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Magnetic North to Grid North  

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Firestix
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Best free software to convert magnetic north (1983) to grid north?  

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GeeOddMike
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Bill93
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Try the NGS tool kit program

https://www.ngdc.noaa.gov/geomag/magfield.shtml

to get geodetic, and then apply mapping angle theta for the SPC or UTM or whatever you are using.

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Nate The Surveyor
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Just a few thoughts.

We usually put into each job, in the field book, a note that looks like this:

1-2 357-1/2 ø SC # 4

What that meant was point number one, to point number two, a non-declinated magnetic azimuth of 357 and 1/2 degrees. We would put these into our books, periodically, at various locations of a job.

It mainly served to tell us which compass, because their declination were not identical.

But, it led to the knowledge that 1/2 degree differences, on points 1/2 mile apart were also, fairly common.

These days, we use rtk (Javad) and shoot a shot about somewhere near out base, and the note reads: "145-1/2ø to base"

This gives us OUR declination. (Not necessarily THE declination) for that area.

Carry on.

N

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Firestix
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My situation is that I'm trying to follow an old map that uses 1983 magnetic north.  I, however, am using VRS (Trimble R3).   I can find the control corner (PK nail in the CL of road) but all the other irons that come off of that are less than easy to recover.  I shot the PK, download it, and then entered the bearings and distances from the plat to navigate to the other corners.   Obviously the "2 norths" did not jive.   So now I'm trying to convert the bearings and distances from magnetic north to Grid North.   Any ideas?

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dms330
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Posted by: Firestix

My situation is that I'm trying to follow an old map that uses 1983 magnetic north.  I, however, am using VRS (Trimble R3).   I can find the control corner (PK nail in the CL of road) but all the other irons that come off of that are less than easy to recover.  I shot the PK, download it, and then entered the bearings and distances from the plat to navigate to the other corners.   Obviously the "2 norths" did not jive.   So now I'm trying to convert the bearings and distances from magnetic north to Grid North.   Any ideas?

I'm not working with a lot of info here but I would tend to use the existing road centerline as my basis of bearing for recon purposes.  You never know what is being used for a reference north.  Sounds like some previous work on the job file got things going before this map, hard to say.

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Lee D
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This is one of the few situations where I'd consider using a site calibration. If you calculate everything from the found PK using your best guess of what North should be once you find a second point it should be all downhill from there (assuming the points are still there).

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Firestix
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Lee D,
Found the second point.  It was a solid 1' off!  😑 

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Dave Karoly
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What is 1983 magnetic north?

I suspect the free software will be pencil, paper, and maybe a calculator.

Using the NGDC calculator to get the magnetic declination on the date the magnetic directions were measured (1983?). Combine that with the mapping angle of your project to compute the rotation from magnetic to grid.

I would have to sketch it on paper to figure whether to add or subtract. Magnetic declination is east or west and mapping angle is plus or minus. Once you have the rotation angle and direction (clock wise or counter clockwise) you can rotate the magnetic coordinates to grid.

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Firestix
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What is 1983 magnetic north?

Dave,  I'm asking myself the same question.  The map was is dated, signed, and sealed April 26, 2001 but has a North Arrow on it that is labeled "1983 Magnetic".  

 

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Norman Oklahoma
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We may have a new leader in the clubhouse in the stupid basis of bearings statement sweepstakes.  

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Firestix
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Unfortunately, he recently died ... so I can't question him about it.

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