Magnetic DeclinationPosted by allen-wrench on February 16, 2012 at 3:11 pm
I’ve haven’t had to calculate this yet, but I may have to on an upcoming job. We need to show “true” north and magnetic north. From what I understand, “true north” is sort of a b.s. term because it can be interpreted many different ways.
I understand how to use the NOAA calculator and I understand the concept of magnetic declination, but the correction is always referenced to “true” north, and I don’t know which version of north they mean. I’m sure many of you do this on a regular basis and will be able to help me out.
P.S. – I am certainly impressed by how active this site is compared to another site I frequented as a “point of beginning” (if you know which site I mean) for my survey info. This site is a great resource.
- 29 Replies
- MemberFebruary 16, 2012 at 3:22 pm
My experience leads me to believe that “True” North is the direction from where you are to the axis of the earth on the “North” end of the Earth.
In general Astronomic North as established by a star shot is close enough.
- MemberFebruary 16, 2012 at 3:36 pm
True north is usually expressed as astronomic or geodetic north. To get astronomic north you can observe the Sun, Polaris, or even another star and this is the true north that was used to lay out the PLSS. It is based on pointing north at the north pole or the axis of the Earth’s rotation extended into space. Geodetic north can be obtained by tying into existing NGS control or using GPS. North for it is the north of the ellipsoid.
Of course, today, GPS is the “best” solution. By “best” I mean quick and accurate. It doesn’t teach much about what you are doing so it isn’t as much fun. Back in the day I did astronomic observations all the time. I kind of miss it actually. As far as true north being a b.s. term I would have to say NO it isn’t. There will be slight differences between true north when you observe it astronomically and geodetically but they should be very close; within seconds.
- MemberFebruary 16, 2012 at 3:46 pm
I agree with MightyMoe- GPS is the way to go. You may have already said this, but did your client request bearings be tied to true north?
- MemberFebruary 16, 2012 at 4:00 pm
There is someone in our office that says the GPS defines north as geodetic/astronomic north when it is not calibrated.
Does that mean if you do a one-point control (as we call it, a “here” position) and use assumed coordinates (no projection/no datum) and draw a line at an azimuth of 0 degrees, that line will be geodetic north?
I’ve never thought about what direction north is when it’s not calibrated; just considered it to be a random orientation.
- MemberFebruary 16, 2012 at 4:03 pm
> I agree with MightyMoe- GPS is the way to go. You may have already said this, but did your client request bearings be tied to true north?
It just says show north arrow and “locate” magnetic north. I’m just getting into the details for my own understanding.
- MemberFebruary 16, 2012 at 4:07 pm
In a short answer, it’s close. I’ve had several opportunities to compare my ‘one point’ data against my own, and other’s data on several projects. I generally find somewhere in the neighborhood of 15″ to 30″ seconds.
What I don’t understand is the longer the time between surveys, the difference seems to get greater. If I go back to a job that’s 6 months old, there’s only about 15″ of difference, If it’s been two years…there’s more. That makes me scratch my noggin.
- MemberFebruary 16, 2012 at 4:15 pm
I would tie it to grid north with GPS and report the SPC with positional accuracies. Then I’d put a note that gives the current magnetic declination at the time of survey and make sure the note states that the bearing are not magnetic. It is 2012, right? Make it easier on the rest of us, just in case we have to follow your survey on day.
- MemberFebruary 16, 2012 at 4:15 pm
Be sure you don’t do a “here” shot and then set it up using pseudo state plane coordinates. You could wind up with grid north.
- MemberFebruary 16, 2012 at 4:31 pm
If you are using Trimble (I think Topcon and Lica also work in a similar way) you can have any projection in your job and the inverse function in survey will report grid and true north. As far as no projection, no datum-that’s not something I do anymore (haven’t in many years) but I think it’s giving you true north-but I’m not sure. If I were you I would check that out which is pretty simple.
It will look like this in TGO:
This is a file in a state coordinate system which is the grid az. The number you want would be the fwd az under details. That would be the true north direction from point 2033 (about a 54′ rotation from grid). As you can see the back Az is quite a bit different from pt 2000 to pt 2033.
- MemberFebruary 16, 2012 at 4:59 pm
A true north arrow does not mean much without a Basis of Bearings statement. You are probably not doing a sun or star shot, so my guess is you are using GPS. Many programs will give you grid bearings. If that is what you are showing on your map you need to state what convergence (theta angle) you used, and what point on your project was used to calculate the convergence. Remember that the convergence varies depending on where you are at. I’ve worked in areas where the convergence is 2 1/2 degrees and areas where it was close to zero.
The reason for the basis of bearings statement is so that a subsequent surveyor can recreate it if he/she needs to tie into your work.
- MemberFebruary 16, 2012 at 5:00 pm
You probably already know this; but, just in case I feel compelled to say something.
Establishing “Magnetic North” is much more complex than just going out and holding up a hand compass. To do the job properly it takes many readings from several locations (all tied together of course).
From what you describe it sounds like whoever wrote the description of the work needed had no clue what they really need and meant.
- MemberFebruary 16, 2012 at 5:02 pm
Magnetic declination (variation) is at best a moving target. William A. Burt addresses the issue of diurnal variation in his book “A Key to the Solar Compass,” pages 26-28. Burt clearly shows that “magnetic declination” can vary 10s of arc-minutes during a single day, and therefore coming up with a “daily average” requires numerous observations that would still only be applicable to THAT POINT, THAT DAY!
