rather surprising. I don't recall the specifications, but I thought that within an adjusted net, distances between stations were only different by a few millimiters in several thousand feet. With the accuracy of current measurement technology, and the almost identically sized ellipsoids, one would think that the difference between WGS 84 and NAD 83 would be much smaller than a meter.

Edited by Black Dog Trackers

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BDT,

 

Google can help you too, In seconds I had this:

 

http://gpsinformation.net/main/ngs-accuracy.html

 

http://gpsinformation.net/main/ngs-accuracy.txt

 

And This Tutorial:

 

http://www.ngs.noaa.gov/PUBS_LIB/Geodesy4Layman/TR80003A.HTM

 

Another Tutorial...

 

http://gge.unb.ca/Research/GeodesyGroup/tu...al/tutorial.htm

 

Perhaps you could be more succinct for us. It would seem that the differences between the Datum we have been discussing has really stuck in your craw. We have answered you, I thought Quite well and exhaustively, yet you return with question after question which has been covered already, so what is missing for you? What is it you are trying to achieve or understand? How can we help you? If you don't want help, Ok, what are you driving at?

 

Rob

Edited by evenfall

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To see distances of about a meter (sometimes) between WGS 84 and NAD 83 for the same set of coordinates is also rather surprising. I don't recall the specifications, but I thought that within an adjusted net, distances between stations were only different by a few millimiters in several thousand feet. With the accuracy of current measurement technology, and the almost identically sized ellipsoids, one would think that the difference between WGS 84 and NAD 83 would be much smaller than a meter.

What's potentially confusing is that there are different kinds of differences in the datums. Each datum has its own reference frame consisting of a coordinate system origin and axes directions. In any three dimensional coordinate system, you can get differences in the placement of the origin, differences in the directions of the axes, and differences in measured distances, or scale.

 

If what you are measuring is a shaking, wobbling ball of Jello like the earth, then you also get differences in how you estimate and account for those motions over time.

 

For whatever reason, the keepers of NAD83 decided against adjusting the datum to the international ITRF standard(s) once they discovered their reference frame differences, so we're stuck with the difference between WGS84 and NAD83.

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The first 2 references that evenfall provided indicate that there are Earth-center origin differences between the coordinate systems used by WGS 84 (as re-defined by ITRF94) and NAD 83, and that this is the reason that points are different by about a meter.

 

The Charles Schwarz paper of 1989 states:

However, in the NAD 83 adjustment these coordinates received corrections due to interactions with other observations (mostly classical triangulation and traverses), while no such corrections were made in the determination of the WGS 84 coordinates. These corrections can amount to a meter or more.
If I read this correctly it is saying that the NAD 83 adjustment included positions established by older survey technology in their adjustment.

 

I really have no axe to grind - I'm just curious about why these WGS 84 and NAD 83 produce statistically different results on the order of about a meter. It's just interesting to me from a mathematical cause-and-effect perspective. :unsure:

 

Apparently there are 2 different reasons for the difference - (1) an adjustment difference partly due to the use of different technologies used in determining the positions of the points included in the basic adjustement, (2) a difference in origin and orientiation of the 2 coordinate systems.

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kc2ixe

 

I know it is a PITA but if you carefully re read most of what I have posted in this thread...snip...

Your misssing what I'm saying - you guys may use different metres - but "they ain't metres", no matter what you WANT to call them

 

Call'em NAD83 or WGS84 "units" or 'Length units" - but they are NOT metres

 

It doesn't bug me that you use different "unit" definitions for different elipse definitions - that fine.

 

I just wish you would NOT call them METERS - because they are NOT

 

(aka a Rose by another Name thing - "Names are a powerful thing")

 

In other words - I understood your post, I just think it's messed up that people will corrupt the meaning of an SI unit

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kc2ixe,

 

Sorry, They are meters. In the equations they begin as meters. Every observation we make to justify it used the meter. The factors applied to the equations, equations that use simultaneous adjustments and least squares adjustments, are such that they statistically compare in the Case of the NAD83 the ellipsoid and geoid models as well as the grid of observations, which are the very survey markers you are helping to recover, the math will cause the meter to become averaged if you will, so that everything becomes a best fit.

 

In the case of the WGS84 you have the ellipsoid and geoid models, but they are not the same ones used for NAD83. Also there are physical positions that the WGS compares globally but there are very few as compared to NGS's Database and furthermore their database of physical survey is classified so we cannot know them and use them in comparisons. In any case, the mathematical operations, such as simultaneous equations, and least squares adjustments are applied to the WGS datum as well and as based on the numbers used there it scaled the meter differently than NGS.

 

In terms of the purity of the meter, the meter used, is as thought of in the a-priori state, and was a pure meter to begin with, but the earth ain't flat, and all the equations will have to have some room to move. Scaling factors are in play and I am sorry but it is something we simply have to accept. As a for instance there are other things we have to accept.

 

I like to think I do good work in the field but I can not avoid certain truisms, Nobody can. Like that leveling paths are path dependent and using a different path can skew my results. Like I can take more than one distance reading with EDM between a couple of points and not get the same results, Which is to say more technicallly, Open Traverses rarely ever match. (it is almost safe to say that they never match) Further, my measurement in reality is really a straight line measurement attempting to measure a curved surface. Not exactly a best fit, oh and geodetically incorrect, but we use what we have.

 

Geodetically my measurement is not correct as a stand alone measurement. If I make enough of these measurements and base them triangularly so I can tie points together I can begin to see the curvature of the earth, and when I take them all into a big picture with some applied calculus the "averaging" process will blend the measurements to decide what lengths are the best fit. The new numbers may not concur with my initial observations exactly but they will be close and considered more accurate. This is in essence what happens to the length of the meter.

 

I also want to call attention to the fact that the Meter began as a Geodetic observation. Here was the original formula. 1 meter equals the distance from the north pole to the equator on a path that passed through Paris, France. Then it was divided by ten million. There is your meter. From there they wanted to have an empirical copy and the "Powers that be" changed the length a bit in the process of doing so. You can google all this if you want a better understanding. Today, it is close to what it originally was , but not exactly.

