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Wintertime

"No geodetic control at this mark"

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I found a couple of previous threads in the benchmark hunting forum that discussed this situation, but I still don't really understand it.

 

I was trying to find the NGS datasheet for a certain reference mark (DB2102). I entered the PID and got a form that said "This listing contains control for which complete digital data sheets where [sic] not provided." At the bottom of the datasheet, a code next to the station's PID and name referred back to a line on the form that said "No geodetic control at this mark."

 

How could a mark that had "no geodetic control" and for which the proper information was never submitted have been assigned a PID?

 

And for that matter, what's a station in central California doing with the prefix "DB"? That prefix is supposed to be for extreme southeastern California. I know of a station with the prefix "DF" near Yosemite, too; that prefix is assigned to Georgia.

 

Patty

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Hi, Patty,

 

I don't have any "inside information" on those messages you are seeing, but on numerous occasions, I have found disks where the message said something similar. Usually this applies to reference marks, but occasionally I find an "unfindable" station disk.

 

As has been mentioned before, both professionals and volunteers sometimes are too quick to declare a disk "destroyed" . I admire the entries of an oldtimer (recently retired) at NCGS. His reports sometimes say that a station was "NOT FOUND BUT MAY STILL EXIST". That's a great way to preserve your dignity if a disk is later recovered. :cry:

 

-Paul-

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Hi, Paul.

 

There is nothing on the data sheets for these marks indicating that they were declared destroyed. The wording implies that they were never accepted by the NGS in the first place--and yet they have PIDs assigned. That's the confusing part.

 

Patty

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Its not confusing to me.

 

They are not meant to be control points, they are reference marks to another station and thus have no control established for them. Weather they got a PID or not is irrelevant NGS probably wants they labeled with PID's for inventory. Reference marks and Azimuth marks are no longer established.

 

This is probably one of those things that if you are unfamiliar with the field operations you would not find this easy to understand. The main station is the control, the rest of the peripheral marks are useless without the main station.

Edited by Z15

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Its not confusing to me.

Well, no, I wouldn't expect it to be confusing to you! I said that I was confused.

 

They are not meant to be control points, they are reference marks to another station and thus have no control established for them.

Okay, I can understand that.

 

Weather they got a PID or not is irrelevant NGS probably wants they labeled with PID's for inventory.

Meaning...? What would be the purpose of having them in inventory if they provide no useful information?

Reference marks and Azimuth marks are no longer established.

Oh, interesting, I didn't know that!

 

The main station is the control, the rest of the peripheral marks are useless without the main station.

Sure, that makes sense. I just didn't understand why NGS would give them PIDs if the aren't really useful for anything.

 

Patty

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Here's another, perhaps simpler hypothesis. The geodetic control for a mark is the relationship it has with neighboring stations. In the days of classic triangulation, each mark was connected to a least 2 others and often times to many others. Then the whole system of marks were adjusted so that the errors at each station was minimized and each mark was given the best value for its position.

 

When marks were resurveyed in the 30s, 40s 50s and beyond, the position of a mark was verified by taking sighting to the neighboring marks.

 

Now if in a specific area, all the neighboring marks were lost, destroyed or not visible (trees growing, buildings built, etc.) and if the mark itself was suspect, than it's position was basically unusable. Without being controlled by other marks, it had basically lost it's position. But it's not destroyed, it just has no geodetic control.

 

 

Patty

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Here's another, perhaps simpler hypothesis. The geodetic control for a mark is the relationship it has with neighboring stations. In the days of classic triangulation, each mark was connected to a least 2 others and often times to many others. Then the whole system of marks were adjusted so that the errors at each station was minimized and each mark was given the best value for its position.

 

When marks were resurveyed in the 30s, 40s 50s and beyond, the position of a mark was verified by taking sighting to the neighboring marks.

 

Now if in a specific area, all the neighboring marks were lost, destroyed or not visible (trees growing, buildings built, etc.) and if the mark itself was suspect, than it's position was basically unusable. Without being controlled by other marks, it had basically lost it's position. But it's not destroyed, it just has no geodetic control.

 

The same scheme works for bench marks (vertical control), except leveling is done in closed loops so each stations has at least two (one on either side) to provide control.

Edited by Papa-Bear-NYC

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Now if in a specific area, all the neighboring marks were lost, destroyed or not visible (trees growing, buildings built, etc.) and if the mark itself was suspect, than it's position was basically unusable. Without being controlled by other marks, it had basically lost it's position. But it's not destroyed, it just has no geodetic control.

 

It does not work like that. Just because other marks in the area are gone does not automatically mark the last one questionable. Who determines that anyway?

 

They have a PID for NGS internal use. Remember this db was setup before GPS was online. Now GPS is a much better way of establishing control and most of these old tro stations are proving to be less accurate then thought of in the past.

Edited by Z15

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