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Adjusted Coordinates That Are Not Correct


TillaMurphs
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Just curious. We ran across a recovery where we are certain the Adjusted Coordinates are incorrect.

The mark is RD1646: NGS LINK GC LINK

 

What could lead to the coordinates in the datasheet being incorrect?

 

Thanks,

 

The 86-foot difference could just be a case of two errors adding: your GPS coords could be affected by the terrain and trees, and the datasheet coordinates are only third order, so they could also be off by several meters, too. More likely the GPS coords are the main culprit, because from your photos, it doesn't look like a good location for GPS.

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I commented briefly on this before. If the published coordinates are in some impossible location, then perhaps they ARE in error.

 

However that monument didn't look 'right' to me, as if it could have been disturbed. As I recall there are some other ties that it might be possible to check, otherwise the best check is a good position on it somehow.

 

And as mentioned, your handheld GPS can get off, particularly in marginal environment with any degree of trees and vegetation and perhaps a poor constellation at that moment in time.

 

Hard to tell.

 

- jlw

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I agree with you gentlemen that my GPS error could be a big part of this. But I still wonder…

Here was my thinking (which may be totally wrong):

I searched for this at two different times; 1 month apart. Both times, the adjusted coordinates showing on the GPS took me to basically the same place (within 10 feet or less). I figured that if I ended up at the same place on 2 separate visits then my GPS must be relatively close?*

I figured that in a worst-case scenario then my GPS might have been off by maybe 40 feet? Now, if a third order setting could be off by 10 feet it seems like this would have a worst-case difference of around 50 feet. That is still a ways from being 86 feet off.

 

jwahl – I totally agree that this stone could have been moved (based on the tilt in the top surface). However, it did have nails and surveyors tape within a foot – so – somebody marked it at this location at some time. And – it is at the highest part of the summit in a logical location.

 

Also, thanks for pointing out the third order control. I did some research and there are a couple of other third order horizontal control stations in the county. It is rare but not unheard of.

 

I know I am missing something and I am sure you guys can correct me on my reasoning. Thanks.

 

(* For what it is worth, and I know these are not absolute numbers, but My Garmin nuvi said “18 foot Accuracy” at this location. Historically “18 feet” has been a medium fix – where “12 feet” is the best I ever see, and “30 + feet” is in an area with major obstructions.)

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When I looked at your photos after reading the other thread, my initial thought was the same as jwahl's: that the rock holding the disk appeared that it may have been disturbed and moved.

 

Not conclusive by any means but the monumentation report describes it as being set in an outcrop while your photos show what looks more like an imbedded boulder.

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For what it's worth, here is a NAD83 composite of the USGS Topo map, an aerial photo, and the plotted coordinates of the datasheet and the handheld coordinates. The Topo map was converted from NAD27 to NAD83, the photo was already georeferenced to NAD83, and the stations were plotted in NAD83.

 

It looks like the USGS had some different coordinates for Wilson than those published in the NGS datasheet.

 

RD1646.jpg

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Oooo. Holograph – thanks for that. The topo lines showing the oblong for the summit look very much like the terrain that I saw when I was at the site. And – the center of the triangle symbol is located, relative to the oblong topo line, close to where we found the mark.

 

Very interesting. Unlike the datasheet, the USGS information seems to match what we found.

 

Thank you.

 

I now come back to the original question. As a generic query (rather that referring to this specific site) do datasheets sometimes end up with incorrect Adjusted Coordinates via a common number transposition, conversion mistake or other paperwork error?

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I've seen two mistakes in supposedly accurate horizontal coordinates.

 

MH0702 looked wrong. It turned out to be confused with MH0753 and NGS determined that the description was swapped.

 

ON0828 and AE2132 appear to be the same disk but the coordinates do not match. It appears most likely that MN DOT data somehow got a 1 meter mistake in the Y coordinate of their post-processed GPS data. NGS had not made any correction last time I checked.

Edited by Bill93
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Here are some of the things I am curious about:

 

RD1645 RTE WILSON R STA 1843+19.6 381.146 METERS 24836

 

...ON SLOPE BETWEEN WILSON RIVER HIGHWAY AND CONSOLIDATED

'TIMBER COMPANY LOGGING RAILROAD..

