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"hunting" The Wily Benchmark ...

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One can never predict what rogue survey marks he may encounter while traveling in the backcountry. Best to be prepared!




—“Hunting” an elusive USGS benchmark, ET 6.—





—The mark hides in the brush.—





—It tried to get away but we tracked it down.—





—One shot, one kill!—





—Wait a minute, this is stamped ET 5!—


On a cold, windy day in Northeastern Pennsylvania, Rich and I set out to hunt down ET 6, a USGS electronic traverse station we'd spotted on some of our maps. It's located on State Game Lands near Archbald, PA. We had a pretty good idea where to find it, and figured we'd combine the benchmark hunt with a little squirrel hunting as well. Well, the squirrels were nowhere to be found (I suppose they were smart and stayed huddled in their nests, out of the cold) but we did track down the benchmark. Though we were frozen solid by the time we reached the mark, we managed to take a few photos and measurements, and we made an interesting observation: on our control diagram, this mark is labeled ET 6. The disk itself is stamped ET 5. We're waiting for word from USGS on where the error lies. This was quite an unusual "hunt"!



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That is a beautiful rifle you have. What brand? I, too, own a couple of very sweet shooters. :laughing:



The rifle in the photos is a Sako Finnfire, and the one that I use is a Ruger 77/22. Both are rimfire .22LR (.22 caliber Long Rifle). And both belong to Rich. I don't own one ... yet! :rolleyes:


Glad you enjoyed the story and the photos! :rolleyes:



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Am I doing something wrong? :laughing:

Not really, you just don't have all the information. :rolleyes:


This mark is not included in the NGS database. We knew about it only because we spotted it on a Geodetic Control Diagram we have obtained from NGS. This diagram shows horizontal control placed by USGS as well as NGS. No coordinates are given, but we worked back and forth between this diagram and a topo map to determine the probable location of the mark. In this instance it wasn't too hard because the mark is actually indicated by a triangle on the topo map, though that's apparently not always the case for these marks.




—A sample section of the Geodetic Control Diagram. The mark we found is labeled as ET 6, in the upper left-hand corner of Quad #165.—




—The mark is indicated by the triangle (it's not terribly obvious!) and elevation of 2218'.—


There is a mark not too far from us that's labeled ET 5 on the diagram. (It's shown in the lower right-hand corner of Quad #165.) I can't help but wonder whether that mark is stamped ET 6!



Edited by Zhanna
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Zhanna, it looksl like you've been having fun hunting benchmarks, even if you didn't shoot the one you intended to. ;-)


I notice you refer to ET6 as an "electronic traverse station." Is that what the plus sign inside the triangle means? I found one of those in Yosemite a while back and was wondering what that symbol meant. That particular mark is not in the NGS database, so I couldn't look up its description there.




I had a very productive trip to Yosemite a couple of weeks ago, recovering nearly 20 benchmarks. Most of them had been found during a 1986 USGS survey, but they didn't file recovery reports with NGS, so I'll be doing that.



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So this is a triangulation station disk that happens to have a "+" in the middle?


Not necessarily, its a generic disk to be used for all control.


All disks have some some of point of reference, weather Bench Mark (Elevation only) or horizontal point (X,Y and or Z). Its just happens that USGS uses a "+",


Examples from dustyjscket.com




Edited by elcamino
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Maybe it also should be stated for those who do not know the instrument has a plumb bob that centers the instrument on the +, dot ,X, [],triangle,star of david or other point on the B.M. that it is set up over.


As for setting the order of stations it is beyond me.


Control Topographic

I know you start from a known point consisting of 2 parts (1) horizontal control,for which Triangulation and/or traversing the control stations are located in plan,and (2)Vertical control,for which by leveling the benchmarks are established and the control stations are located in elevation.

The Control provides the skeleton of the survey which is later clothed with the details,or locations of such objects as roads,trees,houses,streams,ground points of known elevation,and contours.


