Jump to content

Underground Disk?


Recommended Posts

I was wondering what these were used for and if an attempt could be made to find the under ground disk. The description says it is 40 inches below the surface of the ground, would this be right under the surface disk and if I cannot dig to find the disk, would you have any idea of what it might look like? URL=http://www.geocaching.com/mark/details.aspx?PID=CR0860]Here is the Benchmark[/url]

Edited by TXcachehunter
Link to comment

The underground mark is set directly below the surface mark. What they did was set the undergound mark first, then build a plumbing bench out of wood stakes and set the surface mark at the same precise position, usually with a cushion of sand between the ug mark and the base of the concrete post.


There would be no reason to dig up the underground mark unless the surface mark were gone. Thats what the reference marks are for, to find the ug mark.


The ug mark could be a survey disk like the surface mark or it could be something else. In my work I had the occasion to dig up 3 of these (and set many more), 2 were survey disks and the 3rd was just a brass pin or bolt in a mass of concrete. The ug mark is set in about 1 or 2 shovels of conrete, aprrox. 1 gallon were the ones I found.


When I referred to having dug these up, it was when in our offical capacity as state gov't surveyors we had found the surface mark destroyed and we going to restore the surface mark for use as a GPS control station. We always did this with the blessing of the NGS state advisor. We in no way would ever concern ourselves with the UG mark and no one here should do the same. Its dangerous to dig or probe the ground as you are not aware of any buried utilties like HIGH VOLTAGE POWER and if you should do any damage, you are laible for all consequences, no matter the cost. So please do not go digging for the UG mark, leave that to those who need to use it because once disturbed, its of no value to anyone.

Edited by Z15
Link to comment

Not Trying to overstep my bounds here, but Z15 is a former Surveyor, and a lot of us here in the Forum know this because he used to mention it in his Signature.


I am not trying to blow his cover as of late, but I want to clarify that the reason he knows about Underground marks is because he has had to work with them professionally. He is not saying that he dug these up as a Geocacher, and he is not likely to advocate that you should either. So if you have it in your desire to dig up an underground mark, please reconsider.


Like Mike said, a survey party placed an underground mark in a deep hole, and then just above it they positioned what was to be the primary Mark for that location. From there, a different survey party came and performed a number of surveys at that location to obtain the needed data to perform the Geodetic positioning.


The old practice, since abandoned for better ones had the underground mark acting as a back up to the surface mark in case it is lost. We professionally assume that if a previous party put in an underground mark and the surface mark is in position and not lost or disturbed, that the underground mark is there and in position. If the surface mark is fount to be lost or disturbed then the Underground mark serves as a backup. We dig for it and check it's quality.


It is instructive for you to know that there is no way for you to dig a hole and check the status of an underground mark without disturbing the survey quality of the surface mark so please do not do this! There is a $250 fine for tampering which in today's terms really could mean that if you are found doing this you could be liable for the costs incurred to replace or repair what has been undone. I am saying it has happened. The best advice is to be happy that the station is there and that you know it is a Triangulation station with an underground mark.


Thanks for your interest in these stations and I hope we have helped you understand what is going on with them.


Oh, and sorry Mike if I have blown your cover! :-)



Edited by evenfall
Link to comment

I am not advocating but what about a stainless steel probe?

I use it to verify the underground mark is still there.


If it says 12", I measure this from the tip on the probe.

It is usually 2-6 inches deeper,but not always.


We use to do this for finding buried things..............

The ground (where once disturbed) is never packed to the same density as the original soil density.


It will also meet resistance at the mark on the cement.

Link to comment

The "surface mark" or principal disk may often be buried up to 18" and you may need to use a metal detector or probe. It would be very unusual for the "underground mark" to be that shallow.


Just look for the surface disk and assume the underground mark is beneath it. Not even the NGS checks the underground mark when the upper one is found.


What ever you do, don't disturb the earth enough that a mark could shift its position.

Link to comment



There is no need to probe. It is there. If the surface mark is disturbed we will immediately know when we survey. Besides, If your probe is anything like mine, it is good at finding Rocks too.


No one is going to alter the underground mark. Really.


Frost heaves and other forms of land movement, perhaps from soil saturation, liquefaction, earth tremors, what have you might disturb an underground mark but the surface mark would immediately denote this.


Please remember that the surface mark that was placed above this is, on average 9-12 inches in diameter if round, and is sometimes square in the same size range. They are often and most usually 3 feet long, which when buried will make the bottom 3 feet deep.


Just trust that the underground mark is there. It is :-) If the surface mark is gone, you may be hard pressed to locate the subsurface mark without surveying the point in.



Link to comment

One thing to note, once the surface mark is destroyed and a new mark is set from the UG marks, it automatically gets downgraded in accuracy. For instance, if it was a 1st order station, it would be a 3rd order at best. Thats because the surface mark was the survreyed point and once its gone, the accuuray would not pertain to a reset mark. Thats just the way it is, no matter how good the reset was the original is gone and so its the quality.

Edited by Z15
Link to comment

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

  • Create New...