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What's your opinion of this one?


Hoosier_Daddy

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I have been a surveyor for years now, but only relatively recently have I gotten into hunting for benchmarks officially like many of the members of this forum. I recently found this one http://www.geocaching.com/mark/details.aspx?PID=KA0253 , but as you can see from my posts, I have a few questions as to whether this disk is still reliable.

 

I would be interested to get the opinion of a few others on this forum regarding disk D43.

 

This disk has been set vertically in the wall of an old train station. This is the first time that I have come across one of these, though I have seen pictures elsewhere on this forum, of course. I found myself wondering why anyone would set a vertical benchmark this way instead of horizontally. It seems to me that having a mark set this way is just asking for a bad reading. My rodman stuck a pocketknife in the groove and set the level rod on the blade to get a reading, and that worked just fine for the nature of the work we were performing. If I were doing something where more accuracy was required, I am not so sure that I would trust the stability and levelness of a knife blade... What was the reason for setting a bench this way, and what is/was the reccommended method of getting a good reading off one?

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I'm not a surveyor, but it seems to me that putting the mark on the wall keeps it off the floor / ground, which is vulnerable to re-modeling: new concrete, new ground levels, etc. In fact, your own measurements (nicely done, BTW) show that the ground level has changed. Hmmmm... if the mark was on the ground, would it have been destroyed? Maybe.

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Rambling thoughts:

If the building has not been totally reconstructed and pieces of stone moved around, the sidewalk could never have been only 1 foot below the disk because that would put the window at a ridiculous height.

 

Can you blow up the old picture and compare features to determine how high the window was back then?

 

The bricks could have been sandblasted and tuckpointed giving them a new look without disturbing the disk.

 

Of course for really careful work, I would assume you always connect to two benchmarks and check agreement.

 

I have heard of the pocket knife trick before. Seems like it would be trivial to get 0.005 foot that way and great care might get you something like 0.002. If you need it better, would this work? Set up your total station a few yards away and as close to level with the disk as practical (maybe work on your knees). Measure the distance. If you are nearly level with the disk (within say 5 degrees), the distance accuracy doesn't have to be extremely good. Shoot the vertical angle to the center of the line on the disk and calculate the vertical offset to get H.I. Or to avoid calculation, pick a turning point at exactly the same distance as the disk and read the rod with the same vertical angle as needed at the disk. Then level the instrument line of sight and go on with the level line.

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Here is a quote from "MANUAL OF GEODETIC LEVELING", USC&GS Special Publication No. 239, 1948, page 43. It is on-line at: http://docs.lib.noaa.gov/rescue/cgs_specpu...35no2391948.pdf

 

"Buildings-In general, public buildings, buildings of a memorial character and

churches have a longer life than do commercial buildings. On the other hand a new and

modern bank building in a city or large town is apt to remain undisturbed longer than

an antiquated city hall or county courthouse, unless the old structure has a historical

significance which may insure its remaining untouched for that reason. Railroad stations

in growing communities are frequently demolished or rebuilt to provide added station

facilities for increasing traffic.

A bench mark set in the wall of a building is less apt to be disturbed than one set

in a step or portico, since these portions of the structure are more frequently changed

than the building itself. Bench marks should not be placed on steps or other projecting

portions of buildings unless these portions are supported on massive foundations to prevent

settlement or heaving from frost action. Bench marks are frequently destroyed

or rendered inaccessible by reason of additions being made to the buildings on which

they are set. For this reason placing bench marks on the sides of buildings where additions

are possible should be avoided."

 

For a brief history of USC&GS and NGS leveling see: http://celebrating200years.noaa.gov/founda...ng/welcome.html

 

GeorgeL

NGS

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The sidewalk has obviously been replaced. The cement part of the wall has both an upper and a lower water table. Looking at the old picture, it seems that the bottom water table comes up to about knee height and is definitely taller than the upper water table where the benchmark is currently mounted. Therefore I believe the present sidewak is a few inches higher up on the building than the very large sidewalk in the old picture. A mark could've been a foot above the old sidewalk and still be on the lower water table.

 

I would think that a secure mounting would have been drilled into the cement, not put in a seam between cement pieces. I think it's possible that the benchmark may have been moved when the sidewalk was re-done. There might be the remains of an old mounting hole near the present sidewalk surface level somewhere nearby.

