Many Scaled Benchmarks Moved 100 feet or More

[+Klemmer]

Maybe this was obvious, or covered before, but I didn't realize this until today. Earlier this year, the NGS database (almost all marks, I believe) underwent a major "National Readjustment". You can read the details here at NGS if you would like. This happens 5 or 10 years or so.

 

Adjusted marks are already so accurate, and might move an inch or two, which is irrelevant to us when using our consumer grade GPS receivers.

 

Scaled marks, on the other hand, seem to be rounded to the nearest second AFTER readjustment. Usually no problem, we follow the text "To Reach" description, and another 100 feet off is not a problem. Yeah, maybe.

 

Picture this: I get my data by the county load from NGS. All the latest coordinates. I'm good. My BM hunting buddy gets his from Geocaching.com. Guess what? He's looking on one side of the road, I'm looking on the other. A random example of just Longitude (first one I picked):

EU0262 Latest datasheet on NGS website: W114° 35' 35"

EU0262 Old datasheet on GC website: W114° 35' 34"

EU0262 Latest coords after conversion by GSAK W114° 35.583'

EU0262 Old data from the GC website W114° 35.567'

 

The "Old to New" difference in position is about 100 feet. If both Lat & Long "readjusted" a second each, the slant range difference could be 141 feet. This is (I think) in addition to the inherent 600 feet of inaccuracy from scaling the marks from a map when they were created. The Datum is NAD83 in all cases (although slight different revisions of it).

 

I suppose the same is true for the scaled elevation of most adjusted marks. I'll leave that to the reader. Not very important to finding a mark, anyway.

 

Moral: Better watch out where you get your data from these days. Oh, and better follow those text directions!

 

So - not only is GC missing lots of newer marks that are in the NGS database, but many thousands (hundreds of thousands?) of their scaled benchmark positions no longer agree with the NGS database. One more reason they should figure out a way to update the website. Hey forum moderator (is it still Max?) - you wanna maybe point Jeremy to this thread?

Edited December 5, 2007 by Klemmer & TeddyBearMama

[Wintertime]

Moral: Better watch out where you get your data from these days. Oh, and better follow those text directions!

Nice detective work, Klemmer!

 

I hadn't noticed the change in the NGS coordinates, but I'm so leery of scaled coordinates that I barely pay attention to them anyway, instead using the text directions as you suggested above. For example, some friends and I found HR0701 about 900 feet from its nominal location, and a couple of years ago I found another one in Yosemite that was more than 1200 feet off.

 

Patty

[andylphoto]

Good catch, and good information. I guess the question is then...are the new coordinates better, or are the old coordinates better, or does it vary on a case by case basis, or does it really matter because as Patty said no one here pays attention to them anyway, and finally, could I make this sentence any longer or use any more commas?

 

I guess for me it probably doesn't matter a whole lot. Whether I get the coordinates from GC or Monkeykat's viewer, I'll load them into MacGPS Pro, compare them with a TOPO map and adjust them myself if there is a benchmark in the vicinity.

 

Good information to have in any case though!

[CallawayMT]

 

Picture this: I get my data by the county load from NGS. All the latest coordinates. I'm good. My BM hunting buddy gets his from Geocaching.com. Guess what? He's looking on one side of the road, I'm looking on the other. A random example of just Longitude (first one I picked):

EU0262 Latest datasheet on NGS website: W114° 35' 35"

EU0262 Old datasheet on GC website: W114° 35' 34"

EU0262 Latest coords after conversion by GSAK W114° 35.583'

EU0262 Old data from the GC website W114° 35.567'

 

Moral: Better watch out where you get your data from these days. Oh, and better follow those text directions!

 

 

Larry,

 

I too have not noticed this, but probably never would have. So why do we need to watch where we are getting our coords? Which is more accurate? I think that was good your friend was in one area and you in another, gives you a better group chance to find the mark.

 

And yes, do follow the text directions.

 

CallawayMT

Edited December 5, 2007 by CallawayMT

[+GEO*Trailblazer 1]

This is what I was trying to say about the NAD 27 and other adjustments I have noticed.