 

Geodesists have folk amongst themselves who are also powers that be as well and they are the people who decide how they are going to describe the earth with a particular Datum. Once they decide, technology marches on and the soon find themselves doing it again. The meter is simply an agreement that happens to be a mathematical construct for measurement. It is the basis for the measurements. It really is. Not much different than a 10K ohm resistor which fits nicely into equations on paper, but can have as much as a 10% variance in reality. In the case of the Meter, after the math is applied, things change. You simply have to accept it, everybody does.

 

Rob

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I'm just curious about why these WGS 84 and NAD 83 produce statistically different results on the order of about a meter.  It's just interesting to me from a mathematical cause-and-effect perspective.  :unsure:

 

Apparently there are 2 different reasons for the difference - (1) an adjustment difference partly due to the use of different technologies used in determining the positions of the points included in the basic adjustement, (2) a difference in origin and orientiation of the 2 coordinate systems.

BDT,

 

Yes there is a difference in origin and orientiation of the 2 coordinate systems.

 

Both Datum use different ellipsoid models.

 

Both Datum use different Geoid Models.

 

One Datum has a grid filled with thousands of survey markers it is trying to accurize as a whole, the other does not.

 

One is trying to be a best fit for North America, the other the Earth.

 

These are reasons that even Least squares wise they are statistically different, but I think that it is likely more miraculous that they are as similar as they seem to be given the empirical differences, not just the statistical ones.

 

The Why of it really does come down to the basic differences as stated above.

 

Over powering the entire set of answers to this discussion is one answer that overrules all of the other answers. The Cause: Before all of this, the Geodesists (in each agency) had to come to agreements as to how the best way to tackle the problems would be. This is the result of the agreement in two different Datum. The Effect: The left hand did not account for the right. Didn't then and still doesn't.

 

Rob

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Your misssing what I'm saying - you guys may use different metres - but "they ain't metres", no matter what you WANT to call them

 

Call'em NAD83 or WGS84 "units" or 'Length units" - but they are NOT metres

 

It doesn't bug me that you use different "unit" definitions for different elipse definitions - that fine.

 

I just wish you would NOT call them METERS - because they are NOT

 

(aka a Rose by another Name thing - "Names are a powerful thing")

 

In other words - I understood your post, I just think it's messed up that people will corrupt the meaning of an SI unit

Kc2icxe,

 

You can just as easily say that nobody uses "real" meters, anywhere in the world, for any purpose whatsoever, because nobody ever has a 100% accurate standard, nor 100% accurate measurements.

 

Suppose you had your own tape measure, and set about measuring everything on your property as accurately as possible. Your "datum" would be your tape measure.

 

Suppose your neighbor set about measuring everything on his property with his own tape measure. At some level of precision, you would inevitably find that his and your measurements of your common boundary would differ.

 

You would have some choices -- you and your neighbor could jointly re-measure everything at great expense, or you and your neighbor could determine some conversion factor that you could use to convert your measurements to his system, or his measurements to your system.

 

That's all that's going on -- neither of you are using a "real" meter, because there's no such thing. In your system, you will think your neighbor's meter is too short or too long, and he thinks yours is too long or too short.

 

-------

 

edit: ArtMan correctly points out that there is a real definition of a meter, my point is that no one has or will ever be able to make a real instrument capable of real measurements that exactly correspond to the definition of a meter -- all instruments have error.

Edited by holograph

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FYI

 

 

Metal Tapes: Surveying, Oil Gaging, and General Purpose; Metal Scales (10030C)

 

The calibration of metal tapes and scales is carried out in a laboratory that houses two permanent working standards, a laser–based displacement interferometer and a 50 m (200 ft) stainless steel bench. Measurements are primarily performed using the laser–based displacement interferometer system. A measuring cart consisting of a cube corner reflector and an attached microscope is used to manually locate the scale graduations. The laboratory is maintained at 20 °C ±  0.2 °C but a control system can vary the chamber temperature ±5 °C for special tests. Calibration of metal tapes is made with the tape under tension and supported on a horizontal flat surface. Unless otherwise requested, the five points are usually the 1/4 section, 1/2 section, 3/4 section, the end point, and usually the unit of measure. So, for example, for a 100 ft tape one could have the 1 ft, 25 ft, 50 ft, 75 ft, and 100 ft. For a 30 m tape, we usually give the calibrated values for 1 m, 5 m, 10 m, 15 m, and 30 m. On request, each interval calibrated on a surveying tape can have computed lengths for two (single catenary), three, four, and five equidistant points of support. Calibration of metal scales is made with the scale supported on a horizontal flat surface.

 

The laser–based displacement interferometer standard is capable of calibrating tapes and scales with scribed graduations and under ideal conditions with an expanded uncertainty of 2L  10-6., where L is the length of the artifact being tested. Calibrations made with respect to the stainless steel tape bench are normally reported with a relative expanded uncertainty of 10L  10-6. A NIST serial number will be engraved on each calibrated tape for identification.

 

back to top| back to index of dimensional measurements

Special Tests of Surveying Leveling Rods and Long Length Artifacts (10040S)

 

NIST provides special tests for calibrating leveling rods. In the past, leveling rods were calibrated by either of two methods. One method involves comparison of the rod to a 3 m standard at the intervals of 1 m, 2 m, and 3 m. Other intervals can be accommodated. A second system provides automated measurement at multiple intervals and automatic report generation. Both systems incorporate a 7 m one–dimensional measuring machine with a helium–neon laser interferometer interfaced to a minicomputer. The automated system uses a motorized carriage and a photoelectric microscope for automatic edge detection of the graduations. Measurements can be made on virtually any type of linear scale and leveling rods with scribed, engraved, or painted graduations. The expanded uncertainty for high–quality Invar leveling rods is 50 µm and 100 µm for used rods. Current studies suggest that these may be lowered in the future to near 20 µm and 50 µm, respectively. The length of intervals will be reported as measured at 25 °C unless otherwise requested. The report is supplied in either written or electronic form as requested. The second method is in disarray and only the first method is currently viable.

 

Upon request and by particular arrangement, NIST is capable of performing special tests of a variety of long length artifacts. These tests are limited only by the breadth and weight of the artifact proposed for testing.

 

The technical contact(s) should be consulted before the leveling rod and long length artifact tests are requested.