 

In the old picture, a black spot kinda looks like a benchmark disk below the double windows and down near the sidewalk. B)

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I'm just a benchmarker, so all I have to go on is what I've encountered.

I don't think I've ever seen one put in the seam between the granite, though, I'm sure that it has been done. Usually, the are set into drill holes in the granite:

KV0162

The stain does show that it has been there a while, but it does seem that the granite and brick have been sand-blasted (?) or otherwise cleaned. I would assume that both are original.

There does not seem to be any drill hole in the lower water table.

Ground level/sidwalks do change. The early photo does not show it there were a step at the doorway. The sidewalk may have been raised. In which case, it might be the lower water table where the disk was originally set. (Finally resolved one of my local quandries: Rockaway. The sidewalk was raised! The top of the step is now level with the doorway. Benchmark in the sidewalk is either under the present sidewalk, or destroyed when the sidewalk was rebuilt.)

Datasheets do contain errors! :rolleyes: They might have meant 3 feet, but wrote 1 foot. I've seen enough compass errors to concern me. Sorry, that's northeast, not south. The tracks run north-south. The disk cannot be north of the tracks! I've reported a typo in coords. That's 40 seconds West, not 10. And PapaBearNYC found the one that was 6.1 feet south, not 61 feet south. So, errors do occur.

Yes. It is definitely not a reset. But, yes, it might have been moved. Use with caution.

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The building looks original to me, although the brick may have been cleaned (water blasted is more likely than sand blasted in my opinion). I have seen marks set in joints between stones, as it may be easier to chisel into mortar than into rock. The weathering stain under the disk makes it seem old also. Here is a picture of the station from the mid-1950s:

05-04blvdstation2.jpg

(from www.monon.monon.org).

I can't tell if there is a disk below the ticket windows in this pic, but it does show that the level of the sidewalk was not significantly changed, and that the building looks to be original. In the end I would guess that the statement that the disk was a foot above the walk is incorrect or incorrectly entered.

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I have heard of the pocket knife trick before. Seems like it would be trivial to get 0.005 foot that way and great care might get you something like 0.002. If you need it better, would this work? Set up your total station a few yards away and as close to level with the disk as practical (maybe work on your knees). Measure the distance. If you are nearly level with the disk (within say 5 degrees), the distance accuracy doesn't have to be extremely good. Shoot the vertical angle to the center of the line on the disk and calculate the vertical offset to get H.I. Or to avoid calculation, pick a turning point at exactly the same distance as the disk and read the rod with the same vertical angle as needed at the disk. Then level the instrument line of sight and go on with the level line.

 

Bill,

 

The level runs would have never been run with a transit, they were only run with high order differential levels. You would not want vertical angles to be a part of differential leveling, too much error. I doubt that these leveling crews even had a transit with them.

 

The best way to level from these vertically set marks is to set your level up with a zero HI, you have to do some wiggling to get them there, but your error comes close to zero if you are shooting the center of the horizontal level mark.

 

CallawayMT

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What was the reason for setting a bench this way, and what is/was the reccommended method of getting a good reading off one?

 

For the benefit of us laypeople, is it accurate to say that benchmarks which are mounted vertically are primarily for elevation, and not horizontal position? (The ones I've found are SCALED horizontally, but are ADJUSTED vertically.)

 

As for getting a good focus on the close-up shots, here's a technique I use with my SONY cameras:

 

Position the camera four to five feet from the disk and use the ZOOM feature to get a close-up shot.

 

For some reason, I've never been able to get a clear shot with the lens being only inches from the disk--although on a recent team hunt, the other three folks were "right on top" of the disks and their photos were excellent.

 

(Another factor for consideration. My way is easier on the knees.) :rolleyes:

 

-Paul-

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I know any super precise work would not use a vertical angle. But my point was that if your station can read to 5" and you are 20 ft away you can probably get better accuracy with the method I described than trying to hold a knife level within 0.001 ft (0.012 inch), and faster than trying to get the instrument at the height of the mark within a similar tolerance, which would be pretty frustrating.

