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First Day Of Hunting.


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Went out with the Fiancee today for the first time hunting benchmarks. I've done geocaches before but just noticed this aspect of things. The Fiancee found it much more interesting than geocaches, so we might be doing it more.


Anyhow, this was a disappointing area. Living in one of the fastest growing counties in America leads to a lot of marks done in by progress, I'm afraid. I only found two marks today, one a NGS mark that had been found before, and one a "temporary" mark (or at least, I think I did). All the rest were busts.


I'm particularly interested in people reviewing JV4487. It's the only one I reported as destroyed, as I'm fairly sure that it is. It's parking lot now for a good distance in both directions. It's the only one I'm considering reporting to the NGS, which is why I'd like it checked before I do that to make sure I have enough information.


Anyway, if anyone wants to do a find by user on me and check out today's logs and make comments that would be welcome. Might as well make sure I'm doing the right thing now.

Edited by bicknell
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Welcome to benchmarking.


Just a suggestion before you submit to the NGS. We looked at your picture and couldn't tell if there was any access plates nearby that may have covered the disk. The description says a disk was going to be set in the same spot. Where we live there are several in the middle of the street that are accessed by 8" or 10" steel plates.


If there is no cover then the disk is probably gone. We're not sure how much proof the NGS requires for a destroyed classification, though.


Have fun benchmarking,


John & Shirley

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I'm not sure how far in a circle to make a search for an access plate. We wondered around for a few minutes in the parking lot (initially the GPS wanted to have 150' accuracy so we wandered around a bit trying to get it to settle down). We didn't see anything for a circle of 30-40' or so, other than the storm drain in the picture. I did look down in it, but didn't see anything other than a drain.


More to the point, even if the benchmark disk is visable, is this still a usable spot since digging up the underground disk isn't going to happen without digging up a parking lot, hardly practical for a standard survey.

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I almost forgot, but I want to bring it up here to see how to report this error as well.


In the description for JV4487 and JV4490 they reference distances from "the intersection of route 7 and route 28" saying the marks are 0.1mile to the west.


Well, the intersection of routes 7 and 28 is some 4-5 miles to the west. It's been there for at least 20 years. I suppose 28 may have been here at one time, although I have no knowledge of that fact.


More likely to me is that Drainsville Road and Route 7 (I think clearly the intersection described) is route 228. It's likely 228 was misentered as 28, as the two are only a few miles apart. Maybe there is a chance 228 used to be 28, I have no way to look that up.


In any event, anyone starting at 7 & 28 today is going to be miles off course and confused. I know it can't be the current 7 & 28 because one marker is the loudoun fairfax border, which is by drainsville road.


So, I think these are also descriptive errors that should be reported somehow.

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If there is a cover plate and/or the disk is visible it can still be used.


The FAIRFAX-LOUDOUN BOUNDARY COMMISSION was to set both an underground mark and a surface mark, so if you check with them, they can tell you if the work was actually performed. If they did not set the marks (get a copy of the report for the NGS.). The tempory setting is destroyed and would probably qualify for inclusion in the NGS records.


If they did set the marks you will need to get clarification from the NGS for its status.


Hope this helps,





PS: You did a good job photographing the 1953 reset.

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Did you check near the intersectionof 7 & 28?


If your coordinates for JV4490 are correct then you may be able to locate the mark with a metal detector, since it is a sparkplug in a block of concrete. A metal rod will be handy for probing the spot where the detector sounds off.


Even though the description says the coordinates are adjusted, it may be worth a quick check to go by the description and measure from the intersection.




edit for speeling...

Edited by 2oldfarts (the rockhounders)
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First of all, Bicknell, as a fellow Northern Virginia benchmarker, let me welcome you to the club. I guarantee you that it will be in turns enjoyable, frustrating, instructive and possibly habit-forming.


Loudoun County is indeed one of the fastest-growing counties in America, and its benchmarks tend to disappear faster than you can say "strip mall."


From your post and the 1954 benchmark description, I'd wager this one is almost certainly long gone. It appears to have been intended as a termporary marker. It's unclear if a permanent benchmark disk and associated underground marker were ever placed, but if so it's not in the NGS database.


Incidentally, this mark is erroneously listed as being in Fairfax City.