National Institute of Standards and Technology

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Bureau of weights and measure

 

At one time in my life we had to get our instrument calibrated here.

Down to micro kilograms.

 

I was a measurement tech and got poisioned with this stuff....Mercury.

CALCULATION OF THE ADVANCE

SEE CHAPTER 4 TOO.

 

When it comes to precision and accuracy it all based upon the system to which it is related.

To try and compare one thing with another is futile.

It has to be used in it's proper context.

 

Just my opinion,you guy's are way beyond me on the precision diffrences btween 2 systems.

 

EDIT:

FIXED LINK I HOPE

Edited by GEO*Trailblazer 1

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While I knw nothing about ellipsoids, some observations from a few years of caching:

 

1. Jeeep and Garmin conversions WGS84 and NAD27 differ. NADCOM explains this by saying that you cannot use a straight formula. obviously Jeeep and Garmin use different fomula. To get exact conversions, you must do spot sampling as USGS did into the tens of thousands across the US. The differences between NAD27 and NAD83/WGS84 are quite different depending what part of the country. Therefore, Garmin's conversion which use a formula is in error as are all other mfr's. (I'm not aware of any data base conversion they store in their units. It would take up too much memory).

 

2. Assuming the same situation occurs in NAD83 ellipsoids vs. WGS84 ellipsoids, simple formula computations prove nothing without doing an extensive spot sampling. (Why can't they use the sampling data from the NAD27 conversion?).

 

3. Unless you are using maps, it really doesn't matter if the conversions are wrong. As long as marking a waypoint regardless of which Datum you're using is then usd by the seceond party to find the spot in the same DAtum, the GPS will get you to it since the calculation formulas use to find your location are the same amond the different GPS models.

 

That's a layman's guess. Any comments?

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That's all that's going on -- neither of you are using a "real" meter, because there's no such thing.

As much as I respect holograph and his contributions here, I'm afraid his post might be misinterpreted.

 

Of course, there *is* a real meter, just like there a real kilogram, etc. These fundamental units have exact, agreed-upon definitions.

 

But any physical representation of a meter is not going to be exactly one meter. That's why the international standard for measurements has evolved from tangible artifacts, such as the alloy bar with a pair of engraved lines that were once -- by definition -- one meter apart.

 

The meter is now defined in terms of the distance light travels in a vacuum in 1/299,792,458 of a second.

 

The National Institute of Standards and Technology gives the hsitory of how the meter has been defined over the past two centuries.

 

-ArtMan-

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Just a point of clarification, USGS (The U.S. Geological Survey) had nothing to do with the deveoplment of the NADCON datum transformation program noted by Alan2 in point #1. The development, maintenance of the National Spatial Reference System, including the primary realization by monuments, definition of horizontal and vertical datums and the various products and services that support them (e.g. NADCON) is the responsibilty of the National Geodetic Survey (NGS), formerly the Coast & Geodetic Survey (C&GS). This may be nit-picking but those of us at NGS are senstive to this.

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The Meter used for Geodesy begins with being calibrated to NIST standards. All of our equipment has to undergo rigorous calibration standards frequently to remain in certification. That highly calibrated Meter definition becomes mathematically scaled as the numbers get crunched by the various factors involved. If this does not seem acceptable, well, take a math class and brush up. It does happen, and it is not avoidable. There are simply some constructs in Applied Mathematics which cannot be avoided and this is one of them.

 

No Matter how anyone slices it. The length of a Meter is nothing more than the product of a negotiated agreement. The agreement has changed over the years and for all we know, could still. This is another construct we have to accept, because there is nothing any of us can do about it.

 

Comparing anything argumentatively to assumed accuracy between brands of Consumer grade GPSr is similar to wanting a screen door on a Submarine. Why would we want to try to do this when in the specified reality we are truly blind when under 10 feet? We simply cant. Bill 93 took a stab at it while back and he concluded that he got mixed results with one unit over time as I recall, but he did take a scientific approach to the problem and that was very enlightening. Consumer grade gear is as dependent on the quality of the GPS Constellation as anything else, except that the Accuracy is simply untrustable under 5-10 feet. Please remember this discussion of Datum difference is dealing with lengths in the Centimeter and Millimeter range.

 

It looks to me like re reading this thread would help some of us get back in the game and on the same page. This thread has very carefully laid it all out on the table. It is very representative of what the deal really is. We have tried to present the way it is, Not what we would prefer.

 

In the final analysis, all the Mathematics of Geodesy become secondary to the politics of what the Geodesist decide will be the best standard for their intended purpose. They go with the best models they have at the time, which they thing will work for the purpose. It is a snapshot of that time as well. I can only imagine they are striving for the ultimate explanation too. There is no one Be all, end all solution to any Geodetic Question. Please trust me when I say as someone from the field that it is a lot to keep up with and a lot to keep in mind. It was just as much to learn and understand. It really never stops. I could get handed a new piece of gear tomorrow and a new Datum to use with it in a year. Then I have to look into what I used to believe was the best of what is thought to be true and see how it may have changed. See if there is now a better observation and explanation than what I had. It is simply just the way it is.

 

To Add to what Dave had said, The DOD NGA, (Department of Defence, National Geospatial Agency and it's predecessors) Had nothing to do with the development of the NAD83 Datum. In the Civil field we use NGS control, we go with what they say, and they take the responsibility for their control. It is very good control. Arguably the most accurate control in the world. We do not cross compare it. It is simply not useful to do so.

 

We could of course argue the differences until say, whenever, but what would be the point? The guy who pulls all the strings is the king and he could get up from the table and choose to say, ok, all Geodetic measurements on our system are to be transformed 3 cm to the northeast. Then we would be arguably no closer or further away that we already are, when you base it on base it on what is currently known and take the entire grid on the whole. Yet it would be the new rule.

 

The earth is not a perfect ellipsoid. It has an ever changing Geoid. The GPS Constellation is tweeked for accuracy in real time. There are atmospheric distortions. Ground control for DGPS is meant to help curb atmospheric distortion, yet was derived from and based on the problems already considered. Then there is calibration and the laws of diminishing returns... Will we ever find the one true spot? No.

 

See? It is pointless after a point to keep beating on it.

 

I Hope that helps some of us sleep better.

 

Good Luck.