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As for getting a good focus on the close-up shots, here's a technique I use with my SONY cameras:

 

Position the camera four to five feet from the disk and use the ZOOM feature to get a close-up shot.

 

For some reason, I've never been able to get a clear shot with the lens being only inches from the disk--although on a recent team hunt, the other three folks were "right on top" of the disks and their photos were excellent.

For what it's worth, I've discovered that when my camera (a Canon PowerShot) is in macro mode, it has to be zoomed all the way out, or the closeups are out of focus. I frequently take one shot of a benchmark from a few feet away zoomed in using normal mode; and another shot from several inches away using macro mode, but zoomed out. Maybe your friends were doing something similar. Of course, cameras that don't have a macro mode won't be able to focus when they're really close to the mark no matter how you zoom them.

 

(Another factor for consideration. My way is easier on the knees.) :rolleyes:

And thus deserves high recommendation for that reason alone! B)

 

Patty

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What was the reason for setting a bench this way, and what is/was the reccommended method of getting a good reading off one?

 

For the benefit of us laypeople, is it accurate to say that benchmarks which are mounted vertically are primarily for elevation, and not horizontal position? (The ones I've found are SCALED horizontally, but are ADJUSTED vertically.)

 

As for getting a good focus on the close-up shots, here's a technique I use with my SONY cameras:

 

Position the camera four to five feet from the disk and use the ZOOM feature to get a close-up shot.

 

For some reason, I've never been able to get a clear shot with the lens being only inches from the disk--although on a recent team hunt, the other three folks were "right on top" of the disks and their photos were excellent.

 

(Another factor for consideration. My way is easier on the knees.) :rolleyes:

 

-Paul-

For closeups I've had the opposite experience with a Canon. I use the closeup feature (also called macrozoom, with a flower icon) and shoot from about 1 foot away. I find using the zoom feature screws this up badly. I think zooming tends to use the less accurate part of the lens so you get the best optics in a closeup with no zoom. Don't confuse macrozoom (another name for closeup) with zoom (typically used for long shots).

 

I find in closeup mode I can see features on the disk in the final image which I cannot see with the naked eye in the field.

 

As with any product, YMMV and of course knee quality my trump image quality B)

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What was the reason for setting a bench this way, and what is/was the reccommended method of getting a good reading off one?

 

For the benefit of us laypeople, is it accurate to say that benchmarks which are mounted vertically are primarily for elevation, and not horizontal position? (The ones I've found are SCALED horizontally, but are ADJUSTED vertically.)

 

As for getting a good focus on the close-up shots, here's a technique I use with my SONY cameras:

 

Position the camera four to five feet from the disk and use the ZOOM feature to get a close-up shot.

 

For some reason, I've never been able to get a clear shot with the lens being only inches from the disk--although on a recent team hunt, the other three folks were "right on top" of the disks and their photos were excellent.

 

(Another factor for consideration. My way is easier on the knees.) :rolleyes:

 

-Paul-

For closeups I've had the opposite experience with a Canon. I use the closeup feature (also called macrozoom, with a flower icon) and shoot from about 1 foot away. I find using the zoom feature screws this up badly. I think zooming tends to use the less accurate part of the lens so you get the best optics in a closeup with no zoom. Don't confuse macrozoom (another name for closeup) with zoom (typically used for long shots). I find in closeup mode I can see features on the disk in the final image which I cannot see with the naked eye in the field.

 

As with any product, YMMV and of course knee quality my trump image quality B)

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Here is another quote from USC&GS Special Pub. #239 regarding marks set in walls (page 45). It mentions using a tape when leveling to a disk in a wall.

 

"Benchmark disks may be set either horizontally (shank vertical) or vertically (shank

horizontal) but, when they are set vertically, they are much less convenient to use since

a leveling rod cannot be placed on them and it is usually necessary to use a tape in lieu

of a rod in leveling to or from a disk set vertically. If disks are set vertically care should

be taken to see that the tablet, because of its weight, does not loosen itself before the cement

has set firmly."

 

From "Geodetic Leveling", 1981 page 3-60, at: http://www.ngs.noaa.gov/PUBS_LIB/Geodeticleveling_nos_3.pdf

"On a vertical surface, it (the control point) is at the intersection of a pair of crossed lines (the exa t center of a bench mark disk fig. 3-76)."