As it marks the Fairfax-Loudoun County boundary, it should be listed in one of those jurisdictions. I'm not sure what the rules when a mark straddles jurisdictional lines. For example, TRI STATES 1942 (LY2604), which is located where NY, NY and PA meet, is listed in Orange Co., NY. I guess it has to go somewhere. (This station, near Port Jervis, NY, has got to be one of the most-logged benchmarks, with 37 reports!)


I would encourage you not to report your results to NGS for a while, until you get comfortable with the process, nomenclature, etc. You can always go back and report later, if you are so inclined. Many of us do report to NGS, especially when we find a mark previously reported as 'not found,' but because Reference Marks and Azimuth Marks are easily confused with the main mark of a station, my suggestion would be not to worry about this aspect of benchmarking for now.


Anyway, welcome to the club, and happy new year!



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I may try to find info on the "boundary comission", however, for a bit of timeline....


The mark was supposed to be set in 1954. There was pretty much no development in the area until around 1970 or so. I don't have an exact date for this strip mall, but if it was before 1980 I would be extremely surprised. I suspect 1985-1990 as the most likely timeframe it was built.


So, my thinking is, even if the surface mark was set, it was likely dug up and paved over when the strip mall was built 30 years after the fact. I may be wrong, but I can't imagine a builder making a strip mall go to great lengths to keep a survey marker that was 30 years old and build in a trap door for it.


I've been wrong before, which is why I'm asking now. I'm new to this, and still not quite sure how all this works.

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In Baltimore the other day I found a mark where a new sidewalk was poured around it, rather than on top. I have rarely seen that sort of effort taken to preserve benchmarks that are in the way of a construction project.


While I think that, if the boundary disk were set as planned, there is a small possiblity that it has survived, in my opinion, the original mark (nail) is almost certainly gone. If you do find a disk - again, this is my interpretation of the rules we go by here - it's all well and good, but it is not the benchmark described and set in 1954. In other words, a fantastic find, worthy of photos and much bragging, but IMO you would properly log it as a NOTE, not as a find.



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Personally, I rather hope that the NGS does not accept as gospel the report of sporting benchmarkers. It would be better served that NGS could use the submitted information to send out a "monument duster" to verify the report. Some of the descriptions given in those "how to reach" sheets are quite old, and the older points don't have accurate Lat/Long values. Those that do may be in NAD 27, and there is no direct conversion from NAD 27 to NAD 83. There is only an approximation approach to conversion. While the reports of sport benchmarkers are a great tool... It would be a disservice to local professionals that may need those points, if they were reported gone, when in fact they were still there.



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Almost all recovery of NGS station recovery is done by Volunteers these days. The amount of survey in the field is massive but the NGS is not funded by the Department of Commerce as such so as to be able to pay a force who recovers these and has not for years. In fact NGS has very few Paid Survey Crews these days either. In much of the work, Most States employ Surveyors with equipment of high enough quality to maintain the geodetic needs of the states and a lot of this is brought to NGS Standards for obvious reasons and reported by liaison through State/NGS Geodetic advisors. Today most states and may counties and cities are involved. Todays equipment enables this work to be done to extremely high accuracy without the use of large crews. It has become a GPS World.


Whomever chooses to report what they find to NGS becomes a part of the all volunteer force. Even I as a surveyor on the clock can submit my findings to the mark recovery page on the NGS website but even on the clock, neither I nor any employer will be remunerated for my efforts at any level though I would technically earn my wage at work during business hours. But the recovery would be something my employer would be paying me to do, which is something they would never recover the costs of. No Matter, I do it on my own time for free. I like the challenges, some of these are tougher than a chess game in that I can lose track of time doing it, and I like it. I imagine if it were easy, I probably wouldn't enjoy it as much.


No station ever monumented of any kind is updated in the NGS prior to being looked at by NGS Personnel. So to a degree, it is looked over, but they are relying on you to do your best. They are hoping that we will do so with integrity. They do not allow the submission of destroyed marks through the mark recovery website.


The only kind of station that NGS will allow to be destroyed without empirical evidence of destruction is a Landmark type station. This can only be done via email and the criteria is strict. Stations which were Discs, or otherwise monumented as points that could be occupied with an instrument set up, cannot be destroyed in the database unless you can furnish the destroyed object.


Basically put, NGS likes that you are interested and wants to help you become your own "Mark Duster" They are very happy about the contributions from Geocaching.