.. WALK E ALONG TRACK ABOUT 0.5 MILE TO NE END OF LONG TRESTLE, THEN GO NORTHWARD ABOUT 600 FEET TO STATION.

 

Also the topo shown by holograph isn't matching the road unless the road is half in the shadow. Shifting it to match the road looks to me like it puts it about on the HH2 coordinate plotted.

 

- jlw

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Here are some of the things I am curious about:

 

RD1645 RTE WILSON R STA 1843+19.6 381.146 METERS 24836

 

...ON SLOPE BETWEEN WILSON RIVER HIGHWAY AND CONSOLIDATED

'TIMBER COMPANY LOGGING RAILROAD..

.. WALK E ALONG TRACK ABOUT 0.5 MILE TO NE END OF LONG TRESTLE, THEN GO NORTHWARD ABOUT 600 FEET TO STATION.

 

Also the topo shown by holograph isn't matching the road unless the road is half in the shadow. Shifting it to match the road looks to me like it puts it about on the HH2 coordinate plotted.

 

- jlw

 

My experience has been that topos have varying degrees of accuracy for different features. Also, nothing says that the photo's coordinates are the ground truth. The photo orthorectification process distorts pixels based on elevation, so it is only as good as the elevation model.

 

Here is a larger clip. Notice how well the topo and the photo are aligned in the upper left corner. For instance, the field boundary matches the boundary line on the topo, and the road through the field is exactly aligned with the topo. It's harder to see, but the line of the creek seems to align well, as well as the fork in the plantation road at the upper right. In fact, the only feature that seems to be misaliged is the road through the middle of the figure.

 

RD1646b.jpg

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I found one (intersection) station where I believe the adjusted coordinates are in error. This also is a third-order station.

 

http://www.geocaching.com/mark/details.aspx?PID=RL1588

 

Being that it is an intersection station and unlikely ever to be used, I posted a log with my findings and left it at that--I didn't think it was worth putting a whole lot of time into research to prove that this was the tower described. To my knowledge, there was only one company in the 70s providing microwave services (for TV) in the Upper Peninsula, and this tower certainly dates from that era or before, and is (or at least was) owned by that company.

 

Obviously I can't say with certainty that there wasn't another tower, also with three microwave reflectors, there 35 years ago. It doesn't seem likely though.

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Here are some of the things I am curious about:

 

RD1645 RTE WILSON R STA 1843+19.6 381.146 METERS 24836

 

...ON SLOPE BETWEEN WILSON RIVER HIGHWAY AND CONSOLIDATED

'TIMBER COMPANY LOGGING RAILROAD..

.. WALK E ALONG TRACK ABOUT 0.5 MILE TO NE END OF LONG TRESTLE, THEN GO NORTHWARD ABOUT 600 FEET TO STATION.

 

- jlw

 

Hi Jerry,

 

I really wish those two objects (RD1645 and the trestle) could be helpful. However, they are nothing that we have the ability to make use of. The terrain is rough, there is a large elevation change and there are many trees and dense brush. There is no way we could measure from them with the tools that we have.

In addition, I believe that neither are useful existing physical points. RD1645 is an “unmarked” highway station. The logging railroad is long gone – removed more than a half a century ago. There is zero indication of where the Consolidated Timber Company line used to cross the highway. The 1945 burn took out many of the wooden trestles on this line. Even if some portion of the trestle were to remain, the “about 600 feet… northward” mentioned in the description would probably have too many variables to be useful in determining if the disk is where it is supposed to be.

On both of our visits to the location we planned to look for the trestle area. However, on both occasions we ran out of time for a through search. We do plan to go back to look for trestle remains - more for our interest in the old railroads than for help with the station mark.

 

Thanks for your interest.

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I am a latecomer but I also agree with those who feel the disk has been disturbed. I have seen disks set at angles but not that great an angle. Further, you kept saying that the disk was in a "rock" and it is listed as being in an "outcrop". In my mind that is a big difference. Was your "rock" actually part of an outcrop or could you shift it at all?