 

If the NAD 27 is about 110 feet off and the new is 141 feet off how far did all the readjustments move data?

 

I used the Originals Markers(Azimuths,bearings and distances with other local Monuments) to come up with the NAD 27 from Meades Ranch-Reset.

[DaveD]

Only those points in the NGS database that had been determined with high accuracy GPS participated in the NAD 83 national readjustment. The intention of this effort was to reduce distortions between the conventional passive control network (referred to on this board as bench marks) and the network of continuously operating reference stations (CORS). This adjustment contained approximately 70,000 stations in the NGS database and the average positional shift was only in the range of 2-3 cm. This will have no impact on any geocaching activities.

[+GEO*Trailblazer 1]

The intention of this effort was to reduce distortions between the conventional passive control network (referred to on this board as bench marks) and the network of continuously operating reference stations (CORS).

 

Hi DaveD,good to see ya again at least here.

 

Are the distortions you describe the small inaccuricies in the data itself?

Or the difference in ground observed and GPS derived coordinates.

I wish I knew how to articulate what I am really trying to ask.

 

Maybe when I have been around it as long as you I will come about the right way to ask these techie questions.

 

I understand these things in my mind and can see how the data and field work go hand in hand.

 

I also see that this would really have no impact on us GEOCAC Benchmark hunters,we will find it if it is still there usually,regardless of the coordinates.

 

So reading in between the lines...

Passive Control is the Benchmarks

Active Control is the CORS stations.

Is this correct?

[DaveD]

Passive Control = bench marks

Active Control = CORS

 

The small distortions came from a number of different error sources. The data covers GPS observations performed from around 1989 until 2005. During the early days (before 1995) the constellation of the satellites was not complete and the accuracy of the orbital parameters were no where near as good as they are today. In addition, many of these surveys had been pieced together over a number of years and whenever you do that you will almost always encounter a certain amount of distortion. This was an effort to remove, to the extent possible, all of the various error source. Since most of these errors were small the resulting positional changes are small e.g. 2-3 cm.

 

I'm not sure what you mean by "ground observed and GPS-derived coordinates." The coordinates NGS computes for these points were all derived from GPS measurements and are of course related to the physical monuments on the ground.

[+Black Dog Trackers]

This change in scaled coordinates that Klemmer & TeddyBearMama discovered is rather surprising to me. I was gonna comment on it earlier today (but had no time then), saying that I thought only the horizontal coordinates of GPS stations were going to be re-adjusted in the National Readjustment. DaveD has since come in and confirmed that only the GPS stations were affected, and that that amounts to only about 1 in 10 marks of the whole NGS database.

 

So, I wonder what caused the change in the scaled coordinates? It would seem that it is not from the National Readjustment, but something else instead.

 

Someone, I think it may have been Wintertime, said that instead of using the coordinates of vertical control (with scaled coordinates) stations with a GPS, use those coordinates with the coordinate tickmarks printed on a topo map. In other words 'de-scale' them! This seemed to be good advice, especially when the verbal description was inconclusive or no longer usable. However, if the coordinates of these unadjusted marks have been (presumably automatically) re-adjusted recently for some reason, this advice will unfortunately no longer hold true. Not good!

 

===================

 

Geo*Trailblazer 1 -

 

I would say that both things you mentioned are involved; small inaccuracies, and the difference between coordinates derived from ground survey nets and coordinates obtained from GPS observations. All measurements and observations have small inaccuracies, but the latter difference is more to the point of the National Readjustment.

 

Re-reading between the lines:

• Passive horizontal control are those horizontal control marks whose coordinates were based on GPS observations
  
• Active control is the CORS stations
  
• Relative horizontal control are those horizontal control marks whose coordinates were based on conventional ground survey methods (sighting to other stations) these are not true "Bench Marks" but here, we call them "benchmarks", as well as the Passive control and even Active control types above
  
• Non-horizontal control are those vertical control marks with scaled coordinates that are the true "Bench Marks" that here we also call "benchmarks"
  

My understanding of National Readjustment was that only the first of those 4 types above would be re-adjusted. Klemmer & TeddyBearMama has pointed out that horizontal coordinates of the last of the 4 types above has also been changed. Odd.

