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Generic Dude
Posts: 11
Joined: 1/22/2009
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Magnetic Variance on GLO Plats |
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I am confused about the magnetic variance labels on the old GLO plats. The general layout sheet that shows the whole township has the variance labeled on every section line east and west. It's different for each mile.
I'm confused how they came up with these variances. Could someone with a good knowledge of history please explain this to me? Why so much difference from mile to mile?
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Wednesday, February 11, 2009 at 9:38:27 AM |
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Richard
Posts: 8
Joined: 10/23/2008
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Re: Magnetic Variance on GLO Plats |
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Dude,
I don't know how they did it in the GLO, but the Coast survey practice was to establish the astronomic meridian using the sun or Polaris and compare that to the magnetic meridian determined using a declinometer, a kind of surveyor's compass. Take a look at http://www.lib.noaa.gov/researchtools/subjectguides/function.html#GEOPHYSICS for numerous articles on "magnetic declination."
Richard
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Wednesday, February 11, 2009 at 1:54:23 PM |
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Isleno
Posts: 40
Location: Gonzales, La USA
Joined: 10/20/2008
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Re: Magnetic Variance on GLO Plats |
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| The magnetic variances were measured by the surveyor. A "true" north line was established by a stellar observation, usually Polaris. The solar compass was then used to observe the line and the needle card was adjusted so that the needle indicated north when the compass was aimed on line. It is different for each case because it was measured in the field. Many surveyors had a "true" line established at the base camp and they would use that variation everywhere in the township.
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Wednesday, February 11, 2009 at 1:57:23 PM |
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Generic Dude
Posts: 11
Joined: 1/22/2009
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Re: Magnetic Variance on GLO Plats |
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I guess I should have worded my question better. I understand the method by which they determined true north, and the application of variances, but I don't understand why it would be noted on each mile of the township. What I thought they did was determine the variance a few times per township and use pretty much the same number for the whole thing. Upon further review of the notes, it says that they used a Burt's Solar Compass.
It almost seems like they figured declination at each section corner. The problem is, why so often, and when the declination value varies so wildly within the township, why not just use an average number for that township?
Thanks for the help everyone.
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Wednesday, February 11, 2009 at 2:18:30 PM |
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jwahl
Posts: 8
Joined: 1/31/2009
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Re: Magnetic Variance on GLO Plats |
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Well it may depend on some more info, since there may be more than one reason. One reason you may see variations listed on each mile is that the survey was done with a solar compass. (which appears most likely your cae. With that instrument, an orientation on the sun was made at most setups and the magnetic variation could actually be measured and recorded. It was not used most of the day, but if the sun was obscured, cloudy weather, or within an hour of noon the instrument could not get an orientation then the needle could be used. They could also actually run by fore and backsights for periods of time. This is common in some areas of the country such as Michigan where the solar compass was used by it's inventor Wm Burt.
There are cases and points in history where bearings were not returned on E-W lines random and true lines but the line was corrected back by adjusting the declination using simple rules. So the guy would run east and at a certain distance say 80.12 chs, fall 24 links north of the section corner. He could then adjust the declination a specific amount based on the falling and run back compass west supposedly then on true line. Later on this practice was changed to correct the bearing and actually run back on a bearing.
Recently someone showed up with a survey that reported various declinations along the line, this was another way of reporting changes in bearing.
Reading the notes usually provides a clue was to what was done, and sometimes the equipment being used is reported in the notes also.
- jlw
PS. Why is another question. In Michigan it seems like they combined the Public Land surveys with geological surveys. And the wild variations that Burt and others noted actually were one way iron deposits in the UP were discovered. I think there are probably some good on line resources about Michigan surveys and Burt. There is also a book about him that is pretty good I think it is called "They left their mark".
Another possible reason to report it was so that other surveyors who might come along to subdivide sections or retrace the lines with an ordinary compass could do so more easily.
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Friday, February 27, 2009 at 9:28:36 PM |
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