There is in fact a very exact conversion to calculate the datum shift between the NAD 27 and NAD 83 Horizontal Datum. It is a program called NADCOM and it is available on the NGS website both as a Javascript online and as a download to run on PC. The NGVD 29 and NAVD 88 Vertical Datum can also be converted as well , both similarly, with a program called Vertcon. NGS has a large number of programs that will convert a number of data type for the interest of geodesy.


Two points of interest, I can hunt them in either Datum and they are accurate to better than less than 6 inches, most usually to way way better than 6 inches accuracy. In either datum with or without conversion. All Active Horizontal control in the database has been converted to NAD 83 so it is current data and this is stated as such as such in the datasheet, the superseded control is there as well.


You can play with these programs by going to http://www.ngs.noaa.gov/ then look at the Geodetic toolkit and the software downloads.


As to disservice to professionals? well I see this mistake happen but it is a human system, so it happens. I don't choose to see it as a disservice as Surveyors sometimes don't find them too, then we find them later, or next time. If you like, you can report only ones you find, and not the ones you don't. At least you are helping freshen the info on ones that are known to you, and not making a mistake on any that you just are not sure about. By just reporting the find only, everyone helps out a lot. I am a surveyor, I do use these and am happy to see the updates.


It is cool if you try to use the old description to see if it still works. If Not, A GPS Waypoint made from the Coordinates will take you straight to horizontal control, if the description is no longer working, you may choose to provide to write a new one based on the way the object can be found today, if you are recovering to NGS. If you are just playing the Game then a simple found it will suffice. Many of the recovery narratives on Datasheets are a great guide to use in how to best write a description if you want.


No one is under any obligation to report what they find, or don't find to NGS, unless they choose to do so. The Main thing here is to have fun with the challenges, the hunt, the history, the sightseeing, and the fellowship in the forum.



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Here are a few thoughts on JV4487 DESIGNATION - LOUDOUN FAIRFAX BDY HERNDON.


Ok, this was a Nail driven in the top of a 1x2 stake in 1954. This stake was likely rotted off by 1959. Stakes are untreated wood so, it was worm food. I have revisited a lot of wooden survey in my day and well, the weather in a few years is not at all kind to it. Even if this were still a field, there is no chance of it existing 10 years later at best.


The FAIRFAX-LOUDOUN BOUNDARY COMMISSION was to have created a station here but if they did, they never reported it to NGS as a radial search of the NGS Database for this location shows 4 stations within a half mile of here and it is not listed. So this boundary commision is not required for the recovery of this station. it was not updated further than this to the NGS, ever.


As to the asphalted parking lot, if this station was a wooden stake that would now be 50 years old, and it is gone. To build such a parking lot, all the topsoil would be stripped from this location and crushed rock hauled in to create a stable compacted sub grade for paving prior to laying the asphalt.


This station will likely qualify as a destroyed for geocaching, and a Not Found for NGS Recovery. Again, you are not required to file what you find , or don't find, with the NGS, Unless you choose to.


One other quick tip, which you may already know. The best satellite constellations are during bankers hours, so for a lock with highest accuracy, with weather not a factor, between 8:30 AM and 4:30 PM will bring you the best look at the GPS Birds. If too many are directly above you, or too low on the horizon things can become problematic. Above offers a weak spread, and the Horizon can offer an attenuated signal. I am not saying that other times of day are bad, just that this time of day is optimum.





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Sorry to disappoint you, but there is, in fact, no accurate of conversion of datums between NAD27 and NAD 83. This true especially from NGVD29 to NAVD88 vertical datums. Here is a quote directly from the NADCON readme file.


NADCON conversions between datums are approximate values based on models of real data. NADCON should be used only when data does not exist in the data base (NGSIDB) for one of the datums required. The accuracy of the transformations should be viewed with some caution. At the 67 percent confidence level, this method introduces approximately 0.15 meter uncertainty within the conterminous United States, 0.50 meter uncertainty within Alaska, 0.20 meter uncertainty within

Hawaii, and 0.05 meter uncertainty within Puerto Rico and the Virgin Islands. In areas of sparse geodetic data coverage NADCON may yield less accurate results, but seldom in excess of 1.0 meter. Transformations between NAD 83 and States/Regions with High Accuracy Reference Networks (HARNS) introduce approximately 0.05 meter uncertainty. Transformations between old datums (NAD 27, Old Hawaiian, Puerto Rico etc.) and HARN could combine uncertainties (e.g. NAD 27 to HARN equals 0.15 meter + 0.05 meter = 0.20 meter). In near offshore regions, results will be less accurate but seldom in excess of 5.0 meters. Farther offshore NAD 27 wasn't defined. Therefore, the NADCON computed transformations are extrapolations and no accuracy can be stated.


