Vertically Placed Disk Question

[+frex3wv]

I have a pretty good idea (and picture in my mind) how the surveyors used equipment over horizontaly placed disks - but am unsure how they did readings for those set in the sides of buildings etc.

 

Any info would be appreciated!

 

Thanks in advance,

[68-eldo]

I certainly am not an expert on this, but I imagine the surveyors rod has a metal projection at the bottom that fits into the line that goes through the center of the benchmark.

 

I believe the line in the center is the point measured to in the survey.

[+m&h]

These vertical marks were set during leveling runs, and mark precise elevation. It is possible to place a knife blade, or even a machete blade, into the horizontal tick mark on the disk in such a way as to be able to place a leveling rod on the side of the blade. This does not usually make for the greatest accuracy. The mark may also be observed directly from a level set up so that its instrument height is zero relative to the disk. There are also leveling tapes available for this precise purpose, held against the wall with an index mark at the disk's tick mark, and checked for verticality with the vertical hair of the level scope. We have seen USC&GS descriptions of some of these procedures, and thought to look them up for this reply, but haven't found them yet.

 

Fortunately, instruments don't have to be set up "over" these marks.

 

Cheers.

[NGS Surveyor]

From “Manual of Geodetic Leveling, USC&GS, 1948, (http://docs.lib.noaa.gov/rescue/cgs_specpubs/QB275U35no2391948.pdf)

 

From page 43:

“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.”

 

From page 49:

“In stating the character of a mark, such as our standard bench-mark tablet or other

similar mark, the mark should be referred to as a “disk”, since that is all that is visible

to the person trying to recover it. If the mark is set with the disk in a horizontal position

so that the rod can be stood on it, it is not necessary to state that the disk is horizontal.

However, if the mark is set in the wall of a building or is placed in a vertical position in

any type of structure, so that a rod cannot be stood on it when leveling to or from the

mark, this fact should be stated in the description.”

 

From page 65:

“Quite frequently the first backsight or the last foresight of a section is made on a

tape. This is the case when the bench mark is in the wall of a building and it is impossible

to hold a rod on it. The record should indicate clearly what readings were made on a

tape and should also include the number of the tape.”

 

For a modern look at the problem see the photo at: http://www.flickr.com/photos/12262796@N06/3707001715/ , which shows a commercially available invar strip (with bar-code) being held on a knife blade centered on the mark on the VERTICAL CONTROL MARK disk.

 

GeorgeL

NGS

[+m&h]

Thanks, George!

 

Cheers,

[68-eldo]

Ah – I had not thought of a direct reading on the mark. I did think of using a device similar to a knife blade but I figured that function would be built into the rod.

[Papa-Bear-NYC]

Here's how they did it 100 years ago in New York City:

 

 

That cross piece is a spirit level with an attachment to fit into the grove of the disk or bolt.

 

From this volume:

 

[+frex3wv]

Hey guys -

 

Thanks for the responses and info - very interesting as always!!!!!!!!!!!!!!!!!!!!!