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County Error Followup


Bill93
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A while back in the general benchmarking forum a list was posted for PIDs with coordinates that were in a different county from that listed on the data sheet. I've been (armchair) researching some of those in Iowa. NK0783 is quite interesting, not just the usual oops-adjacent county mistake.

 

NK0783 'E 38 RESET' has a to-reach that my map shows to be very close to the to-reach and coordinates of

NJ0633 'E 38' that is marked destroyed, although it is by road instead of RR. The latitudes match within 9 seconds. The longitude is one degree minus 4 seconds different, and that's why it showed up in the wrong-county list. Sounds like a degree transcription error and a few hundred yards different plotted position on the scaling map.

 

Ok, you ask, when does it get interesting? Well, another PID for the same name NJ1006 'E 38 RESET' has a to-reach starting from a different town that turns out to get you to the same road intersection, and coordinates that match exactly with the original E 38. This 'good' reset data sheet is dated 1984, and the messed-up one was dated 1984 MONUMENTED and 1989 GOOD by NGS indicating that the wrong data came second.

 

The two descriptions differ 0.16 vs 0.2 km from the road intersection and refer to different features for the east-west reference, and have over 5 ft difference in north-south distance from the road. I'm wondering if they describe the same disk or if the reset got done twice.

 

The wrong longitude also leads to a slightly different GEOID03 value, only 0.41 meter,but other marks within a few miles of each of these show variations much smaller than that.

 

What I don't understand is the difference between the types of NAVD88 heights listed: VERTCON on NK0783 and RESET on NJ1006. Somehow, with a geoid difference of 0.41 meter they ended up with elevations only 0.04 meter different. Is this evidence that we might find one or two reset disks?

 

The location is an hour and a half drive from my house, and I don't have reason to go that way often. But I'm sure looking for an excuse, now.

Edited by Bill93
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Somehow, with a geoid difference of 0.41 meter they ended up with elevations only 0.04 meter different. Is this evidence that we might find one or two reset disks?

 

Bill,

 

There is a chance that a monument resetting crew placed a monument.

 

There is a chance that a leveling crew set out from one location and ran a level loop over this reset, which closed, and the results were put through the least squares process and it became a PID with datasheet.

 

There is also a chance that a different leveling crew set out from a different location than the first and ran a level loop over this reset, which closed, and the results were put through the least squares process and it became a PID with datasheet.

 

The left hand may not have known that the work had already been done by the right hand, but because all the post processing was not yet finished in house, they may not have known it was already in process. Further, the post processing may have been done by different people, and not remembered. It could have slipped through with no one knowing or realizing. So two PID's for the same station seems plausible.

 

Leveling is path dependent, which means that if you and I live a mile apart, and there is a Bench Mark half way between our houses and there is also a Bench Mark at each of our houses, and we each level our way to the one in between, there is a good chance that our findings, when compared at the middle Bench Mark may not match. The least squares is what evens these anomalies out and helps everything fit together.

 

In the case you mention, this is .04 Meters or .131233 ft, rounded to 13/100ths of a foot. One tenth 3 hundredths if you will. Call it a sliver less than 1-37/64th inches difference... it is conceivable that 13 hundredths difference could close in two large level loops... But it is a large difference.

 

Until you lay eyes on the actual place, it is a lot of guessing. But there should only be one E 38 reset in the "38" level line.

 

Good luck Bill!

 

Rob

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I think I've figured out a little more than I had when I first posted. For starters, the geoid height at those lat-long positions is irrelevant to determining if the given elevations make sense. Only the change in geoid height for the datum conversion at one location or the other is important.

 

The datum conversion for the original mark NJ0633 (NAVD88 - NGVD29) is 328.964 - 328.982 = -0.018 meters. VERTCON gives this same value at that lat-long.

 

It looks like the (good) reset described in NJ1006 changed the height by -0.214 meters (lowered). There is no NGVD29 height listed for NJ1006, so the reset calculation was done after the old location NJ0633 was converted.

 

It looks like (bad) NK0783 was entered as the first reset, because it is given NGVD29 elevation; probably the conversion was not done on NJ0633 yet. Then its NGVD29 height would be the original 328.768 plus the move of -0.214 = 328.768, rounded to 328.77 on the data sheet. So far so good.

 

The VERTCON value for the erroneous longitude is +0.017 meter. So NK0783 got converted by this amount, to 328.768 + 0.017 = 328.785 rounded up to 328.79 on the data sheet. Or else rounded first 328.77 + 0.02 = 328.79.

 

It would make sense that someone found that they didn't have data for a reset mark, because the longitude error kept it from showing up, and created NJ1006 using the reported move.

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The conclusion from this is that the same physical move for both the "E38 RESET' data sheets could result in the published elevations. There is no evidence there for a second disk.

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A 1978 manual on the NGS site (NOAA Technical Report NOS 73 NGS 8) briefly discusses reset procedures. It indicates that the only field work needed was two measurements of the difference between original and reset positions. Running new level lines from other marks was not mentioned there. I'm guessing that they typically did not run new lines and redo the least squares unless the mark was of particular importance.

