Bench Mark magnetic material?

I suspect (but really know) that it may be because Bench Marks are often used (observed) during Ground Magnetics Surveys. It doesn't take much magnetic material to create a nontrivial anomaly in a proton magnetometer reading. Of course many OLD BMs are set on Iron Pipes or steel rods driven to refusal, and I am uncertain about the new Stainless Steel Rods. Just a guess really.

edit. It might also screw up a Gravity Readings as well, and Bench Marks are the key to Gravity Surveys.

Loyal

The magnetic survey ideas makes sense. I'm not sure why magnetic material would affect the gravity meter, and am pretty sure it won't change the actual gravity by a measurable amount.

Well, I've added iron to very very few bench marks. and a few triangulation stations which tend to be the one that are seriously buried. Since it's just along side it is easily remedied with a pair of vice grips if anyone cares.

Bill93, post: 403371, member: 87 wrote: The magnetic survey ideas makes sense. I'm not sure why magnetic material would affect the gravity meter, and am pretty sure it won't change the actual gravity by a measurable amount.

Well, I've added iron to very very few bench marks. and a few triangulation stations which tend to be the one that are seriously buried. Since it's just along side it is easily remedied with a pair of vice grips if anyone cares.

My speculation concerning Gravity Measurements is along the lines of what effect a magnet may have on the Gravity Mater itself, not an effect on the gravity field. I vaguely remember something about that back when I was doing Gravity Surveys in the 1970s, BUT I'm not sure what I had for dinner yesterday...

Loyal

I've driven rebars by a few benchmarks or monuments that did not have any magnetic material in them. They took a long time to find, and I thought I was helping.

Just a few comments.
Last Thursday, I dug out a Section corner. It was a 1/2" rebar, a foot deep. very hard clay and rock soil. It was giving a mag signal, but not real good one. I set a magnet on the top, (See this: http://www.ebay.com/itm/lot-100-50-1-2-X-1-8-Neodymium-Disc-Strong-Rare-Earth-N48-Small-Fridge-Magnets-/182311618618?var=&hash=item2a729e883a:m:mq5RFaPvwEwVrMypFSSriqw ).
Then, I placed a fresh plastic cap on it.
Boy, does that thing SING now! Then, refreshed the witness trees.
I'm leaving footprints, for future surveyors, so that folks will not have to fuss about what I did...

Having said all that... what do you think about placing one of these little magnets beside the bench mark, so that it can be REMOVED, during uses, for specific gravity measurements?
Now, we can FIND it easy, and we have a mechanism, that can be moved away.
I'm thinking of a piece of PVC, with caps, and a magnet inside, that can be placed BESIDE the bench mark, and can easily be moved?

Several yrs ago, I dug out an AGC survey marker, (Arkansas Geological Commission) that my dad had set years ago, with my help. For whatever reason, there was NO magnet.
It also had a steel fence post driven about a foot north of the actual marker. Then, the landowner filled in the area, with 12-16" of fill.
Along comes surveyor A, and he uses his metal detector, and FINDS the broken off base to the fencepost, and USES it as the section corner. (of course, he does not describe it, because he is a minimalistic ... nevermind!). So, he used the fencepost base as the section corner.
Along comes Surveyor B. He does the same thing. Digs out the broken off steel fencepost, calls it the Section corner.
I come along, dig down, and I KNOW there is an aluminum AGC marker down there. I dig. And, Dig some more. THERE IT IS, all nice and purdy!
So, right before I leave, I place a MAGNET right beside the AGC marker. So, I am thinking, here we have done alot to perpetuate this position, and sure as feces, somebody is gonna muck it up.
I'm just musing out loud here.
What do you think of more extensive use of magnets, to perpetuate corners?
As cheap as they are, (and everybody with kids, or grandkids needs a bag of 100 of those high powered magnets!) Why not?

N

It seems sensible to increase the findability of any mark. Certainly for cadastral uses I've heard no objection. As noted above, my irons would be easily removable, as would be your magnets. Obviously you don't do it on one labeled "Magnetic Station" for fear of leaving some residue.

The NGS contemplates the possibility of magnetic material. From their document
http://www.ngs.noaa.gov/web/tools/updates/windesc5/dformat.documentation.htm#A.10
I copy this portion:
A.10 Magnetic Property Code
The magnetic property code is a one-character code which indicates the magnetic property of the mark or monument.

Magnetic Property Code Definition
A Steel rod adjacent to monument
B Bar magnet imbedded in monument
H Bar magnet set in drill hole
I Marker is a steel rod
M Marker equipped with bar magnet
N No magnetic material
O Other (see description)
P Marker is a steel pipe
R Steel rod imbedded in monument
S Steel spike imbedded in monument
T Steel spike adjacent to monument

Every time I find a non-ferrous mark I always drop a little rare earth magnet on top before re-burying.
What I hate the most is steel fence posts near the monument. How the heck am I to figure out where to dig if the fence post is within a foot of the monument? Much prefer carsonite posts.

Loyal, post: 403368, member: 228 wrote: I suspect (but really know) that it may be because Bench Marks are often used (observed) during Ground Magnetics Surveys. It doesn't take much magnetic material to create a nontrivial anomaly in a proton magnetometer reading. Of course many OLD BMs are set on Iron Pipes or steel rods driven to refusal, and I am uncertain about the new Stainless Steel Rods. Just a guess really.

edit. It might also screw up a Gravity Readings as well, and Bench Marks are the key to Gravity Surveys.

Loyal

While would be wary of setting a gravity meter on a monument with a magnet, it is not likely to have an effect. See note above taken from the Model G meter. This venerable instrument has been superseded by much more capable units most likely less susceptible to local magnetic sources.

See http://userpage.fu-berlin.de/geodyn/instruments/Manual_Lacoste_GDl.pdf

GeeOddMike, post: 403478, member: 677 wrote:

While would be wary of setting a gravity meter on a monument with a magnet, it is not likely to have an effect. See note above taken from the Model G meter. This venerable instrument has been superseded by much more capable units most likely less susceptible to local magnetic sources.

See http://userpage.fu-berlin.de/geodyn/instruments/Manual_Lacoste_GDl.pdf

Thanks Mike,

I couldn't remember the details, only that it was mentioned at some point.

Loyal

Help me out then.

I have struggled with this part of benchmark recovery when I run OPUS shared solutions. I have not, up to this point determined the magnetic property of a mark. There is not an UNKNOWN in the description list, so I've selected NONE - a very inappropriate designation. I often do not need to bring the Schondstedt out to find an easy to find monument. I'll need to put a detector in my bag. Suggestions?

FWIW,

I consider this field to be appropriate for the mark setter only. While, as you indicate, one can determine it with a detector.

I would choose "Other" and state magnetic status unknown in the recovery. As you also state, writing "None" is misleading.

As for Bill93's original post, I would choose another monument if possible due to paranoia about subsequently finding it had an impact on my survey. While at one time the USC&GS was the agency charged with magnetic surveys, this function was transferred to the USGS.

There is an interesting report on the impact of magnetic sources like railroad lines in leveling. At one time the suspended prism levels were manufactured with materials susceptible to local magnetic sources.

See: https://www.ngs.noaa.gov/PUBS_LIB/EmpiricalCalibrationOfZeissNI1LevelInstrumentsToAccountForMagneticErrors_TM_NOS_NGS45.pdf