Oh, and remember, these accuracies are at the one sigma level ( 67% of the time) That means that the other 1/3rd of the time they could be off hundreds of feet (not likely but still statistically possible)

This may be accurate to you as a geocache hobbyist, but as a surveyor, you should be alarmed if you use this conversion in your work.


The same holds true, even moreso, with VERTCON. It's accuracies really stink in some places. I am in Florida, and I have personally seen this "conversion" used by surveyors, and checked their results. Not good enough for surveying. Now in areas where there are a lot of common datum points (ie having both NGVD, and NAVD dataums, the conversion is closer than out in the boonies where the common points are sparse. But even then, close ain't good enough for a surveyor. Actually, its scares me to learn of the number of surveyors that use VERTCON to transorm their vertical benchmarks to get a job done. I wouldn't do it, and what's worse is that they sign and seal this work. Not me, not gonna happen.


Since you are a surveyor, let me give you some more technical fodder. The reason that the transformations cannot be accurate is that the source references are not related, linearly, or otherwise. The horizontal datums generate from two totally related ellipsoids. NAD 27 is based on the Clarke Ellipsoid of 1866, with its initial point located at Meads Ranch, Kansas. NAD 83 is Earth centered and based on the Geodetic Reference System 1980 (GRS 80). Factor in the inherent survey "distortions" (an NGS derived term...heheheh, I read FUBARS) The two just can't be accurately related. Again, from the standpoint of geocahing, using small hand held code receivers, its close enough to use. For surveying....no cigar (where is Monica anyway) Now, even with high end dual frequency, dual P-code survey grade receivers, you can "localise" into a NAD 27 project and get very good results... but that is a topic for a whole new discussion, and has little to do with transformations.


Vertically, its even worse, NGVD assumes that "Mean Sea Level" all around the country is 0.00' (we learned that the Atlantic and Pacific Oceans are not at the same elevation when we cut the Panama Canal, hence the lock system [we may have known sooner.. but that is the popular tale]). Well when the huge least squares adjustment for NGVD occurred (by hand, mind you) it was constrained by having to hold 0.00 at all the tide station control points. That coupled with the survey "distortions" made it supposedly inaccuate. I suppose on a national scale it is, but in a local area, its only as inaccurate as the original leveling. (the adjustment errors are distributed accross the country). Now when NAVD was created, get this, it holds one (1) point as fixed, at Fathers Sound somewhere up in the Cold Country North of here, and adjusts a lot of the same runs, but they are minimally constrained (only one control point). So again, there is no direct corrolation between the systems. If NGVD had used only one fixed point, one could just apply the difference between the two systems and presto, an accurate transformation. ( a linear relationship)


Since we can see how the systems differ in nature, we can understand why they cannot be accurately related. However, because of pressure from surveyors and engineers, geologists, et al, the NGS came up with its best solution to translation between the datums. NADCON, and VERTCON. They basically find the closest common points to the desired shift position and "best fit" the solution between them. Obviously, the closer to a known dual point, the more accurate the shift value. Not a true least squares solution, but a good idea. Not including the surveying "distortions". ( heheheh I love that word)


As for NGS, well they have taken quite a beating over the years, going from over 6000 employees to around 600. They did at one time have "monument dusters" that drove around verifying marks everyday, but that died by the wayside as their personnel dwindled down. I know that they have used private sources to dust monuments for years and I was not trying to bash sport benchmarking, but I look at the posts here and see folks remark on how the Power Squadron missed locating a mark, and find pleasure in scoring a find over them ( I would and do too). My concern lies in reporting a loss. How many professionals read the find list and look at the list of recovery dates and see a reported destroyed or loss and don't look themselves? Most of them. Here is the rub, now they have to travel farther away to get a known point which equates to money. Increased fees to the clients for additional work. Should they scout ahead of time to know their control, yes in perfect world, but then again, if they pull their control from the net, and its marked gone.. they go looking for another... if that mark is still there, its a loss. Not trusting those notes myself, unless they were from a professional duster ( a DOT, County, or reputable surveyor) I have always gone to look myself. I have found some that the Squadron said were gone, and saved the taxpayer a whole buncha money. Your statement to only report the finds makes great sense to me... a note saying the monument is there helps me more than a note from that same someone that says its not. But that is just my two cents.


