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Local Magnetic Variation


Gnikhog
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Earlier this week while recovering MZ1477, I discovered an unusual localized magnetic variation of about -13° (over an above the normal local declination of -15°) at the station. Having found the station first, I was getting pretty frustrated when I could find neither RM 1 or RM 2 (24.7 and 25.6 meters distant respectively). Fortunately RM 3 was set in a boulder above ground and I was able to find this even though I had been looking for it some 4.6 meters away. As it turns out this was what led me to figure out what was going on. I couldn't believe that my compass work was this poor so I stood at RM 3 and took an azimuth reading of the station - it was exactly 180° from what the datasheet indicated - just as it should be. Next I went back to the station and took an azimuth reading of the now visible RM 3 and noticed the -13° variation. When I applied this local error to the azimuths for RM 1 and RM 2, they were quickly uncovered. Interestingly, this magnetic variation was repeatedly measured to be about -5° at RM 1, 0° at RM 2, -4° at RM 3, and -13° at the station using this forward and reverse azimuth methodology. (I acknowledge that my compass skills contribute an error term as well.)

 

I could see nothing visually (out in the forest) to create this very localized magnetic variation. Perhaps a local vein of lodestone or magnetite? Anyone had any similar experiences? What methods do professionals use to avoid these types of problems or would they be avoided all together through the use of distant (multi-kilometer) azimuth marks?

 

[by the way, this is not related to the azimuth errors in the box score for RM 1 and RM 3. The azimuths (and distances) provided in the 1937 station description are correct. I know that you know that they are 180° from todays 0° = North convention! Yes, I did send Deb Brown a note on the box score errors.]

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What methods do professionals use to avoid these types of problems or would they be avoided all together through the use of distant (multi-kilometer) azimuth marks?

 

Yes, I did send Deb Brown a note on the box score errors.]

Surveyors don't use compasses for creating geodetic quality survey data. They used an instrument called a Theodolite to survey all those angles in to very high accuracy, using trigonometric methods.

 

Deb may not concur with errors you may have noted with a Compass in an area you noticed magnetic anomalies. The Box Scores are full of data which has been subject to a least squares adjustment as well and fit the model. There is no basis for trusting a Compass over Survey Data with Applied Mathematics.

 

Cool observation though.

 

Rob

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Thanks Rob, you confirmed what I suspected that the pros do (or don't do in this case): they don't rely on the compass for anything critical.

 

Also, my note to Deb was not about the magnetic anomaly, it concerned a transcription error from the original monumentation text regarding the RM azimuths to what appeared in the box score. Two of the three box score RM azimuths are off by exactly 100°. The third is correct. Fortunately this datasheet contained the azimuth and distance data for the RMs in the original station description … not an uncommon practice for marks set and documented by the Massachusetts Geodetic Survey in the 1930's.

 

/John

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You mentioned the area was "in the forest". Is there a possibility of a buried pipeline, or other man-made metal objects underground? (I assume no overhead powerlines). I have noticed a huge (30 degrees, maybe) compass swing while walking on a fireroad, stopped, said "huh?", walked back, and sure enough - high pressure gas pipeline crossed the fireroad (these are thick walled, and about 18 inches in diameter). . I could see the surface markers (those "tent-like" signs on short poles - for aerial monitoring / checking), every half mile or so, coming from one side, going to the other side. No signs near the fireroad, which were maybe removed / destroyed by recent spring road repairs.

Edited by Klemmer & TeddyBearMama
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John,

 

Klemmer makes a great point. A pipeline will wind up as a magnetic anomaly, especially if it is fuel. It is also notable that magnetic north is not static nor consistent across the country. Because of its continued movements, what you wrote on a paper in the past would be rendered inaccurate over time by nature. So it isn't stable enough for use.

 

Good eye on the transcription errors. All the old data was transcribed to computer by hand, and these errors, though rare, are being found all the time.

 

Good Hunting!

 

Rob

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