Magnetic North to Grid North
Best free software to convert magnetic north (1983) to grid north?
Try the NGS tool kit program
to get geodetic, and then apply mapping angle theta for the SPC or UTM or whatever you are using.
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.
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.