Inasmuch as the Earths magnetic poles MOVE as a function of time, even that data would NOT be applicable to THAT POINT, on the same day, of a different year.
Compound all of that with the fact that the declination of ONE POINT on a given project, may or may NOT be the same as it is on ANOTHER POINT on the same project (even on the same day). I have seen 6 DEGREES of variation in as little as 200 feet, and 4 or MORE degrees is routine in a quarter of a mile (at least in my project areas).
Generating Geodetic North using GPS is a breeze, and one should be able to do that within a few (~3-6) arc-seconds without too much trouble. You can then get a “pretty good” ESTIMATE of Astronomic North by applying the Horizontal Laplace Correction computed by DEFLECT09 to your solved Geodetic Bearing:
But bear in mind that the DEFLEC09 values are based on a MODEL, and you need to factor in the relative uncertainty inherent in you geodetic (GPS) positions as well. If you REALLY want Astronomic North, then you need to do an Astronomic Observation.
I guess it all depends on what you REALLY NEED, and how far you are willing to go to get it. Plus or minus Half a Degree on a given point (w/ date stamp), is pretty easy. Getting any “tighter” than that, is another thing.
Your mileage will of course vary.
P.S. And don’t forget a “basis of bearings” for BOTH your Geodteic/Astronomic AND Magnetic bearings (i.e. metadata). The ol’ “how I dun it.”
- MemberFebruary 16, 2012 at 5:15 pm
Does your client even know the differences in all these Norths you are referencing? What’s the greatest convergence out there for the GRID -> TRUE? Here in MA I’m at an edge of the state and it’s about 1 d, 3′.
These are my north arrows for some GIStastic maps I created of a quickly eroding barrier beach (5′ per day for 400 days). The magentic north here gets rotated to the date of the location, which is referenced on the map itself.
I also added grid lines so the average GPS user can work with the data a little. But I am a jerk, my grids are on the SPCS, so the average GPS user will need a little help to change his system…
- MemberFebruary 16, 2012 at 5:47 pm
I take back what I said earlier and refer you to Loyal’s post. He said what I wanted to say only much better than I could have.
Y’all have a nice day.
- MemberFebruary 16, 2012 at 7:01 pm
magnetic north is a myth. don’t waste your time chasing it, you will only die tired. go grid, go now and don’t look back.
- MemberFebruary 16, 2012 at 7:03 pm
cptdent’s link is a good start, but be cautious in using that data on anything that you are going to CERTIFY to in a professional sense.
The World Magnetic Model (WMM) upon which that data is (apparently) based is maintained by the National Geospatial Intelligence Agency (NGA), and updated every 5 years, and is NOT intended for “SURVEY Grade” applications.
Here’s a cut-n-paste from the above link:
“It is important to recognize that the WMM and the charts produced from this model characterize only the long-wavelength portion of the Earth’s internal magnetic field, which is primarily generated in the Earth’s fluid outer core. The portions of the geomagnetic field generated by the Earth’s crust and upper mantle, and by the ionosphere and magnetosphere, are largely unrepresented in the WMM. Consequently, a magnetic sensor such as a compass or magnetometer may observe spatial and temporal magnetic anomalies when referenced to the WMM. In particular, certain local, regional, and temporal magnetic declination anomalies can exceed 10 degrees. Anomalies of this magnitude are not common but they do exist. Declination anomalies of the order of 3 or 4 degrees are not uncommon but are usually of small spatial extent. See EMM home page for a model which includes crustal fields down to 50 km wavelength.”
The EMM Model mentioned above (Enhanced Magnetic Model), is maintained by the NOAA National Geophysical Data Center, and this model gets the wave lengths down from ~3000Km to ~56km. That’s still NOT Survey Grade in my opinion.
That is NOT to say that this data isn’t useful, it IS, you just have to understand its limitations and proper applications thereof.
- MemberFebruary 16, 2012 at 8:17 pm
I have an old book at the house on finding magnetic north. It’s dated probably in the 1940s or 1950s, and it really is quite involved for an undertaking. Book is probably 3/8″ thick, and it’s detailed.
Contact me directly if you’re in dire need of the kosher way of doing things …
- MemberFebruary 16, 2012 at 9:53 pm
One GPS Point Cannot Define North…
…Astronomic, geodetic or grid. You need at least 2 GPS points.
As far as magnetic declination goes, astronomic or geodetic differ by such small amounts that it does not affect reported magnetic declination.
Paul in PA
- MemberFebruary 16, 2012 at 11:10 pm
> You probably already know this; but, just in case I feel compelled to say something.
> Establishing “Magnetic North” is much more complex than just going out and holding up a hand compass. To do the job properly it takes many readings from several locations (all tied together of course).
> From what you describe it sounds like whoever wrote the description of the work needed had no clue what they really need and meant.
> Larry P
This is exactly the case. Some architect threw it in the requirements for a site topo of maybe 2 acres. I guarantee they don’t know why they requested it. You know they’re not going to orient their apartment building to anything but the property lines, so magnetic north is just a waste of time. I’d love to ask them what they plan on doing with the magnetic declination adjustment.
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