Will compass isolate marker?

[eahousley]

I have noticed when reading some of the original datasheets for local BMs that many indicate that they contain a bar magnet. If I am near a marker, will a compass isolate its location?

 

I recently visited a marker that had a county witness post; so I knew the marker was probably within a couple of feet. My experience in this county is that they plant the post behind the marker 6 to 24 inches. It appeared that the ground had been disturbed near the marker which resulted in it being covered up. Because the ground is frozen and snow covered, I decided that I'd have to come back in the Spring with a shovel. However, I'd like to limit my digging.

 

Sure, I know I can poke around for it with a coat hanger or screwdriver, but I've been curious about this compass idea for other situations. How close do you think I'd have to be to it to affect a compass needle?

 

-- It's from aliens. I seen um. --

[survey tech]

The magnets are not large enough to deflect a compass beyond a distance of a few feet and most markers do not contain magnets anyway, although those in your area may be an exception to this.

[+Kewaneh & Shark]

Surveyors commonly use metal detectors to help locate subterranean survey markers, most of which, are made of iron of steel. Rebar and iron pipe are the most commonly used around here. More often than not, benchmarks are made of materials that cannot be found by a metal detector, i.e. brass caps, concrete posts, wood posts, and even stones, although many may have a steel of iron core that the brass cap is attached to. The magnets were placed with the benchmarks to help locate them with a surveyors' metal detector. (The metal detectors go crazy when they get near a magnet.)

 

Using a compass to search for a magnetized benchmark is a great idea. Mining surveyors used to use a tool called a dip needle in order to find ore deposits. A dip needle had a free-floating needle like a magnetized compass. The difference was that while a traditional navigation or orienteering compass is held horizontal, the dip needle case was held vertically with the needle floating horizontal to the ground, and the needle would dip up or down when it got near an ore deposit - down near an iron ore or other ferrous metal deposit and up if it was near a non-ferrous metal deposit like copper, silver, or gold. I would imagine that you should be able to use your comass in a similar manner.

 

Experiment with a refrigerator magnet and your compass to see how close you have to be to the magnet before the compass gets affected and how. You may have to be fairly close, and if the magnet buried with the benchmark is very deep (they usually aren't) your compass may not be affected.

 

Keep on Caching!

- Kewaneh

[+patw]

All of the magnets which I have found in underground bench marks have been of approximately 4x2x1 inches. They resemble those sold in packs of four in Radio Shack stores. They emit a much stronger field than a refrigerator magnet and would be more appropriate for your experiments. Good luck!

. . . . patrick & shirley

 

patrick & shirley