Buried Magnets

[+PFF]

I notice that many data sheets in my region indicate that a bar magnet was buried beside a station. In a case where a metal detector is getting confused by the #@&*% pull-tabs from beer cans, etc., would it be helpful to test an area using my compass?

 

A bar magnet, it could cause "north" to swing south, or even 90 degrees. This would be a pretty good indicator that the station is at this particular spot. Were the magnets buried for this purpose, and was the alignment specifically done to give an "artifical" reading which would be unmistakable?

 

-Paul-

[+mogle1]

The metal detectors used by surveyors are specifically designed to detect magnetic fields. A magnet beside a monument would make the detector "scream", especially at a high setting.

 

The old dip needle used by surveyors to find property corners was a compass in a case with a strap. You turned the compass until the arrow appeared (using a built in mirror) to be pointin up. Then you kept it in that orientation as you passed it over the area that an iron rod (property corner) was supposed to be. If the arrow moved to a downward position you knew you were over the corner and you started digging..

I sure do enjoy the way we do it now. With the dip needle you had to be real close to the actual position of the corner because it took so long to cover a small area, now I can search a 50' circle in no time at all.

Edited February 16, 2006 by mogle1

[Bill93]

I've looked on ebay for magnetic detectors, and so far haven't been willing to pay the $300 and up for a used Schonstedt or other major company's professional model. Are there any economy models that sense the static magnetic field (not treasure hunters that detect conductive metals) that are worth buying if you can settle for a little less sensitivity?

[Photobuff]

I've no doubt those Schonstedt units will scream near a magnet, but they'll also find re-bar and any other ferrous metal just fine. Problem is, they won't find a bronze benchmark very well. Not sure about buried magnets. The problem with magnetic fields is that they drop off tremendously with distance. It would take a very large magnet to be detectable with a compass more than a few feet away. The whole idea seem silly- just get a cheap metal detector that discriminates between ferrous and non ferrous metals, since how many benchmarks have a magnet buried with them? My experience has been that pop tops and aluminum cans produce such an obvious signal that there's little trouble telling them from benchmarks, that is, the signature is different. Hint- when you find a flattened can or other surface debris, don't move on, remove the can and check again. There might be a benchmark under it. What I do have trouble with is RR debris. If you have a lot of buried spikes and track plates, the area will be a pain to search no matter what. Those areas don't usually lend themselves to probing with a rod either.

[Z15]

The survey disks we used had a small round magnet glued to the underside of the survey disk.

 

The reason it is noted on the datasheet is that certain equipment can be affected by a magnetic field, such a gravimeters. You can't have anything magnetic in the mark, even a metal witness post will skew the data.

 

They indeed were for locating with a metal detector. Schonstedt was the only model worth it. Over the years were had other models but they all were junk and could never be repaired. In 30 yrs, I only had one Schonstedt fail and it was because one of my crewmen abused it by jamming into hard packed snow in sub-zero weather.

Edited February 16, 2006 by Z15

[+mogle1]

 

The reason it is noted on the datasheet is that certain equipment can be affected by a magnetic field, such a gravimeters. You can't have anything magnetic in the mark, even a metal witness post will skew the data.

 

This brings up a question. I've never seen a partial monument, the've either been there or completely missing. Do they have rebar reinforcement in them?

 

They indeed were for locating with a metal detector. Schonstedt was the only model worth it. Over the years were had other models but they all were junk and could never be repaired. In 30 yrs, I only had one Schonstedt fail and it was because one of my crewmen abused it by jamming into hard packed snow in sub-zero weather.

 

That crewman of your must be pretty stout, the Shonstedts I've seen and used are mighty tough.

Edited February 17, 2006 by mogle1

[mloser]

None of the monuments I have seen have been reinforced. I have hammered a couple apart too, so I have seen inside them. That includes the 1930s-1940s precast ones as well as the cast-in-place ones.

[+PFF]

That crewman of your must be pretty stout, the Shonstedts I've seen and used are mighty tough.

 

Never underestimate the ability of a co-worker to screw up a piece of equipment!

 

-Paul-

[Z15]

That crewman of your must be pretty stout, the Shonstedts I've seen and used are mighty tough.

 

They have a electronic circuit board in them.

 

Never underestimate the ability of a co-worker to screw up a piece of equipment!

 

Thats for sure. We always had what they called CO-OP's. College student summer interns and our equipment always took a hit. One summer just as the Coops came on board we got a new 2001 Dudge 3/4 ton 4x4 ton PU. The PS assigned one of them to drive it. Within in a month, he had cut a tire, bent the rear bumper, trashed the inside with MacDonalds garbage and montain dew bottles full of tobacco spit not to mention getting it stuck some place he had no business driving it into.

Edited February 17, 2006 by Z15

[+mogle1]

Never underestimate the ability of a co-worker to screw up a piece of equipment!

-Paul-

 

That's why I am liking the single person robotic survey system. I break it I have to fix it, therefore I am very careful with my all my equipment.

When Europe throws some more satelites in the air and that L3 frequency gets added I may go to RTK surveying. My single frequency system is pretty amazing with the accuracy it gets, so I can't wait.

 

Sorry, I fell off subject there.

[+Woodbutcher68]

That crewman of your must be pretty stout, the Shonstedts I've seen and used are mighty tough.

 

They have a electronic circuit board in them.

 

Never underestimate the ability of a co-worker to screw up a piece of equipment!

 

Thats for sure. We always had what they called CO-OP's. College student summer interns and our equipment always took a hit. One summer just as the Coops came on board we got a new 2001 Dudge 3/4 ton 4x4 ton PU. The PS assigned one of them to drive it. Within in a month, he had cut a tire, bent the rear bumper, trashed the inside with MacDonalds garbage and montain dew bottles full of tobacco spit not to mention getting it stuck some place he had no business driving it into.

 

When you make it idiot-proof, the idiots will prove you wrong!

[+Airmapper]

Hey Woodbutcher68, read my sig line.

 

I helped a surveyor find a traverse point nail, he had his detector, and the nail was well covered, the person who placed it didn't so much as flag a nearby tree, let alone put up a marker steak. It amazed me how far that detector picked up the nail. He said it would work with Benchmarks as well. (He said it could pick up Brass, even if the mark didn't have a magnet.)

[kc2ixe]

...snip... marker steak. ...snip...

 

Maybe some animal came along and at the steak - Was it raw, or cooked? (Medium rare please, with garlic and parsley)