Thanks for you time, hope I offended no one.





I didn't reread this, so if I have a spelling error, or a lost train of thought, I apologize.

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NAD83/27 Question.


Coming from Geocaching I'm used to everything being done in the default WGS84 datum that's the default on the receivers. Looking over yesterday's paperwork and reading this thread I noticed all the benchmarks I've looked for so far are in NAD83, and it appears a few are in NAD27. I only have a high level overview of why they are different. Long story short, I'm guessing I want to set my GPS to the same datum that the coordinates were recorded in, at least I figure that should be more accurate than converting them to WGS84.


Is that the right way to do this? We're going to look for a few now, and thankfully with multiple GPS's we're going to try one in WGS84 and one in NAD87 and see what the differences look like. My guess is the unit must be doing some internal conversion, and I just hope it's accurate.

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Doc, the reasons you give support the caution the NGS exercises in declaring a mark destroyed. I also dislike "Not Found" reports that don't explain the circumstances. All you can conclude from those is that it wasn't visible from the car.


But a good "Not Found" report may have considerable useful information that the professional doesn't have time to dig out. I try to contribute what I can in a "Not Found" (see one of my better ones at NJ0544) and would encourage everyone to do so. It might give approximate years when highway or regrading work was done (NJ0753), notes on building changes, highway renumberings, railroad changes (MG0612). If the user reads the "not found" report he may make a much better judgment as to whether it is worth his time to search for himself, and have a headstart if he does decide to do so.

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Your unit will probably work internally in WGS84, and use relatively simple conversion algorithms to go back and forth to enter or display data for other datums. Those are probably as good as the typical 6 ft or greater accuracy of a handheld unit's measurements. The difference between WGS84 and NAD83 in my area are less than the precision of my unit. NAD27 shows differences of tens of feet around here.


The on-line conversions may be more accurate than that, if they take into account more subtle factors. If so, the "right way" is to use the best conversion possible to get WGS84 coordinates before you go to the field, and when you come back use that conversion to get NAD83 to report your results in. The improvement may not be much over the easy way, which is to simply make sure your unit is set to the datum that matches the information you are entering or reporting, and let it do the conversions.

Edited by Bill93
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To all,


Remember NAD 83 is based on the GRS80 Ellipsoid... When you are talking about NAD 83 and WGS 84 you are comparing apples and oranges.


Both GRS80 and WGS84 are geocentric ellipsoids. (center of the earth [albeit a mathematical, and unmeasureable point])


The GRS 80 (Geodetic Reference System) ellipsoid is very similar to WGS84 (World Geodetic System) ellipsoid. So similar in fact, they are virtually the same. You certainly will not see a difference on your hand held "code" based receivers.


Originally, WGS84 was supposed to be based on GRS80 and therefore identical... but slight differences were found, so they are slightly different


Some have argued that since NAD 83 is North American and WGS 84 is Worldwide... that is the difference. Actually, if the GRS 80 ellipsoid was extended worldwide, there is still very little difference. Again, nothing you will see in hand held units.


Set your unit to WGS84, and forget about it, unless you are just interested in reading about the differences and learning a thing or two.


The bigger differences we see are not in the GRS80 and WGS84 ellipsoids, but rather, in the relentless readjustments that NGS is hellbent on making. 83/83 then 83/86 then 83/90 then 83/99 (around here anyways) Some areas have different adjustment years. Its a staggering pain in the a** to surveying. Around here, we get about 0.2' difference in 83/90 and 83/99. Not real good for surveying.accuracies. Makes the publication of local coordinates rather difficult. Even better, the HARNs don't fit each other, that is why they were/are trying to readjust those.


Rather than detail mathematical philosophies, let it be said there are two sides of the fence on control, some at NGS are on one, some on the other. I am on one of those sides.



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Bill93 wrote:


.....a good "Not Found" report may have considerable useful information that the professional doesn't have time to dig out. I try to contribute what I can in a "Not Found" (see one of my better ones at NJ0544) and would encourage everyone to do so. It might give approximate years when highway or regrading work was done (NJ0753), notes on building changes, highway renumberings, railroad changes (MG0612).