This then goes into Primary control,secondary control and the orders first,second,third,fourth.

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Here's my guess on the chicken and egg. While it is possible to place a disk at a previously determined position, it is a lot easier to put one in the general vicinity of where you want it and then do the precise measurements later. Particularly when they set a concete post in a dug hole it is good to allow some time for the dirt to resettle before relying on the position to be stable. The 1930's disks were probably set by a crew traveling in advance of the party that measured the elevation or did the triangulations.

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Notice the lines on the diagram that have numerous hash marks. These indicate lines where the distance between monuments was measured electronically - hence the use of the term Electronic Traverse (ET) common to many USGS marks. The lines and marks in red were observed by USGS, while the lines and marks in black were observed by USC&GS/NGS. Stations of that era were often observed with Electronic Distance Measuring Instruments (EDMI) that used either microwaves, laser or infrared light sources. Both mirowaves and lasers have gone out of use today. Surveyors today use instruments that measure with infrared light and typically have fairly short (< 2miles) measurement capabilities.

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Yes the Sako is a fine weapon and lots of us have taken squirrels with The Ruger I

have two myself. But what are You gonna do if one of them decides to Charge? Huh

then what? You need no less then a 30-30 or a 30-06 I use a Remington 700 BDL

and always carry my Ruger GP101 .357 as back up.

Those suckers have mean teeth and can leap 20 feet in a single bound I have seen one

run down a black bear and chew it to bits and let me tell You it wasn't a pretty site to


Sometimes they travel in packs! the leader is surrounded by others called RM's

that stands for Really Mean' and they will go for the jugular.

So please be careful with those small caliber fire arms. :D

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This is a fun post Zhanna!


As Geocachers we refer to all survey marker as benchmarks which is fine, we all know what we mean but in the strictest of terms the terminology is more specific.


When is a Benchmark not a Bench Mark? Chicken, egg, geodetic surveyor?


In the strictest terminology, and as far as NGS survey markers and other agency control that meets NGS criteria are concerned, surveyors refer to these monumented locations as Stations. There are different kinds of course but for most of us who are dealing with Geodetic Monuments we will primarily find Triangulation Stations which are Horizontal Control, latitude and longitude, and Bench Mark Stations. A Bench Mark to a surveyor means one thing, Vertical control. It is not a generic terminology, rather it is a point of known elevation. Other types of stations are used to measure Gravity and Magnetic fields. These Stations are also often triangulated and double as Horizontal Control. Azimuth Marks, which were set to aid in getting a Station "dialed in" as to it's orientation so measurements could be taken with less effort, and the Math would be the same each time this was done. These are most usually only found on first order horizontal stations but there are other instances too. Then of course there are the Reference Marks which point to the station, and all of these station monument types can vary as to the era the marker was set.


Zhanna has a wonderful website about many things survey, and she has a page that shows drawings of the various Marker types the NGS has set down through the years. If you would like to look at it, it is located Here.


These Stations are monumented with a plan in mind. The Surveyors, specifically the Party Chief knows what they intend to do in advance. A survey party would usually be interested in performing specific types of survey. Vertical teams don't do Horizontal work and vice versa. it is specialized. A Bench Mark monument will be set in the ground, drilled in a sidewalk or even vertically in a wall by a crew that places monuments in a place that has been pre determined. Then a leveling crew would come through the area to ascertain and document what the actual elevation is at the surface of the station disc's highest point. A monument crew would also be sent to establish a Triangulation Station, In the ground or drilled into a stable surface if other, but never set vertically. This would be a big excavation in the time when an underground mark was also set as well as a surface one, not to mention the RM's and Azimuth... In either and any case, the station has to be in place prior to the time the measurements to it are made, and the monument will be of the type that reflects the type of data which is to be ascribed to it.. You can't measure something that is not there. Then a Survey crew will come, center their instruments over the station mark with perhaps a plumb bob in older times or an optical plummet on newer instruments and begin the planned triangulations.


