Magnets in PVC sleeve BMs

[+patw]

While logging some bench marks in Florence, OR, I found several iron rod marks in pvc sleeves with a small magnet beside the rod. These were small, rectangular magnets, about two inches in length, like you would find at Radio Shack.

 

What is the purpose of these magnets? Would a compass respond to them if they are some eight inches below the surface? Is someone using a metal detector to find the BMs and, if so, do the magnets help?

 

Curiouser and curiouser . . . . patrick

 

patrick & shirley

[Z15]

To be able to locate in the future with a magnetic locator. If the magnet is there, its because the rest of the materials are non-magnetic, i.e. rod is aluminum or stainless steet.

[+patw]

Yes, I figured it was to facilitate a latter search. Would a metal detector respond to a magnet better than to a comparably sized piece of metal?

I thought the more common rod used for bench marks was of iron(?)

Why do we not see these magnets more commonly? I have recovered rod bench marks in pvc sleeves in Florida, New York, Kentucky and New Jersey but found these magnets only in Florence, Oregon (?)

BTW, I read every one of your posts as well as those of survey tech, whether the topic itself is of immediate interest or not. Usually, it is of interest once I've read your inputs. Thanks for your contributions to the community!

. . . . patrick

 

patrick & shirley

[Z15]

Well, the use materials is up to the agency/surveyor setting the mark(s). Like most things, you have choice of many different materials. Steel rods as the lowest cost, aluminum is more expensive and stainless steel is the most expensive. Its usually boils down to cost. For instance I worked for the MDOT for 31+ yrs, one of our survey chiefs (I worked for 3 different) insisted we use 3/4" aluminum Bersten rods driven a minumun of 9 ft (below frost in this area), while another was alwas concerned with cost and would only use concrete posts or disk set into permanent objects (rock, bridges etc), even though we could prove to him using the driven rods cost less due to effort to construct. Other agency's were using concrete (for cost reasons only) so he would also. My crew of 2 could set 6-8+ marks in one day while his crew could only manage 2 or 3 because this had to hand dig the holes 5 ft deep by 12" dia. to meet, all we had to do was to use the gas driver and hammer them in, dig away the top 2 feet and install the PVC pipe and cover, most would take about an hour.

 

If you will notice there is a statement on the data sheets about the magenetic condition of the mark. They need to know if anything magnetic at the mark, for gravity surveys and other things they may want to do in the future that could be affected by this. You know what can happen to computer disks and anything magnetic?

 

While iron seems reliable, it is not for precision control. Iron will corrode, then attach itself to the surrounding ground and move with the frost (if you have frost) or the surrounding earth. What you may have found what you assumed was all iron (Steel) was proboably in fact copper coated steel rods. These were a low cost alternative to stainless steel and concrete but we tried to get these but no one is suppling them, we went with alumunum and it still cost about $9/3ft.

 

Anytime you deal with professionals (Engineers are the worst), there will always be a difference of opinion of how things should be done. We had a term we used amoung outselves, "Why do it right when we can do it wrong"

 

[This message was edited by elcamino on December 23, 2002 at 07:03 AM.]