Alright lets say I grab a deed from the 1800's and they had a Magnetic declination as North..?ÿ We only find ONE natural monument today. Can we hypothetically retrace (AKA Reestablish) that boundary using GNSS today.?ÿ
We all know we can go from Magnetic to Astronomical to geodetic (LAplace Corr)?ÿ north and then to Grid (Convergence) (Based on NAD83 SPC) but what would the steps be to also take into account the survey was done in the 1800's and that means we would be jumping between two different Magnetic models for starters.
So just a little brain teaser for a rainy day!!!!
https://www.ngdc.noaa.gov/geomag/calculators/magcalc.shtml#ushistoric
This addresses the changes between dates. Note the highlighted section. Note as well that access requires registration.
Output in CSV format shown below. Registration is not required to run the tool. The registration request is an attempt to justify their support for the tool.
As can be seen in the image file, output is provided in 5-year increments. FWIW, in 1880 the declination was 15 degrees and 3 minutes and in 2023 it is 11 degrees and 16 minutes. Of course the tool provides CSV in decimals not DMS.
Love that tool. Yes it addresses the dates issue for sure. One thing you mentioned is dms vs decimal degrees is one of the common blunders i have seen on the math computation side. Another is failing to consider the convergence angle to get to grid. I have seen the magnetic declination done correctly but then they proceed without adding or subtracting the convergence angle for geodetic to grid. The laplace is usually such a small amount not significant in the big scheme for this scenario but almost always missed in the computations. I was riding a ride at our local fair with the kids and just started pondering this. So its all pure hypothetical scenario. Thanks for replying for sure. Always great to possibly have others that might could use that tool or get exposed to the tools out there. I have used the app mobile version when doing recovery several times just to kinda stay on the bearing when retracing to aid in evidence discovery.
Mathematically yes, practically no. Local needle attraction. Use of Jacobs staff at best. Who knows what method at worst. Not to mention transcription errors. The math solution is last resort and in this case possibly not a resort. If there is no evidence of establishment which is something that is very rare then I might suggest the math solution as part of a boundary agreement if the result looks reasonable. I don't believe the math is enough evidence to certify a survey without the owners consent. Being in a PLSS state may also be a consideration relative to the north call because the PLSS line would control deed north before the compass.
Glad you're discussing this. I'm currently researching some CCRs concerning a certain section line and its origin.
Please note ORIGINAL SURVEY BY J.M. GOULD 1848 VAR. 4°30' E.
Using a NOAA's magnetic field calculator, my input and output were
Latitude: 28°42'46" N
Longitude: 81°34'04" W
Elevation: 0.0 ft GPS
Date Declination
( + E | - W )
1848-01-01 4° 9' 45"
2023-01-01 -6° 36' 13"
Does this mean the magnetic bearing is now 2°26'28" W?
First of all, I am no expert on magnetic variation nor bearings. I am also familiar with the form and the source of its contents.
I do see that the CCR section indicated as Original Survey 1948 indicates a variation of 4.5 deg East. This differs from the tool output you show of 04 deg 09 min 45 sec (or 4.75 degas).
As these are modeled values, it would be best to know which model was used.
Both these values are declinations NOT bearings. They are applied to the bearing to account for the difference between the compass needle pointing and “true” values.
See image below regarding how to correct a bearing for declination.
Also on that tool the magnetic model clearly states 30 minutes. Something to consider. While we can apply that numeric declination correctly from math. We must consider the uncertainty of the model itself. And we must understand how uncertain the actual measurement was with a compass. I have used a compass on and off for most of my career both land surveying side and geodetic. Nope its not geodetic accuracy but it sure has saved me from math blunders for a simple quick check when doing computations. Love using one on rural boundaries for aiding in searching for evidence and walking the bounds. Its a tool like any other one. I imagine it has helped me find corners on many occasions and back when we comped by hand our traverses saved my rump on control blunders. Ya see i am not the most edumacated so i made a lot of mistakes calculating.