 

Also, page 3-62, Section 3.7.5 "Leveling to Awkward Control Points" has some interesting information and drawings.

 

GeorgeL

NGS

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PFF, In answer to your question copied here,

 

"For the benefit of us laypeople, is it accurate to say that benchmarks which are mounted vertically are primarily for elevation, and not horizontal position? (The ones I've found are SCALED horizontally, but are ADJUSTED vertically."

 

Mark set vertically are for elevation only and SOME marks that were set horizontally were for vertical. All of these marks set to record vertical information are called, using the surveyor's definition, "BENCH MARKS". Note, a subset of these placed around tide gauges are called TIDAL BENCH MARKS. They often have a circle and slash stamped in the center of the disk and may be "U.S. Coast & Geodetic Survey" or a "National Ocean Survey" or a "National Ocean Service" disk.

 

Marks set with the disk horizontal could be for vertical or horizontal positioning, or for gravity, magnetics, etc. The old U.S. Coast & Geodetic Survey had quite a few different disks including BENCH MARK, TRIANGULATION STATION, TRAVERSE STATION, TOPOGRAPHIC STATION, HYDROGRAPHIC STATION, GRAVITY STATION, MAGNETIC STATION, REFERENCE MARK, AZIMUTH MARK. Surveyors call these "survey marks" or by the type stamped on the disk such as TRIANGULATION STATION.

 

Regarding scaled versus adjusted, many vertical marks have scaled positions because the horizontal position of the mark has never been surveyed (the position was obtained by scaling off a map). There are a few vertical marks that have surveyed, accurate horizontal positions just as there are a few TRIANGULATION STATIONS that have surveyed, accurate vertical elevations.

 

Hope this helps,

GeorgeL

NGS

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I'm always interested in how things got the way they are (I'm sure most of you are too) and when I find disks that are both horizontal and vertical control, I've noticed a common scenario in the areas I've worked.

 

A lot of horizontal control surveys were done in the 1930s. The stations were most often on high points (obviously) and typically consisted of a triangulation station disk and 2 or more reference mark disks. So far so good. The station is most often named for the places it's sitting on (like "BOOT HILL" :huh: ).

 

Then in the 1950s, it seems like a large number of level lines were run in the area resulting in many bench marks (vertical control stations) being set. These typically have a letter and a number (like "B 241"). The 241 was the series name for the leveling and some series went all the way from A to Z.

 

Now the connection to this thread comes in: Often when a level line passed near a triangulation station, the level line guys would use one or more of the pre-existing disks (usually the reference marks) and used it in their vertical control survey.

 

As an example look at tri-station KU3890 "PALISADES". On the data sheet you will see the station (a copper plug) was established in 1898, and in 1930 it was resurveyed and two RMs (standard disks) were established.

 

Then along comes a leveling party in 1956 and they "usurp" the 2 RMs and establish two new vertical control bench marks: KU1644 "PALISADES RM 1" and KU1645 "PALISADES RM 2". These disks now serve a dual purpose: RMs for the tri-station, and bench marks.

 

Now the amusing part is that when the datasheets were put together, the two new PIDs using the RMs for vertical control have "scaled" positions, because the weren't surveyed horizontally. And they actually managed to put them on the wrong side of the George Washington Bridge! Ironically, their position is known fairly accurately, since they are known to be a specific distance and azimuth from the 1898 tri-station - a second order station. Sounds like the person who entered the data for the 1956 survey, didn't read the data from the 1930 survey.

 

Of course new marks put in today using GPS, are often both horizontal and vertical control stations, since the instrument will determine both metrics from the get go.

Edited by Papa-Bear-NYC
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Papa-Bear - Thanks for the info about KU1644 and KU1645. I've sent a notice to the NGS database team to change their positions.

Thanks Dave. You're the man for getting things fixed.

 

But although these marks make a good example of how marks got reused for a different purpose, they are unfortunately not much use for leveling any more. One seems to be buried and the other has been "decapitated" (the disk was cut off and only the stem remains). See my NGS logs.

 

But for the benchmark hunter, they make a good visit to a spectacular area overlooking the Hudson River. Besides, just the old 1898 copper plug is worth the visit.

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