I'll echo the remarks of Bill regarding documentation. Sometimes, I take it a bit too far, like on EZ-0975, where I followed the chain of title back to the individuals who sold the land to the bank. The bonus was that I discovered an old survey showing the long-gone house, referenced trees, and even the mark, itself!


When I say this one is not found, you can take it to the bank. (Opps, that's a pun, isn't it?) <_<



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bicknell -


Welcome to the benchmark hunting community.


You stated that your fiancee found benchmark hunting to be interesting. Good God, man, marry her immediately!! She is, obviously, a woman of high character and surpassing intelligence.


Like ArtMan (above), I am located in northern Virginia (Fairfax Co.) as is another veteran hunter, Black Dog Trackers. The three of us have picked over Fairfax and Arlington Counties and DC pretty well. Hope you have much success in Loudon County. You have already figured out, however, that benchmark hunting is particularly challenging in an area of great recent development. The crews that widen the roads apparently have no respect whatsoever for benchmarks. Many casualties throught the region. I have had to range pretty far afield (NJ, DE and MD) to find significant concentrations of surviving benchmarks.


I agree with Artman - get a few months of hunting under your belt before reporting any NOT FOUNDs to NGS. You'll be surprised at how "smart" you get by reading several hundred datasheets, benchmark hunter comments and forum posts. Hint - always review the original datasheet before you start out to hunt a mark; post-2000 updates can make all the difference.


Once again, welcome aboard & good hunting.



Edited by seventhings
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Maybe we could meet up with you guys one day for some pointers? We've found at least one that really looks like it should still be there but we couldn't find it so perhaps a couple of new and a revisit or two would be educational.


What's your success rate with the waypoints being right. That is, if you just walked to the marks in the area with your GPS in hand would you more or less be standing on it? I have yet to find one where I thought the description was off from where the GPS point was placed, but with a huge lack of finds who knows. The one disk I did find (which was already found) was about 20-25 feet off from the GPS coords.

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Hi Bicknell


I can speak from experiance on your query about the accuracy of gps vesus the description "if you just walked to the marks in the area with your GPS in hand would you more or less be standing on it? I have yet to find one where I thought the description was off from where the GPS point was placed".


Check this one out:




Note the gps showing a discepancy of 280 feet. I have read posts on these forums of finds that were off by miles from the coordinates but were found using the description.

I always carry a copy of the NGS datasheet with me when I hunt these marks. Sometimes I get lucky and find them visually without either the gps or the datasheet. Those are rare. Most times I use the datasheet and then a metal detector.



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You stated that your fiancee found benchmark hunting to be interesting.  Good God, man, marry her immediately!! She is, obviously, a woman of high character and surpassing intelligence.

I must agree - I am a woman of high character and surpassing intelligence. <_< We'll probably benchmark while on the honeymoon! :D



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I am in understanding with what you say, in fact you have said many things I have said here many times but not in complete agreement. We see the world a little differently but mostly the same, I take no offense at all.


NAD 83 compensates for a lot of Tectonic plate movement we see here out west, and is specialized as a Datum for the North American Continent and so it is much more accurate than WGS 84 in this country.


I am aware we tossed NGVD 29 out in 1991 and now go with NAVD 88. Not just the former reference of 26 tidal stations as opposed to a single one in Quebec, but they were right to confirm that mean sea level as a stable actuality does not exist. It was basically the Surveyors Loch Nest Monster, an elusive creature. Since we have an unstable apple and an unstable orange, yet they need to be compared by virtue of both being fruit that grew on trees as a commonality, what can you do? This is what Vertcon tries to attempt. I mean if an orthometric height comparison is needed between datum you are bringing Geoid height and ellipsoidal height to the table, Orthometric height differences can then be obtained from ellipsoid height differences by subtracting the geoid height differences. As an orthometric height is a derivative of a geoid height, (which model?) and an ellipsoidal height, (again Which model?)(NAVD 88) We have to define our terms and remember that Clarke 1866 was not used Vertically,(NGVD 29) and sea level is ah.... Tidally dependent on 26 averages that were ahhh not then known to be geoidally (gravitationally) dependant.(NGVD 29) When even differential leveling is path dependent, taken as a whole, it is a tough apple and orange to compare, yet all considered orthometric heights all the same.


