Junction Bench Mark

[ArtMan]

In Richmond, Virginia, station G 467 (GV3391) is described as a "junction bench mark."

 

I saw this on a couple of other datasheets in Richmond.

 

Anyone know what this term refers to?

 

-ArtMan-

[+zygote2k]

probably a chiseled "x" or square in the concrete.

[2oldfarts (the rockhounders)]

probably a chiseled "x" or square in the concrete.

 

It's a "vertical control disk" set Horizontally in the building Facade. At least according to the original datasheet.

 

John

[+BilboB]

5 recoveries since 1985 seemed a bit extensive to me until I read the description and found it was placed at the main entrance to the Virginia Department of Highways Building.

[+jwahl]

I will continue my evening guessing spree. Level lines are run between points, along roads, railroads, etc. but at some point they have to check into other level lines forming loops. I would guess that a BM that is a junction point is one that is common to two or more level lines and thus forms a connection between the network.

 

- jlw

[ArtMan]

I would guess that a BM that is a junction point is one that is common to two or more level lines and thus forms a connection between the network.

That makes sense, jlw.

 

As regards BilboB's comment: all the more frustrating to me that I couldn't find it.

 

-ArtMan-

[DaveD]

jwahl is 100% correct.

[evenfall]

DaveD,

 

Since optical leveling is path dependent, do or rather did these junction points undergo a least squares adjustment so the numbers from each path will average? My experience has been that my field measurements to a similar place via different paths will not exactly match. I was wondering if these junctions are treated differently, By NGS, for geodetically published data than other stations, so as to have an accurate value no matter which established path leads to them?

 

Thanks in advance,

 

Rob

[Bill93]

I have an old surveying textbook with a section on precision leveling that gives some fascinating descriptions of the work done in the 1930's to 50's. The precautions are mind boggling, with multiple measurements, equalized lengths of sight, shade for the instrument, etc. It indicates that they double rodded between marks, read all 3 crosshairs on each sight (both for distance and as a check on reading errors), measured the loops in both directions, and applied every correction known to man on the individual traverses.

 

Then they calculated around each loop and adjusted by rules of thumb (based on number of setups typically) to make them come out exact, and then compared adjacent loops and transferred corrections between loops to make everything match.

 

Least squares was used for parts of the job, but the computational problem with the number of variables they had meant that they could not do a full least squares for all the parts of major networks at once so they had to do some heuristic/rule of thumb adjustments as well.

[+PFF]

.....would seem to place it on the structure flanking the entry steps below the large urn, and on the left side when facing the entrance. But I found no disk. Recent reports from NGS and non-professionals, and an absence of obvious changes, suggest I am missing something obvious.

 

Hi, Artman:

 

Don't feel you are alone. The obvious hits me in the face daily!

 

Looking at the distance from street centerlines, it appears that the disk is on the southwest face of the structure.

 

Based upon the reference to the doors, I believe you will find the disk to the RIGHT (southeast), if your back is to Hwy 60 and you are facing the building. (Disclaimer: This opinion is the result of examining aerial photos and a city map. I have not been to the site.)

 

-Paul-

[ArtMan]

Looking at the distance from street centerlines, it appears that the disk is on the southwest face of the structure.

 

Based upon the reference to the doors, I believe you will find the disk to the RIGHT (southeast), if your back is to Hwy 60 and you are facing the building. (Disclaimer: This opinion is the result of examining aerial photos and a city map. I have not been to the site.)

Paul,

 

Good try, but I don't think so. The scaled coordinates have misled you to believe the building is on the noreast side of Broad Street. Actually, it's on the southwest side of the street, so as you are facing the building standing on the Broad Street sidewalk, southeast would be to the left, which is where I looked.

 

Also, the fact that it is mounted horizontally would suggest it is not placed in the "face," as are many other disks on public buildings.

 

-ArtMan-

[2oldfarts (the rockhounders)]

ArtMan, Try standing at the southeast doorway and look southeast. There is probably a narrow ledge with the benchmark about 5-6 feet from the edge of the door. The alternative is to bring a stepladder since the mark is horizonal and 9 feet above the sidewalk.

 

Good look,

 

John

[+PFF]

Artman:

 

You're correct. I got snookered by the coordinates. However, I have some good news for you. Now, the Power Squadron can't find it, either! And in years past, they were one of the most frequent pilgrims to the site:

 

GV3391

GV3391  HISTORY    - Date    Condition        Report By

GV3391  HISTORY    - 1978    MONUMENTED      NGS

GV3391  HISTORY    - 1983    GOOD            NGS

GV3391  HISTORY    - 19910422 GOOD            USPSQD

GV3391  HISTORY    - 19940312 GOOD            USPSQD

GV3391  HISTORY    - 19961006 GOOD            USPSQD

GV3391  HISTORY    - 19971104 GOOD            NGS

GV3391  HISTORY    - 20000220 GOOD            USPSQD

GV3391  HISTORY    - 20040221 MARK NOT FOUND  USPSQD

 

Ya'll do some investigating and get to the bottom of this. Don't make me come up there! (grin)

 

-Paul-

[+Kewaneh & Shark]

In Richmond, Virginia, station G 467 (GV3391) is described as a "junction bench mark."

 

I saw this on a couple of other datasheets in Richmond.

 

Anyone know what this term refers to?

 

-ArtMan-

From the 'UNITED STATES IMAGERY AND GEOSPATIAL INFORMATION SERVICE (USIGS) GLOSSARY, REVISION B, 23 November 1999' a 'junction bench mark' is 'A bench mark selected as the common meeting point for lines of levels or links of levels.'

 

- Kewaneh

[Z15]

fyi

 

Here is an old date sheet publication that shows the line of levels.

 

[+GEO*Trailblazer 1]

HERE IS MY 3RD ORDER LEVEL LINES.

[Z15]

fyi