 

Rob

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I'd like to know a bit more about the 'meter thing'.

 

I was re-reading the post (May 10) in this thread that caseyb gave from a geodesist. It was there I think where the discussion in this thread concerning different meters started. In that post, it is significant that he used the word meter in quotes. There was a hypothetical example given in which one "meter" was larger than the other "meter", and a "scale factor" between them was calculated.

 

Here's my question: is the "scale factor" between WGS 84 and NAD 83 "meters" a constant for any pair of points A and B (in the U.S.)?

 

I assume the answer to the question is "No" because the the "scale factor" is a statistical variable, and so are the two "meter"s.

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The scale factor is constant, because it scales between the two reference frames. The difference is less than one nanometer per meter.

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BDT,

 

We have written pretty exhaustively about this meter, I think the explanation has been repeated more than once. But maybe we are missing the best treatment of the matter.

 

There is nothing hypothetical about the Meter or the Scale factor. I feel like I am running out of ways to explain this. but maybe this will help. I'll give it another shot.

 

First, The Geodesist was not trying to be hypothetical either, he was trying explain to you how it really, actually is. The quotes around Meter were meant to denote that Scaling factors alter them and we need to keep that fact in mind. We are still supposed to think of the meter as a meter, as it is still the representative unit of length. It just sees a bit of alteration from the mathematical equations and it's various processes. We have to both observe it and accept that this happens to it.

 

Ok, We have agreed that WGS84 has mathematical models which are used for it's determination.

 

We have agreed that NAD83 has mathematical models which are used for it's determination.

 

We have agreed that they are not the same models.

 

Let us agree that the Meter is the reference standard Meter we all know, because All surveyed observations are the meter, and in these mathematical models, prior to the equations that make them Datum used the same meter. It is the only one we use.

 

2 different Ellipsoid models with different mathematical values. Both close but not equal. Lets call the two ellipsoid models X, and Y.

 

2 different geoid models with different mathematical values. Both close but not equal. Lets call them A. and B.

 

Models X, and A were used by WGS84

 

Models Y, and B were used by NAD83

 

When X and A are taken together to create the WGS84 Datum, the numbers mix and integrate and due to this integration, we observe, AFTER the integration, that the meter seems to be a slightly different length than the one we used to begin with. Some part of the formula causes a scale factor to occur. This scale factor is the result from how the integrated number affects the length of the meter when looked at through a global model. It causes the length of the meter to change from forcing it to conform to the curves and shapes of the ellipsoid and Geoid as taken together.

 

Models Y and B are going to have the same thing happen to them as they are integrated into the Datum they are meant to be. Only the factors within this particular model Though theoretically similar in method, are going to cause a Different value of scaling to occur which will be a different length for the meter as an end result.

 

There is no Hypothetical meter. There is a real one and the effect that mathematical results from some very difficult equations cause to happen to it. Even after the fact, It is still meant to be taken as a Unit called the meter.

 

So to your question: is the "scale factor" between WGS 84 and NAD 83 "meters" a constant for any pair of points A and B (in the U.S.)?

 

It depends. If you are comparing the NAD as viewed by the WGS, then the scale factor comparison will belong to the WGS Framework. It will be constant within that framework. If you are comparing the WGS as viewed by the NAD, then the scale factor comparison will belong to the NAD Framework. The scale factor will be constant within that framework. They each have their own Scale factor however and they are not the same if you were to rip each one out and lay them on the table. The scale factor is a product of the integration and each system used different models so the factor would be a net result which would also be, different. As an additional thought, There is no third form of control in the comparison, against which we can compare the other two, and if there was it would suffer the same anomalies. So what would be a good reference standard? There isn't one. These are supposed to be the reference standards.

 

The Meter as A priori is not meant to be taken as a statistical Variable. But once the Formulas are applied it becomes a statistical Variable. The Scale Factor is a product of the Statistical variable that affects the end result length of the meter as an end result.

 

The Pair of points will never be the uniform "same due to different earth centers and which are not linear, the difference is in three axis: Left - Right, Up - Down, Front - Back, not simply the straight line distance between he two. This causes the surfaces of the Datum, which you could think of as an integrated ellipsoid-geoid, to be different in A, the three axis's, locationally, B the Geoid Model, and C the ellipsoid model, all taken at once. In the end they are different and they do not exactly share the same space exactly either.

 

Hope that helped.

 

Rob

Edited by evenfall

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OK, I will re-phrase the question in terms of your last post (so it becomes 2 questions).

 

It depends. If you are comparing the NAD as viewed by the WGS, then the scale factor comparison will belong to the WGS Framework. It will be constant within that framework.
(1) What is the value of this constant scale factor?

 

(2) What is the value of the constant scale factor comparing WGS as viewed by NAD in the NAD framework?

 

I'm still currently of the opinion that these 2 scale factors depend on where you are on the surface of the planet (or even just in the U.S.) and are in fact NOT constants, but if they are constants, what are the values of these 2 constant scale factors?

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BDT, You have already been Answered. I suppose you have a right to your opinions.

 

You're welcome.

 

Rob

Edited by evenfall

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Ok - when I say they "aint meters" - as I said - I understand the diference between the NAD and WGS MODELS and what the models SAY is a location (which is based upon models)

 

What _I_ was trying to say is redefining the meter is a strange way to do it from a "pure" science thing

 

I know all about "tolerance" on measurements - which is why I either specify a tolerance, OR work with the same measuring device to get "matching" when I don't have to work with some one else - what to the say? Measure with a micrometer, mark with a crayon, cut with an axe? :o

 

The only reason I say it funny - let's pull a thought experiment

 

I design a "Perfect" benchmark setting machine - it can set a reference mark ANYWHERE I want to PERFECT accuracy (I spec this just so we don't have to use +- - makes the math easier)

 

I set a mark SOMEWHERE - Lets call "Point0", and it's at Lat0, Long0 elevation 0 (yes I know that I'm in the middle of the ocean at the moment - but HUMOR me - it doesn't matter WHERE it is)

 

I then put a PERFECTLY vertical rod 1 meter tall (just to see over the curve of the earth) and then go and place another 1 meter tall rod EXACTLY 1 KM away - NOT at the base of the rod - but that the point at the TOP of the rod is EXACTLY 1KM from the other rod - (let's just say it's exactly the distance light travels in a vaccum in 1000 / 299792458 of a second)