 

In your picture titled "Rocks" the right side of the rock the disk is in looks damaged. This is noticeable in the next picture titled "Rock" on the bottom of the rock. The existence of surveyor's tape only means that a surveyor found it and possibly used it. I wouldn't say that the majority of surveyors are as careful as we are about what they find. In fact, I would bet that FEW surveyors are as fanatical about survey marks as we are! To them it is part of a vocation; to us it is an avocation. (Or perhaps obsession would be a better word in some cases!)

 

In a nearby county I once found the top portion of a county mark (adjusted), turned upside down and imbedded in the ground, painted orange by a survey crew who didn't take note that what they saw was the bottom of the stem of the disk, or that the concrete was rough and damaged. They had probably used it as is, where is. I hope the accuracy they needed was not so great that they ended up with a major problem!

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Not to get too off topic, but you said "Nuvi". What model Nuvi? I think this could be a big part of the problem also. Not to "hate on" Nuvis, and some of them are very accurate in "pedestrian mode", but if you don't have it on pedestrian mode it will try to lock on that nearby road. And some older Nuvis are about as accurate as an iPhone 3 (i.e. not very).

Edited by HotRod205
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I am a latecomer but I also agree with those who feel the disk has been disturbed. I have seen disks set at angles but not that great an angle. Further, you kept saying that the disk was in a "rock" and it is listed as being in an "outcrop". In my mind that is a big difference. Was your "rock" actually part of an outcrop or could you shift it at all?

In your picture titled "Rocks" the right side of the rock the disk is in looks damaged. This is noticeable in the next picture titled "Rock" on the bottom of the rock. The existence of surveyor's tape only means that a surveyor found it and possibly used it.

mloser (and jwahl from an earlier post), very interesting ideas! Checking to see if the rock was loose or attached would have been a good idea. If it was extremely loose I would have noticed, but, unfortunately I dont remember trying to shift it.

 

I am very intrigued by your thought that it might have been moved and the damage on the rock resulted from the movement.

 

I think I will make a trip back to evaluate the possibilities that you mentioned.

 

However, even if it turns out that it might have been bumped or moved, I still dont see how it could have originally been at the adjusted coordinates those coordinates put it on the wrong side of a knoll/ridge where it would not have been possible to site to reference object RD1645 RTE WILSON R STA 1843+19.6. They are also on the side of a very steep hill.

 

Not to get too off topic, but you said "Nuvi". What model Nuvi? I think this could be a big part of the problem also. Not to "hate on" Nuvis, and some of them are very accurate in "pedestrian mode", but if you don't have it on pedestrian mode it will try to lock on that nearby road. And some older Nuvis are about as accurate as an iPhone 3 (i.e. not very).

HotRod205, I appreciate your feedback and question. We have been very happy with our Nuvi 760. We have used it to search for close to 400 marks and stations so far (not all of them logged) and have never had an occasion to doubt its accuracy (as far as HH2 units go). When we get close to a mark we put it on the satellite page where it displays the real-time coordinates (so that Road mode or Pedestrian mode are not applicable).

I am sure it is not quite as accurate as a Garmin 60 series or a Delorme PN unit (especially under tree cover) but we have checked it against many stations with adjusted coordinates and it has always been quite acceptable.

 

In two trips to Wilson, our Nuvi took us to exactly the same spot for the adjusted coordinates.

 

Thanks for your idea; we are always looking for suggestions to figure this out.

Edited by TillaMurphs
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Interesting development in the case discussed above. NGS deleted AE2132 (GC link) after the corrections were made to the older data sheet for the same disk, ON0828.

 

Now I see they have assigned AE2132 to a disk 300 miles away from the former location. I didn't think they would reassign a PID. Somehow that data sheet has history for its current location going back further than the aforementioned correction, which is puzzling.

 

The embarassing thing is that my recovery notes on ON0828 still say that it and AE2132 are the same disk. No longer true. Moral: be very careful what you put in a recovery report, especially about discrepancies, as things may change.

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