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Magnetic North to Grid North
Best free software to convert magnetic north (1983) to grid north?
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25 Replies
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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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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 ?? 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: “145-1/2?? to base”
This gives us OUR declination. (Not necessarily THE declination) for that area.
Carry on.
N
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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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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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Thermometers in a bin, at hardware store, don’t all read the same. Sign above says “guaranteed accuracy”.
Compasses are a little the same.
Unless carefully checked.
The person you are following, and your own compass may not read the same, if tested.
N
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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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So was Mr. 2001 retracing a 1983 survey?
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Or maybe he was just showing the magnetic declination per a 1983 topo quad? Sounds like there are some more questions to be answered before applying math.
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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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Unfortunately, he recently died … so I can’t question him about it.
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but all the other irons that come off of that are less than easy to recover.
This implies to me that you have found at least one iron?? If not, are there any features shown on the old plat that you can find today (e.g. the road with the PK nail)? If the answer to either is yes, then go out with a compass and get the magnetic bearing between the PK nail and the iron or feature. Use the program that GeeOddMike graciously provided and compute the differences between today’s declination and the 1983 declination and 2001 declination. Then correct your compass bearing to each. I’m sure you’ve talked to the land owner to see where they think the boundary is located.
Also, as someone has suggested the 1983 bearing may be from a topo quad. Look at the USGS Historical Maps web site and see if there is a topo quad for 1983. If so, find out the declination on the map. You can look at other historical topo maps for the area to see how much it has changed over time. Here’s the link.
USGS Historical Topographic Map Explorer
Good luck!
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No, this was an initial survey for land that was deeded to an individual.
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Gene,
There is another PK nail that is referenced in the map. It’s just located in the center of a busy country road where everybody drives fast in a big truck, towing a boat, and on the phone. Luckily it’s less than a 10th of a mile from my home. I may just sneak out there late at night an GPS it.
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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?
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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Not many surveys were done with magnetic north in 2001. A common method is to use the most recent quad map to look up the magnetic declination. This is still done today in some places to meet outdated requirements to have the magnetic declination on the plat, or just out of habit.
A 2001 survey using magnetic bearings would be very unusual. Was it done by a landowner, or an engineer, or an architect?
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A 2001 survey using magnetic bearings would be very unusual. Was it done by a landowner, or an engineer, or an architect?
Your opinion must be based on local custom or your state standards. In NY (and probably every other metes and bounds state), there are plenty of 2001 surveys as well as 2019 surveys done with some sort of magnetic bearing basis.
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So, a little research and a little more info. This isnt a boundary, per se, but a private road put in by a surveyed centerline and an offset on either side. Irons along the centerline have been set (aparently twice) from the same decesed surveyor. I was able to find the pk nail (POB) and the first iron. Unfortunately there is a 1 foot difference from my measurment between the pk and first iron and what the plat shows. We also found another iron in the centerline (with flagging like the first) that is no where near anything on the plat.
Headaches …. just headaches.
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The 2001 survey was, in fact, a resurvey of his own 1983 survey.
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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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