 

Notice something - I have so far used NO geodesy - those two points are EXACTLY 1 KM apart

 

Now we change the frame of reference say from WGS 84 to NAD 83

 

How far apart are those 2 points? If you say ANYTHING except 1KM, you are redefining the meter - aka its NOT a meter

 

I understand WHY it's done - heck - it's a lot easier to say "it's a NAD83 survey meter" than the say "That point's latitude, longitude and altitude is no longer X,Y,Z but D,E,F"

 

My point is - I understand the diference in models (and keep my GPSr on NAD83), and understand WHY there is a scaling factor, and even why the term "meter (metre)" is used. The thing is, I also understand why folks get confused, because

Meter!=Meter!=Meter

 

It's just a Pet Peve of mine when ANY science, for the ease of short hand, uses a term that is used elsewhere, differently (How many Math students take a long time to realize that Y=MX+b is the same as fx=ax+c?)

 

Just one from my field- a kilobyte. Should be 1000 bytes, but it's NOT, it's 1024. That's why I hate the fact we use it. There is/was a movement to use the term kibibyte and megi etc - the "Binary" versions of kilo and mega. I like the idea, but it'll never catch on

 

THAT is my only complaint. aka a complaint about terminology, NOT about what is DONE - call me pedantic

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kc2ixe -

 

I agree. It seems similar to calculating gas mileages on a car but saying that the mileage of the car is always the same (from the car's perspective) and that instead, the size of the "mile" is different on each trip. A trip through the mountains would have a 7,000 foot "mile", etc.; an 'effective mile'.

 

Plus, I think that there is no such thing as constant NAD 83 meter or WGS 84 meter, as as viewed by NAD 83 or as viewed by WGS 84, or whatever. I could be wrong of course, but one evidence is the CORS data that holograph posted back here in this thread with different amounts of distances for 'pairs' of points that are each actually the same point. These are all presumably from the NAD 83 perspective.

Edited by Black Dog Trackers

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kc2ixe,

 

Both WGS84 and NAD83 would say the stations are exactly the same distance apart, since they use the same meter, and you've assumed there is a perfect measurement.

 

More precisely, neither datum would have a role in the measurement scenario you describe (other than the location of Lat 0, Lon 0). The datums say nothing about direct measurements between two points, and you've made no use of the coordinates of the points.

 

It's their reference frames that need a conversion -- each one is using a slightly different coordinate system, and one of those differences is the scale. If you specified the position of each point in terms of the NAD82 and WGS84 reference frames, they would be different.

 

If you determined the coordinates of your two points, and then computed the distance based on their coordinates, you would get slightly different answers due to the scale difference, but you could correct for that with the scale factor.

 

NAD83 and WGS84 are analogous to the "blue" and "magenta" 2-dimensional frames shown below. The two systems have different origins, orientation, and scale. Since the "realized" frames are based on measurements, and measurements alone, there has to be a difference between the two. Both use the same meter, its just that they disagree slightly on the coordinates of the points that have been measured.

 

All three effects have to be accounted for when converting, as well as their change rate over time (d/dt). That's what the Helmert and other conversion algorithms do.

 

The systems below show greatly exaggerated differences. There is a further difference not shown which is negligible, and that is the different ellipsoid. The ellipsoid only has an effect on the conversion from cartesian to spherical coordinate systems, i.e. from (x,y,z) coordinate to (lon, lat) coordinate. Finally, the geoid, not the ellipsoid, is used to obtain elevation from the (x,y,z) coordinates.

 

Effect if only the origins were different but orientation and scale were identical:

translation_factor.gif

 

Effect if only the orientations were different, but origin and scale were identical:

rotation_factor.gif

 

Effect if only the scales were different, but origin and orientation were identical:

scale_factor.gif

Edited by holograph

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From what I have read, WGS 84 and NAD 83 have different origins (different Earth center as Dave Doyle says) and different orientation (I think they had the same orientation before the ITRF, but now they do not). They have the same scale, however, the meter.

 

In terms of holograph's drawings, from what I have read, the NAD 83 - WGS 84 situation would be more like if the axes picture were changed by:

 

1. Combining the different origin and different orientation concepts and

 

2. A distortion applied (like rippled glass) on the axes that magnified some parts of the scale and de-magnified other parts of the scale, with separate rippling for each of the 2 colors. It would be the same scale, but warped through the adjustment of measured observation of many points (with statistical error in the measurements).

 

Distances between pairs of points the same real distance apart would be seen to vary because of two compnents - a systematic component and a statistical component.

 

1. The systematic (non-random) component of the distance is caused by the difference in origin and orientation.

 

2. The statistical ('random') distance component (like rippled glass) is from the adjustment of observed data (with statistical error).

 

The result of combining these is a statistical difference.

Edited by Black Dog Trackers

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BDT,

 

Their ideal, perfect systems have the same scale, but their imperfect realizations are different from their ideals, and the effect is a slight difference in the scale of their realized coordinate systems. As a result, the drawings closely represent the actual types of difference in the realized datums, including the fact that there is a difference in scale. There are systematic differences in all three aspects.

Edited by holograph

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Wouldn,t the constant be the clock?

Not the distance between the point(s). :o

The metre is defined by the second, and has been since 1983

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your still missing my point - I GET that the MODELED distance IS what changes, simply because you CANT always get a direct measuement

 

BUT _I'm_ just complaining about the term metre

 

when someone says "Point X is N metres from point Y" - you may scale to CONVERT from one MODEL to another

 

The thing is - what I'm trying to say is we should be making 3 different statements:

"Point X is N metres from point Y" (direct measurement)

"Point X is MODELED to be N metres from point Y in NAD83"

"Point X is MODELED to be N metres from point Y in WGS94"

 

Of course, in what you are doing professionaly, you use a shorthand - just like all professions do

 

Of course the most accurate would be the "true" measure - all the others - no matter HOW you shift your reference, unless the distance comes out exactly the same as the "true measure" requires a "metre" that isn't - and that is what I'm trying to bring out - and (at least to me) once your realize that "metres aint metres" it actually makes it easier to understand the difference between the different models