If the plan is to monument a third order station, the standards for meeting the requirement is followed. A third order station has had at least 4 separate triangulations performed to establish it's position. Second order stations had 8 separate triangulations, First order stations had 12-16 separate triangulations performed. Then the heavy duty math begins... All the triangulations are then taken together as a least squares adjustment and so on...


A and B order Stations are horizontal control from GPS observations. They may be established as such or upgraded from previous numerical order triangulation, and as an added feature from GPS technology, they can have Bench Marked Elevation Data ascribed to them as well. It is important to treat the data separately. They are first most a form of horizontal control that happens to have elevation data ascribed to them.


As to other Agencies and their monuments, the same criteria and high standards NGS requires of their own settings are required to be met in order to be submitted to the NGS database. In the Case of a lot of the USGS stations that are submitted to the NGS for inclusion in the database, most are Bench Marked Elevation Control points, the USGS did not do as much triangulation, and to be sure much of it was not done to nor meant to be done to NGS standards. Many different agencies did perform Survey to NGS standards and when it was submitted to NGS and found to be up to standard, it was included in the database.


The pictured USGS disc is Vertical Control disc, a Bench Mark for known elevation. It says that it is of course but we should not assume it has any other kind of data ascribed to it either. Why the designer put a triangle around the cross hatch sure seems to send a mixed metaphor when industry standard symbology is considered, but it has been their design for years. Each agency has it's own unique ways and we all get used to it. As Mike said the (+) is the point to which the data is ascribed and all measurements are to be taken. The Disc has the words Bench Mark on it and that is strictly Elevation. The Elevation is no longer stamped on monuments because we have learned that Datums change and the world is not a static place. I would need to see the data sheet as a surveyor in order to see how or if this station will meet my needs.


Now Zhanna, did you see that BM running through the high brush? Was it making noise? Did you have to lead that BM much in order to bag it, or did you startle it then drop it standing still? I can hear the call! (<Quietly with Aussie accent> There we were mate... trying to sneak up on the wiley bench mark. I'm tellin you mate, this is not your ordinary kind of mark, oh no! this one is trying to hide it self on the topo!! We've compared it, and we know it is on the loose, but not for long!! Crickey! We bagged it! Benchmark hunters rule!!) What a High adventure! Hehehe This was a killer post Zhanna, Thanks!


I'll save the Bugs Bunny, Elmer J. Fudd version for another go...



Edited by evenfall
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Zhanna, I checked the USGS data sheet for this station and found that your position checkes the publihsed value by 2.3 m and there data definately describes the mark as being stamped ET 6 WAC. Someone oops somewhere when they published their data. You can send them a correction if you want, but I wouldn't count on ever seeing it in print.

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The pictured USGS disc is Vertical Control disc, a Bench Mark for known elevation.  It says that it is of course but we should not assume it has any other kind of data ascribed to it either.


I'm a little confused. :lol: Could you explain which mark you mean? I was sure that "ET 5" was a horizontal control disk; it's indicated as such on my diagram and there is no mention of "bench mark" on the disk itself. Or were you referring to something else? I have seen plenty of USGS vertical control disks that have the triangle with the "+" inside, and the words "bench mark".



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Hi Zhanna,


Sure, I think I can clear this up. The disc in your photo, labeled ET5 WAC 1963 is a USGS standard Bench Mark disc. Unlike others from different eras which do have the words Bench Mark stamped right into them, this one is just like the one that Patty (Wintertime) posted in a different picture later in this thread. In both pictures, yours and hers, the Station Discs have the words ELEV. FT. ABOVE SEA machine stamped on the discs, and that is a standard stamping on many USGS Bench Mark Discs.