I am also aware that when NAD 83 was adjusted from NAD 27 all stations were not changed with a Nadcon or Vertcon adjustment. You are correct in stating 67% confidence but the ballpark is taken as .15 meter in the continental US and that is 6 inches roughly. With it we should be able to use the NADCON program to Find the survey marker and that is good. If we are a Surveyor, we then know what we need to do. If we actually have to use NADCON or VERTCON, then the survey is considered low accuracy, the onus is on us to state this and the buyer is beware the accuracy. Accuracy is plainly stated by NGS. I am not saying I don't find errors either because I do. But it is not with A, B. and first order work as much as with third order. There was a least squares adjustment based on new leveling and triangulation of the networks during the 80's. I take the NGS term ADJUSTED to mean a least squares adjustment rounded to the fifth decimal. Not bad really, and close enough for most infrastructure. Yes there are inaccuracies, but I am glad I am not doing this with a Wild T-3 and paper with pencil, aren't you? Besides, the entire NGS database is not the NSRS. I have found GPS comparisons to be very good with the NSRS data, and Optically triangulated third order stuff is not taken as NSRS quality. I don't take the Geoid in either 99 or 03 to be a static force so I realize despite the spheroid models being used, the earth constantly moves, the atmosphere and it's distortions never end, the constellation of satellites is up and down, on and off and there is no less than several government agencies in control of each little part of the puzzle. Either way, when you get to the highest accuracy, there still is no one sure place. If you could attain it you would never arrive at the same number twice. Either way, that one sure place is a difference of approximately 1 meter due to the datum shift between NAD 83 and WGS 84. Besides, It is all in constant motion.


This big earthquake a week ago has not done us any favors when it comes to trusting numbers. I would say that many bets are off on Geoid 03 now. It seems hard to trust a Gravity Model after a 98 foot tectonic shift. Beyond that, Both NGA and NGS have done so much accurizing of both their Datum as it is that they really only continue to call them by those names as a political courtesy. Neither one of these are close to the datum they were when so named. We are aware that political needs come before the scientific needs. Always has. Prepare for another update soon.


As for HARN problems from State to State, and the fit problems, well I see that as a bit of a political situation as well. I have heard that not all states are playing the HARN game the same way and until they do, I don't see how we can really ever prove a thing. Again a Political problem is likely the culprit more so than the science. More so, no one will ever officially comment as such and we both know that. If it is the science, we would have to see after we cure the political problem. Science has been found to not best describe reality before so I would not be surprised. In these parts we still have counties that insist all vertical control be done to NGVD 29 Datum. But cross a city limits the vertical control becomes NAVD 88 Have that County road hit a State Highway and you immediately have a Datum Shift. Yup, It's a pain. All Political. You can take my old datum if you can pry my cold dead fingers from it. Imagine the fun I have laying out a gravity pipe job complete with datum shifts. Bleh. Each municipality will extract it's due. Most interestingly this horror tale often comes true brought by a inspector who has never heard the word Geodesy.


The Hellbentness that you say about NGS is true in the respect that they are finding better ways to accurize their Datum as the science and technology becomes available, besides, I am not sure we know all there is to know about geodesy yet. From what I read, there is a lot still being revealed. Don't get me wrong, I am not trying to big deal it but look how far we have come in the last 40 years, 20 years, and 10 years. Maybe the 6000 became 600 but much of the heavy lifting that once was necessary is done, and done by old ways. We have easier ways now. I was a member on a crew of 3-4 back in the Day and most of the time I am now a force of one. I think NGS probably let go of 2000 of the bunch of 6000 when computers took over the calculus. That was a lot of pencil and paper...


Since WGS 84 is used worldwide by other countries and governments, those countries (Europe, Asia) who use it become a local accurizer of that datum, and are equally hell bent, So much so that they are developing their own GPS systems and Datum, so you see it really isn't different, other that the truth of the political matter. WGS 84 is really about DOD and that means Bombs and missiles, and here in this country there is not a lot on the ground to accurize it. NAD 83 is about the north american continent. When it comes to what is going on in the dirt on this continent, I feel the NAD 83 is far more accurate. Either way, I have to tie in to the right numbers. The difference truly is that Plate movement is different in North America, and WGS 84 was not developed to account for it.


The question of accuracy is highly unanswerable the closer we get to it. It is like the last mile of a fiber optic network or absolute zero. You are right, It is nothing which can be seen in the hand held unit of consumer grade accuracy, but it is not correct to have people think that WGS 84 is substitutable for NAD 83 as that is not correct and wrong minded. There is no point in developing bad habits or wrong thinking at any level. It doesn't serve anyone well. And no, I don't use Nadcon or Vertcon for hard survey numbers. Those have to come from field observations as you know. But they have their uses.