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OK - second follow up - and I'll use holographs 3rd example as the "simple" version of what I'm trying to say (aka scal shift only)

 

In that example, the 6,4 point became 7.5,5 - works for me for example data

 

Let's presume in all of these examples that by some "magic" method that we can get a direct measurement bewteen the 2 points as in my previous thought experiment (I'll do my internal math to 4 decimal places, but)

 

Now in the first frame of reference, point 6,4 is 7.211 metres from 0,0 and lets say that this turns out to be the DIRECT measurement too (just to make things simple)

 

in the second scale it's point 7.5,5 or 9.0138 meters from the origin

 

So, what's the REAL distance? 7.211 meters - no matter which

 

In fact, more likely would be some where the direct measure is 8 meters, and nether or 1st "Model" of 7.211 meters nor our second model of 9.0138 meters is RIGHT - in fact, if we "say" in (just to use the first model) that 7.211 Meters is the "correct" distance, we have redeined the meter - as the point has NOT moved - aka "metres aint metres" - we have 3 meters "Scale1 'meters'", "Scale2 'metres'" and "SI absolute metres"

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holograph -

 

The datum is the result of a bunch of points around the country with observed positions (as measured). Each of these observations will have statistical error associated with it. That is where the warping of scale comes from. It will be different in different parts of the country because of the stations there and the statistical differences in their measurements. The realization thus constists of statistical error.

 

The adjustment procedure will distribute the statistical errors but they will still exist and they won't be the same everywhere, vaguely like the differences between an ellipsoid and a geoid.

 

These differences will also be different between the NAD 83 group of points (each with their statistical error in measurement of position) and the WGS 84 group of points.

 

This aspect of the NAD 83 - WGS 84 difference is not systematic because it is not based on systematic errors in observation.

Edited by Black Dog Trackers

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The thing is - what I'm trying to say is we should be making 3 different statements:

"Point X is N metres from point Y" (direct measurement)

"Point X is MODELED to be N metres from point Y in NAD83"

"Point X is MODELED to be N metres from point Y in WGS94"

 

That's exactly what they do say. That's why a geodetic calculation always mentions the datum used.

 

OK - second follow up - and I'll use holographs 3rd example as the "simple" version of what I'm trying to say (aka scal shift only)

 

In that example, the 6,4 point became 7.5,5 - works for me for example data

 

Let's presume in all of these examples that by some "magic" method that we can get a direct measurement bewteen the 2 points as in my previous thought experiment (I'll do my internal math to 4 decimal places, but)

 

Now in the first frame of reference, point 6,4 is 7.211 metres from 0,0 and lets say that this turns out to be the DIRECT measurement too (just to make things simple)

 

in the second scale it's point 7.5,5 or 9.0138 meters from the origin

 

So, what's the REAL distance? 7.211 meters - no matter which

 

In fact, more likely would be some where the direct measure is 8 meters, and nether or 1st "Model" of 7.211 meters nor our second model of 9.0138 meters is RIGHT - in fact, if we "say" in (just to use the first model) that 7.211 Meters is the "correct" distance, we have redeined the meter - as the point has NOT moved - aka "metres aint metres" - we have 3 meters "Scale1 'meters'", "Scale2 'metres'" and "SI absolute metres"

kc2ixe,

 

There was no way to directly measure the "real" distance from the origin to the control points. The measurements they used were incredibly precise, but involved various astrometric obseverations that were adjusted according to models of electromagnetic propagation through the atmosphere, calculation of the effects of relativity, and other theoretical adjustments that are constantly improving.

 

They aspired to accurate measurements using the SI meter as defined. Subsequent improvements in technology showed that they erred, as was inevitable. It's possible to compute a correction factor based on current best estimates of their difference, and that correction factor is all we've been discussing.

 

The point is that until God or some oracle tells us what the "real" distance is, all we have is measurements, and the fact is that geodesists discovered that the measurements that were used to realize NAD83 gave slightly different results than the measurements used to realize ITRF00/WGS84. No one knows what the "real" measurement is.

 

The next time they realize their datums, they will again aspire to accurate measurement, but inevitably technology will improve and it will be possible to measure a bias that could not be measured at the time the datums were realized.

 

If you insist on saying that NAD83 and WGS84 should not use the term "meter," then you have to also insist that no one who has ever measured any real object of any kind should be allowed to call their measurement a meter, because no one has ever made an absolutely accurate measurement.

 

The amazing thing is not that they disagree, but that they agree so closely.

Edited by holograph

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Believe Me Guys, Holo and I as well as those at NGS feel your Pain. This stuff is sometimes hard to get ones mind around and accept, but sometimes we must simplyagree with things that are not easy to understand because ultimately, what is needed is a system that works as well as it can, not a system that is unavailable for use because it is not perfect enough.

 

Real science has warts and all sorts of details we have to observe as well as make exceptions for. It is just the way it is. It is what we have been trying to Illustrate all along.

 

What you can know, and hopefully feel good about is that the NGS is the oldest scientific arm of the US Government. After 200 years, the work they have done is nothing shy of monumentous. They too have had to reconsider, more than once, the things they felt they could hold as truth and fact about the shape of the earth and continent.

 

To this day, it continues to evolve. We may yet think back to this discussion and say, remember when we thought the earth was most like NAD83 and WGS84?

 

Holo, Nice work on the Graphics. I am not sure everyone understands, but your example is right on the money.

 

Rob

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Ok - Guys - last post for me

 

Yes - I know we always talk which datum is used, and you can "assume" the term "Modeled" for the meter

 

As for no way to make "direct" measurements - depends on the distance you are talking about :lol:

 

as for no way to measure perfectly - of course, BUT what I'm trying to say is that the distance between 2 points does not ACTUALLY change when we shift between WGS84 and NAD83 but the distance we calculate those 2 points to be apart DOES, and that if we were by some "magic" method able to measure the "true" distance apart they are, it might not (actually probably will NOT) be exactly the same as EITHER datum, and that the point may, or may not be inside the measurement accuracy and tolerance (and yes I know the diference) of our "magic" method

 

I just don't like the the term "metre" for anything that is scaled from the SI unit

 

Want to call in a NAD83 (or WGS84) Meter - No problem

Want to call it "Point A is modeled to be 1532.001+-2mm meters from point B in the (NAD83 | WGS 84) system" - again, not a problem

heck even

"Point A is modeled to be 1532.001+-2mm meters from point B" when the data sheet states the datum system isn't a problem

 

But just saying "It's 1532.001 +- 2mm" meters from point A to point B unless you are talking the SI unit, unscaled _IS_ a problem, and is why folks get confused why dataum changes are important.