 

===================

 

In the above statements, I'm using what one could call the ArtMan convention (since I believe he first explicitly stated it) that here in these forums, and in the whole geocaching site as well, we are calling both horizontal control marks and vertical control marks "benchmarks" (with no space), but only the vertical control marks are the true "bench marks" (with a space).

Edited December 5, 2007 by Black Dog Trackers

[DaveD]

All non-CORS are considered to be passive control points. This includes all marks for which the horizontal and/or vertical was determined by any combination of GPS, triangulation, traverse, trilateration and leveling.

[+GEO*Trailblazer 1]

Thank you DaveD and BDT.

 

I understand it better now and your last one really clicked Dave,Thank you.

[+Klemmer]

Right, thanks DaveD & BDT. Thanks to all for the comments, actually. So scaled marks could have changed. Not a real big deal, since folowing the text is what we should do anyway. But it is interesting to note that depending on where you get the data these days, folks working together might have different data. Kind of blew my mind. That was my main point in the original post. Also don't forget that the NGS Archived Data is (at this time) still all the old data.

 

So if you want a county worth of data, you should really get it from the "county" retreival method under the the Datasheet page, and not the county from under the the Archived section. They are different, and not just in terms of newer marks, destroyed marks, etc. Of course, the same is true for county shapefiles. It would seem at the moment that state-wide shapefiles would only be available in the archived variety, so would be old data.

 

BDT or 2Oldfarts (or whoever): It this issue (NGS vs GC coords) worth addressing in the GC FAQ and / or forum Readme sticky topic? Or too confusing for the newbies? I tend toward the latter, but........

[+Black Dog Trackers]

Kind of blew my mind.

Mine too, I must admit. I can't see why scaled coordinates would/should be moved. Perhaps it's some sort of 'datum thing' - an improvement of the Earth center location of something.

 

or forum Readme sticky topic

This isn't mine and I don't know whose it is and I think it would be best if it did not remain locked.

 

As to the GC FAQ, we did 2 major projects on it for GC.com a few years apart and I've never tried getting any small changes done to it. There is one already that I know needs to be done.

[2oldfarts (the rockhounders)]

After following this thread and reading all the input, I believe it is a non-issue as far as finding scaled marks. We have found scaled marks over 3/10 of a mile off from the given coordinates, so another 100' or so will have little impact on their recovery.

 

Then too there is the possibility that the corrections may have the coordinates closer to the actual marks than before the 'adjustment' was made. There is no data yet to offer an opinion on that aspect of the change. Maybe some folks here will actually go out and compare the new coordinates to the old coordinates and see if it puts them closer to the mark or further away (on average). They could use some of the marks that they have already found for comparison

 

To unlock the "Me First" thread the OP (UNK1) would need to be a moderator (only mods can unlock threads) or Max Cacher would have to deem it necessary to open it back up. In either case I don't see a need for an explanation of this adjustment in a pinned thread as it would probably create more of a confusion than it would clarify in this instance.

 

John

[+YeOleImposter]

This looks to be just a difference in 'rounding' since the accuracy was obviously not enough to have decimal seconds. So if it was, say 114 35 34.4 it would round to 114 35 34. and with the adjustment it became 114 35 34.6 and was printed as 114 35 35. Only a 0.2 second change, but the data sheet shows 1 second of change.

 

And if the scaled marks coordinates 'moved', then maybe they moved 100 feet closer

 

As to which one to use? For scaled marks the coordinates are to help you find a parking spot, not the mark, so it doesn't matter.

Edited December 7, 2007 by YeOleImposter

[lost02]

Ever since we recovered DU1172 GC NGS I've always checked to see if the coordinates have changed. Haven't found many that went from Scaled to Adjusted, but I've also see many of the Scaled changes that Klemmer noted.