From the USGS Topo you uploaded, we can see the location you refer to with a triangle and the BM EL=2218. That the actual station disc should have been marked ET6, is you and Rich's very cool find, yet the location you found does correspond to a location which is stated as a known elevation. The elevation seems to fit the Topo, and the Station was obviously mis-stamped. It is a Bench Mark, but in this case on the NGS Control Diagram you obtained, it is also shown as part of an Electronic Traverse as well. Without the Data you have, Most others would not be so safe to assume this as they would not know what ET stood for and even then assuming that is no guarrantee. USGS Stations Marked TT were often meant to mean Transit Traverse. In this case, You knew there is more data ascribed to the Station than just Bench Mark Data.


I was saying it is not safe to assume that there is more data ascribed to a station than what the station type on the ground makes obvious. The electronic traverse performed by the USGS was likely performed for checking the physical distances between the stations to compare how the benched elevations fit the topo mapping data. It is a form of "Open Traverse". They are just making sure their elevations hit the contours and the distance between these stations on the ground will scale favorably to those on the maps. The interesting thing about an open traverse is that there is no real way to properly check it for errors unless you perform it over again. We would usually have to perform the open traverse more than once to check for mistakes. To add, there are plenty of ground and heat effects that can harm accuracy when EDM is used over long distances on an Open Traverse. The further you go, the larger the margin for error. It is not like the kind of Horizontal Control we think of when we think of NGS Control. Comparatively the NGS Triangulation is a known point on earth exact to within inches or less, the USGS Open traverse is really just a basic measured distance between two points that help them check to see that the maps fit the territory.


In my post, I was saying as a for instance that we are familiar with NGS Triangulation Station Marks. We see them in the field all the time. We get so used to it, that we automatically assume things we think we can expect at a glance. Yet an old CGS Triangulation station can have Bench Mark Data ascribed to them. Hypothetically speaking, it may have started life as a third order Horizontal Station, then lets say in 2001 a crew sets up a GPS over it and sends in the newly collected data. After the NGS checks the Data, it now becomes a B order Horizontal Station with Vertical Order of say Second Class 0 quality. Yup, It just became a NAVD 88 Bench Mark. Since GPS isn't leveling or traditional triangulation, but can derive data in 3 dimensions at once, an old dog of a certain type can learn a new trick. So what I meant is that there can be more data than we assume is there, but that is never safe to assume anything until we see the data ascribed to the station.


I know of a few CGS Reference Marks that have been leveled to Vertical Order First Class 2. Who would have thought a Reference Mark Disc is also a Bench Mark? But so are many top bolts on Fire Hydrants. It goes to show you never can tell.


I guess I was collectively referring to several things that had been raised in the thread by others as well. I used your for-instance as an example and to stay on topic as I attempted to explain answers to the various questions raised by others. Mike Raised the point that it is a generic disc to be used for all control. I was raising the point that it is never safe to assume we know what the control is until we see the datasheet.



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Once upon a time we did some leveling for a highway re-construction project (US2). The job was to set intermediate (TBM's every 1000 ft) control for the project and all we had to work with was 3 USGS 3rd order Bench Marks. We did 3 wire levels and found the we missed the middle BM by almost exactly 50 ft in elevation. Thinking we made a big blunder we went back out and ran selected parts of the line looking for the error. The line was about 10 miles long, with the BM's about 3-4 miles apart. The odd thing is that we closed within 15 mm in 10 miles but missed the middle BM by 50 ft? We thought this to be a data entry error but could not find it. Well finally after much lobbying, the PS finally called USGS to check. Darn if there was not a typo on the data sheets we had gotten from our design engineers. USGS new some were out there as there was a notation on the corrected date sheet they sent us. A three (3) was transposed to a eight (8), so if anyone were to just use that one BM, everything they did would have been 50 ft higher which probably never happened due to the remote location. The PS was sure we had screwed up.


3 Wire levels are in which there are 3 stadia cross-hairs on the instrument lens, you read the top, middle and bottom wires, subtract the middle from the top and the bottom from the middle and if the difference agrees within a set tolerance (0.004ft) , you are good.

Edited by elcamino
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