 

It's NOT that I don't understand it - it's just that I'm being pedantic about terminology - just like my teachers would slap you down for reporting the wrong number of significant figures in a calculation, and for not making sure your units balanced :lol:

 

It may seem HARDER to be pedantic about units, BUT when you are trying to explain frame shifts to someone who might not understand that you CAN pick different frames of reference, often using the precise names for things makes it easier for folks to understand

 

That's my only point - it's about "Names", and the power of names.

 

(Heck - non measurement one - Here on this forum I'm KC2IXE, the FCC knows me as KG2V, my friends call me Charlie, but no matter what, I'm really Charles - the others are just frames of reference)

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The first two of holographs drawings are pertinent. The third, I believe, is not.

 

There is no real difference in the meter. There is only a virtual/apparent/effective difference.

 

A datum is formed, not only from the position of its origin and orientation of its axes, and the shape of its ellipsoid, but also the particular positional measurements of a group of points including their statistical error. The statistical error of measurement of the points involved in determining the datum causes statistical variance over the surface. This results in apparent (but not real) differences in the meter.

 

So, after the difference in origin, rotation, and ellipsoid shape (the systematic differences) are accounted for, what's left is the statistical difference that one could interpret as a difference in the relative sizes of meters, but really isn't.

 

If one were to insist on modeling this apparent 'meter difference' as a scale as per the third axes drawing, then the distances between each pair of adjacent tickmarks should vary statistically, including the pink differently than the blue.

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Hi All,

 

This is becoming just a little exasperating :-)

 

(chuckling and shaking my head. Head hung a bit low. Looking at my feet even...)

 

kc2ixe, BDT,

 

I wish I could bring you guys to my side of the table. I have Tried, So has Holo. It is not my place to speak for DaveD, Z15, or Holo. We all just work in the field in our own ways. We all know a little. We have not asked for a lot in return for our efforts to help you understand this, but what can I say.

 

Charlie, (if I may call you that this once,) I understand where you are coming from. I do. But If you really would like to better understand this, Please write to NGS and ask them about why Geodesists allow this terminology. I think they are far more familiar with the come and go of it. DaveD is a geodesist, and I hate to say this but Dave seems to be ok with what Geodesy does to the meter. I want for you to have that understanding like I have, but I feel at a loss to help you as best I could, and they are the source for the best answer you can get.

 

BDT, I feel a bit differently on how to help you. I have chosen my words oh so carefully while explaining this and when I watched Holo try to say the same things I have just a different way, I saw you refute points of what we are trying to help you understand. Well, I am at a loss as to how I can help you get it. I want this for you. I have a very strong sense that you want to understand the machinations behind the Geodesy too. I have a plan.

 

The Meter is not apparent, It is real. What happens to it in the datum formation is real and it is incorporated into what the numbers say on your datasheet. It is so. My Instruments interpret geodetic locationing as a construct of an applied Datum. That means that my instrument interprets the meter in the Datum it is told to, via post processing. Welcome to the ways the earth is measured. You can refute me if you like. I have the meter that fits my tape measure and I have the meter that fits my Datum. They are inherently a Meter. Why do you think there is an inherent difference in State Plane Coordinates, and NAD83. The Earth isn't flat! Beware of circular Trig! and that is not the half of it. Ok? :-D

 

There is danger of being wrong when you step out of a well defined box in this game, that is why we use different systems for different applications. We do the "best fit" box a lot. It is not a good Idea to cross compare some of the boxes. It is just confusing and well, not really proactive either. That is why it isn't done.

 

This brings me to the question for you BDT. Mine is, what would be the harm of asking a Geodesist? You are pretty Local to NGS, and a phone call isn't too spendy Maybe set up an appointment to meet with someone who can spare a couple hours to lay it all out. Maybe you can hook up and walk through the process with a full blown Mathematician with a Ph.D in Geodesy and report back to us. I am sure they can explain in detail the problems they had and what they had to agree to do in order to arrive at solutions that would fit. All this is a tough row to Hoe in a Forum. They can show it all to you in ways we could never do here, and aside from that, It would be cool to be able to ask, don't you think? I am sure the ensuing questions and answers as to why the US developed 2 nearly identical Datums that barely acknowledge each other will be an interesting story too. I have alluded to it a bit here myself.

 

Please consider that thought. I am sorry but I fear that when I try to explain to you the things I have been taught, and you refute them, it kinda leaves me with little more to answer you with other than Sorry. That is the way it is. Accept it or don't. If you can't accept it, Ok, But try Geodesist on for size. This is what they do.

 

P.S, DaveD is one of the Geodesists who you would want to ask about Horizontal Datum. He is considered an authority on the matter. He started this thread.

 

I really hope this sorts out for you BDT, you are asking good questions, I just don't feel it is easy for you to accept the answers. Maybe someone else should help fill in the blanks.

 

All the Bests,

 

Rob

Edited by evenfall

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Pedantic is good when dealing with complex topics. I don't quite understand it yet, maybe never will. But I agree with Kc2ixe that in order to fully understand it one must recognize and name the distinction (if there is one) between the international standard meter, a NAD83 scaled meter or a WGS84 scaled meter (or whatever we choose to call them).

 

I am trying to separate the statistical variations from the underlying theoretical framework, and that distinction is not fully pointed out in some of the posts. Maybe next week I'll get time to review the whole subject. There's a lot of meat in this thread to digest.

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One last attempt to clarify the term "scale". The Datums don't scale from the SI unit, they scale from each other.

 

Referring back to the analogy of two neighbors independently measuring with their respective tapes, they can agree on the definition of the meter, but they can only measure with their imperfect instruments. If they don't have access to a third party authority who can calibrate their instruments, all they can do is determine the systematic difference between their measurements.