 

Rob, I agree that any run of a level line has its own errors. And that even if you could do a "perfect" job of leveling then two different starting points in the NGS system probably would not agree perfectly because they were established by least squares fit on imperfect observations. But I don't like the statement that "Leveling is path dependent". This is misleading without qualifications, because the underlying theory of gravitation says that leveling is NOT path dependent. Just the measurement errors differ. If leveling were path dependent, then there couldn't be an idealized level surface and we could have an M.C. Escher waterfall existing in the natural topography.

 

Comments, anyone?

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Bill,

It certainly looks to me like these are the same marks. I would email my observations to Deb Brown. In all likelyhood, the coords and NAVD(88) height for NK0783 are incorrect. She may have to dig up the original paperwork.

 

Rob is correct when he says leveling is path dependent. The earth is not uniformly dense. A plumb bob does not always point to the center of the earth. If I am closer to a mountain than you, what I consider to be level will actually be leaning from the mountain toward the area of lesser gravity. An error is introduced at each step. These errors do not average out even in a closed loop. The only way to eliminate error, assuming perfect instruments, would be to break the path up into infinitessimal steps.

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Not being a geodecist or surveyor, I am having trouble getting my mind around the part about the errors introduced by the lack of uniform gravitation not averaging out when running a level line in each direction.

 

What I am reading, then, is that if I survey a level line in one direction, the errors are not the same as if I surveyed it from the other direction? Similarly, around a closed loop in one direction, the errors are not the same as if I go around in the opposite direction? Is that true even using the same stations in both directions?

 

This seems counterintuitive. It will take some time to sink in.

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If you could follow the actual level surface, which would be the surface of a lake at that height with no waves, you would not be path dependent.

 

When you sight at something distant enough that its vertical doesn't match your vertical, or that of points in between, or where the gravity changes between your sights even if the vertical is the same, then you have another source of observation error.

 

That error can be decreased by shortening your sights (unless the accumulation of instrument-related errors gets to you). This source of observation error depends not only on the path but also the choice of points along that path sighted on. If you run the sights by different lengths along the same path, or by a different path, the variations may cause different observation errors. It is my semi-educated guess that these gravity variation/change in vertical effects only show up in very precise surveys many miles long.

 

If you run the same stations in the reverse direction there is no [different] error introduced by the gravity/vertical anomalies, just a new set of temperature/refraction/plumbing/etc. errors.

Edited by Bill93
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If you could follow the actual level surface, which would be the surface of a lake at that height with no waves, you would not be path dependent.

 

You may want to browse Geodesy for the Layman

 

Bill, the surface of a lake or ocean is not flat (or spherical). Even in the absence of waves or air pressure, it has peaks and valleys due to gravity anomalies. Refer to the picture below. The curve labeled GEOID represents the surface of the ocean.

 

Imagine you want to determine the elevation of the point at the intersection of the dashed line labeled NORMAL TO ELLIPSOID and the solid line labeled NORMAL TO GEOID. Call this point X. Lets call the base of the letter P in the word ELLIPSOID above the MASS DEFICIENCY point P, and assume it is 100 meters. Lets call the base of the letter O in the word GEOID above the MASS SURPLUS point O. Its elevation is 0(zero) since it is at sea level.

 

What is the measured elevation of point X? From point P, can't you see that X appears to be well above sea level? From point O, X appears to be below sea level. From point X, O appears to be below sea level. A closed loop would not yield a zero net elevation change.

 

80003026.GIF

 

It is my semi-educated guess that these gravity variation/change in vertical effects only show up in very precise surveys many miles long.

I am not a geodesist, but I believe you are right. I don't know how much error is introduced per mile in a typical survey due to gravity anomalies.

Edited by TerraVador
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Great illustration. Yes, I intended my lake surface to follow the curving geoid in the illustration.

 

I think we totally agree on what happens, just maybe differ in the semantics. I'm trying to argue that leveling is not INHERENTLY path dependent, but that there are practical difficulties that keep us from doing perfect leveling. And that those difficulties as we have been discussing are less a property of the PATH as shown on a map than of the points (and distances) chosen on that path for instrument setups.

 

I think the CGS standard for the 1930's work was not more than 150 meter sights, or maybe 300 ft (my sources disagree). This restriction helps control the effects of nominal earth curvature, the anomalies we have been talking about, and also refraction.

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I visited the site of the double-described disk yesterday and everything checks out so that I believe that NK0783 and NJ1006 are indeed for the same disk.

 

I'll be logging to NGS this week with the same words as at these GC links (without the "additional comments". Since it is an unusual situation, I'd welcome any comments about whether the logs explain it clearly.

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Bill93. The maximum line lenght permitted by C&GS circa 1930s was 150 as defined in C&GS Special Publication 140, "Manual of First-Oder Leveling" (pg 6). One of the largest soruces of error in leveling is unbalanced sight leghts (e.g. foresight is 150 m, while the backsight is 100 m). The current specification is not more than 60 m for First-Order, Class II leveling with the sight imbalances not to exceed 5 m (we try to keep it at 1 m).

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