In the end, I am not trying to argue the Math with you Doc, nor am I really trying to contradict you, as all your points are good in many ways, but this is the system we have and despite flaws distortions or Fubars, it is what we have and we should use it. It came from marginal instruments and mathematics which had barely been developed to what we have today. We did give up Clarke 1866 for GRS 80 and we may yet give up GRS 80. I know it is a pain and a work in progress, but still I'd like to think of things becoming more correct, not less. We may be able to measure to the gnat someday, if we can find the gnat and get it to hold still.



Edited by evenfall
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Unless NGS has changed it's policy in my absence, it requires that the mark be found, but found in a destroyed condition before it can be declared destroy. A classic example is if the disk and the object it's suck in has obviously been moved from it's position. I believe the theory is that it's much too easy to overlook something so small. So for NGS, you'd probably have to declare it a "not found" and post your reasoning so that any surveyor can make their own call.


Landmark type of objects such as towers require only a photo showing they are gone.


As for the accuracy, a mark that has location (not height) adjusted should be right where your GPS says it is. Those marks are more accurate then a consumer GPS. The number of digits in such a GPS can only define a box of about 6 ft square, so I'd factor that in my accuracy. If you find an adjusted mark way off, I'd first check my GPS and make sure I was on the right settings. If everything checked, and the error was well outside the GPS error range I'd report it. It's possible that someone made a data entry error when the records were computerized.


However, any marks that say location is scaled - look out. Someone read that from and map and could easily be hundreds of yards off.

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What's your success rate with the waypoints being right.  That is, if you just walked to the marks in the area with your GPS in hand would you more or less be standing on it?

I found one (LY0833) this weekend that was off the scaled coordinates by 740 feet. Fortunately it was attached to a big bridge that was hard to miss -- I just had to keep walking in a straight line until I ran into it. Another one (LY0834) along the same level line was off by 1040 feet.


I only use the coordinates to get me to the general vicinity where I can use the description to home in on the station. That's true even when the coordinates are adjusted.


My experience with scaled coordinates is that the GPS reading usually differs by about 100-150 feet. The best agreement I've ever had with adjusted coordinates is about 2 feet.

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So far I think all the ones I've been looking for are adjusted. That makes me feel more confident when the WAAS enabled GPS says I'm on top of it, and shows 6' estimated error, and I look around and realize I'm in a parking lot that extends for 200'+ in all directions that the mark is gone gone gone. :)

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>>all the ones I've been looking for are adjusted


Some of the ones you have logged notes on have adjusted elevations but only scaled positions. For those, the GPS and aerial photos may be helpful but not the last word, and you have to find some landmarks in the description to locate them (unless you stumble over them).


Another pointer: if you take photos for later submission to the NGS, they don't want people in the picture.


You are working so hard for such low success rate that I sympathize with you. Perhaps there is another area not too far from you that is less developed, and would give you more successes for purposes of initial practice? There is a lot to be learned from the experience of seeing actual marks in their settings that will help you work on the tough ones.

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We're planning to take a saturday trip about 15 miles west of here around one of a number of small towns. Most of this area has had extremely restricted develpment and is much more likely to have undisturbed marks.


In particular in my area, many were set 10-20 feet off the centerline of gravel roads, and now many of those roads our multi-lane divided highways. Not likely any of those are still around.


Looks like there may also be some closer to my work, I see some lunchtime trips.

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Hi, Bicknell:


On some of your first trips, go to several of stations which have been logged in the past couple of years. You'll have a higher success rate, which will give you more confidence--plus allowing you to become familiar with the language used in descriptions. By the way, if you find a bright orange cap near a previously-found mark, we can help you get it back to the owner. [Private joke]


A word about coordinates:

I note the Lat/Long in D/M/S (available in NGS or TopoZone), which will translate into Microsoft Streets and Trips. I enter the marks onto a map, using the PID and Designation as a label. It helps me know which ones I'm close to--and which are close to each other. (This saves backtracking!)


I seldom use the GPS, but rely on the descriptive text for the final approach. The exception is for old marks where all references have disappeared. At that point, I'll break out a handheld unit to see where the coordinates fall. Occasionally that is the answer--especially in conjunction with the metal detector.



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