 

You may ask "If NIST can measure a meter to one part in 1e11, why do the datums differ in one part in 1e9, if they are using the same meter?" Well, NIST can't determine the distance from the center of the earth to a point on the surface to one part in 1e11. NIST can't determine it at all. Geodesists managed to determine it to about 1 part in 1e9. The geodesists who determined it for the ITRF came up with a different result than the geodesists who determined it for the NAD83. Since the scale of the WGS84 and NAD83 datums is based on essentially on how far a point is from the center of the earth, the datums have different scales. No one can know how different each might be from the SI meter, because the only way to know that is by knowing the "real" distance from a point on the surface to the center of the earth.

Edited by holograph

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I think one situation here is the relatively sparse participation from the NGS experts on this topic. Not to say that they should join in more than they already have, since after all this is just a benchmark hunters' forum, but it would've helped, I'm sure.

 

holograph -

 

If there are imperfect instruments, then there are both statistical and systematic error. Even if the same one instrument was used by the same one person for all the measurements of all points that established each datum, there would still be a significant statistical error.

 

Your second paragraph reiterates the systematic difference caused by the different origins of the coordinate systems, to which I agree.

 

In short, there is inescapably a statistical effect between WGS 84 and NAD 83. Whether its effect is greater or less than the systematic differences involved in the WGS 84 observations and the NAD 83 observations is unknown.

 

Perhaps the difference between us is that you are focusing more on the systematic component while I am focusing more on the statistical component. :)

Both components exist and contribute to the virtual/apparent/effective difference in meters.

Edited by Black Dog Trackers

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BDT,

 

Yes, we've been ignoring the statistical, random fluctuations, because that hasn't been the central subject of the discussion about datum conversions. Random errors do occur, of course, and you are entirely correct that the NSRS control points will contain random errors that cause deviations from some perfect ideal (the wavy glass effect), and so any measurement made with reference to the control points will propagate those random errors.

 

However, there is also a systematic bias, and it's that systematic bias that was brought up when Casey first mentioned the different datum scales. The mathematical transformation formulas deal solely with the systematic differences between the reference frames, and do not deal with the random differences at specific points -- not because random differences don't exist, but because there is no formula to account for them.

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Sorry I'm late to this thread, but I just wanted to comment on some earlier topics.

 

On what receiver manufacturers use internally for datum transformations, a lot of manufacturers rely on the NGA tables in some version of TR 8350.2 for their transformation parameters.

 

I think some people were curious about an estimate of their local difference between NAD83 and some realization of WGS84. One guy at NGS does have an online conversion. It doesn't seem to state the accuracy in anyway, so consider it for entertainment purposes only. Mostly it's trying to take into account crustal velocities.

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kc2ixe,

I may have missed something in the conversation, or may be repeating something someone else said, but wanted to clarify what I see as your argument.

 

Let's say we have two disks relatively close together (within instrument measuring distance). The disks have adjusted accuracy in a modern datum, take your pick.

 

Now we:

 

A: Measure the distance between the points with an instrument (assuming an error-free measurement system)

 

B: Calculate the distance between the points based on the two disks' coordinates

 

My understanding of your frustration is that the two values will come out different, when you think they should be the same. But, the geodetic B value will always be off, within a certain error based on the datum's model (ever smaller the more modern the datum). The geodesists never claim to be exactly accurate.

 

I think that discussion is immaterial to the actual statements in the datasheets, which when they mean "11.681 meters from the reference Big-Boy statue", they've measured it with whatever instrument and they got that measurement in plain old meters. When meters are quoted, it's always measured by hand, or instrument, is always the standard meter, and thus never is affected or redefined by any geodesy voodoo.

 

There is no different "meter", to my limited understanding, because no geodetic framework claims exact accuracy point to point, nor do they try to redefine the meter. They only try to approach true accuracy.

 

Say that 50 years from now our datum model allows us to repeat this A & B measurement scheme and get a difference of 10 nanometers. That's pretty darn good.

Edited by BuckBrooke

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Wow.... I'll take my credit hour now :D

Still valid after all these years??

Bet you guys forgot about this one :D

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-It would be worth a credit hour to really understand this stuff.

-I'm not sure this discussion ever did arrive at a completely valid conclusion.

-We wish we had forgotten it.

 

The discussion of more than one kind of meter always bothered me. At one point I was confused enough to start accepting a difference for purposes of discussion and now wish I hadn't. There are distances on the ellipsoid grid and distances on the ground, and different starting points for different datums, and statistical error in measuring actual positions, but I accept only one kind of meter.

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Since someone brought this up..

 

You know, I never knew there was a difference between 'survey feet' and normal feet.

 

What's the difference? How does one convert from normal feet to survey feet? (I know there's a formula on the NGS website relating to converting meters to survey feet.)

 

Mike "I have two feet." Morrey.

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there is 2 parts per million difference.

The old definition was based on 39.37 inches per meter exactly.

39.37 inches /12 = 3.2808333... US Survey feet per meter

 

There was a change in international standards based on 2.54 centimeters per inch exactly

(1 ft/12 in) /0.0254 m/in = 3.280839895... International feet per meter

 

39.37 * 0.0254 = 0.999998 exactly

 

It really doesn't matter for any practical application except where huge distances are involved like State Plane Coordinates.

 

The confusing thing is that some states define their SPC in international feet, some in Survey feet, and some don't actually have legislation to say which.

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Since someone brought this up..

 

You know, I never knew there was a difference between 'survey feet' and normal feet.

 

Mike "I have two feet." Morrey.

 

Actually it is the secret issue survey boots that make the difference.

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Bet you guys forgot about this one :D

LOL, no, this was one of my favorite discussions! Very amusing and sometimes educational. I just wish that there was some more input from the NGS during the discussion.

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39.37 * 0.0254 = 0.999998 exactly

Interesting, thanks for the info. I knew there was a difference, and that some states use 'Survey Feet' and some use 'International Feet' - DaveD mentioned this in his webinar back earlier this year. Up until that point, I didn't realize that's what the difference between the tag "sFT" and "FT" on datasheets were! :D

 

So:

1.0 FT = 0.999998 sFT

 

...Great. This makes me .0003mm shorter in Survey Feet.

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Actually it is the secret issue survey boots that make the difference.

...huh.. I thought those boots were for kicking people in the .. er.. :D

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