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Dave-- I've found frequent instances where the magnetic course for a leg of a flight plan seems to change by a few degrees even when the endpoints don't change by much. For example, planning AKO-CYS gives me a magnetic course of 299 degrees, which is the same as what's shown on the chart for the appropriate Victor airway. But if instead I plan TUMBL-CYS, which is a course along the same airway beginning 25 miles west of AKO, I get a magnetic course of 303 degrees, which is different by four degrees! I note that the true course only appears to change by 1 degree (312 to 311), so is this perhaps more than a rounding error associated with the magnetic variation? The change seems a bit large to me, and the variation in the area is only 9-10 degs anyway. Any idea what's causing this? -EDZ P.S. Skyvector is a spectacular product! I am a huge fan and have sent the link to every pilot I know.

EDZ, Thanks for your email. This is a subject deserving a lengthy discussion. First of all, its important to understand where the data comes from. If your waypoint is a VOR (or a VOR fix), variation is "installed". When they installed it, they bolted the beacon to the ground with a certain variation. Magnetic variation wanders quite a lot, and after a few years it can be several degrees off. Since airways and fixes are defined by VOR radials, adjusting the "installation variation" is a big deal and very rarely done. Any heading SkyVector gives you from a VOR is always adjusted to the "installation variation" This is supposed to always match the heading on airways departing from the VOR, but there are a few odd cases where the math doesn't work. If your waypoint is defined by the NFD (National Flight Database) it probably has a magnetic variation declared in the same database. In all cases, the value from the NFD is used. Most named fixes have a variation defined in the NFD, which is an estimated value calculated from a mathematical model. Airports in the NFD may have a surveyed value. Whatever the source of the data, SkyVector will always treat a variation defined by the NFD as canonical. GPS waypoints, or waypoints without variation values in the NFD are assigned a variation calculated by SkyVector. This is done using the IGRF10 mathematical model of the Earth's Magnetic field. Strict adherence to the rules described above can create some strange effects. Some airfields' surveyed variation is 30 years old and off by more than a few degrees. In your example, AKO has an installation variation of -13 degrees. According to the IGRF10, the calculated variation for AKO at 40.155555556N103.179750000W at 4620MSL on Jan 1, 2010 is -8.123 degrees. Hope this answers your question. -dave

Thanks for the quick and detailed reply! It answered my question completely. Using these rules, however, means that the headings on the legs of the flight plan mean different things practically, depending on what the waypoints are: sometimes it's "what you set the OBS to," while other times it's "where you set the heading bug if there's no wind." It might be nice (in some distant-future update) to indicate by color code when installed variance vs. current variance is used, or perhaps have an option to use the current variation for VOR waypoints. (Actually, a more useful option now that I think about it would be to show the true instead of magnetic course on the chart.) Thanks again, -EDZ

I agree. Just think what would happen if surveyors used magnetic bearings to determine property lines, subdivision boundaries, and where to put the freeway! Another huge issue is the problem of many semi-skilled pilots using GPS fixes as waypoints, all getting their data from similar databases. It's surprising that there are not more collisions! The 'old way', as imprecise as it was, was obviously a lot safer. But I'm not a pilot, so what do I know?

I 've heard the same thing, That changing practices, especially GPS-coupled autopilots and well-known fixes are increasing the risk of collision. Its definitely worth being aware of, but I haven't heard any hard-and-fast numbers on the subject. The first thing we need to do is get a name for it. Kinda like CFIT, Something embarrassing and scary.

hey, it took me a couple of weeks but I finally developed an acronym for using GPS waypoints that may bring you into close contact with other aircraft using that same nav technique: I'm gonna call it PRESI = Pilot Relying Entirely on Silicon Intelligence. gimme another month and I might think of a better one, CFIT really has no parallels in the inhabited universe . .

To update what I previously replied: Magnetic variation for GPS waypoints or other waypoints without a database variation is now calculated according to version 11 of the International Geomagnetic Reference Field (IGRF11) Please keep your eyes out for any anomalous results.

Hi Dave: I see you are now using the national geophysical data center IGRF11 model data - I assume the coefficients are just an array from the IGRF2010 model and for your purposes you do not require any other base year models or secular variation data. Did you originally just port the fortran code from the subroutine IGRF11SYN in igf11.f, or the routines shval3 and dihf from the C program geomag70.c, or did you write your own source copied to javascript or php. I note your comments with regards to calculation of magnetic declination (or variation as we say in aviation) and as the magnetic field is quite complex, it would not be unusual to see large variations, which for some AFD data can be off by quite a large amount (double in some cases as I've learned from analysis) as the magnetic field is constantly changing from year to year. It's probably more complex than most pilots realize. I am only curious as i have been reviewing geodetic formula for the same purpose and have been reviewing the NGDC fortran and C code for the purpose of turning the model and equation into code. I know they don't warrantee the code, but it seems quite convoluted for what it does even though the model is complex. I am in the process of converting it now when i thought of SV and wondered if you had done that already. I have a little ems project which will give me a quick bearing/distance guide as an xref to the aircraft gps before it and the crew get going. It also gives me the benefit of google hybrid maps so i can get a decent idea of what the area looks like that i'll be flying into, and a bit of weather enroute from the FAA ADDS and FTP metar sites. I can do all this from my iphone which is nice and quick. Any guidance appreciated. Fantastic project you have going here. Any comments appreciated.

We use the geomag70.c code modified only for integration. We left all the algorithms alone. The code performs well and, more importantly, has been extensively tested. In terms of allocation of resources, we definitely have bigger fish to fry.

I hear ya! Well i'm porting it to javascript and going to try to clean up the code a bit based on the fortran program which is the original beast. The algorithms are straight forward enough but the code definitely shows it's scientific fortran origins :). when I get a working version based on the IGRF11, I'll let you know. You never know, it may be more useful for any future integration plans.

I hear ya! Well i'm porting it to javascript and going to try to clean up the code a bit based on the fortran program which is the original beast. The algorithms are straight forward enough but the code definitely shows it's scientific fortran origins :). when I get a working version based on the IGRF11, I'll let you know. You never know, it may be more useful for any future integration plans.

I have a working version in javascript using the wmm2010 model. uses an external file for the coefficients or a sub function. Returns dec/dip. If you interested, let me know.

Sorry i hadn't logged in but that's me with the javascript for mag dec/dip

Dave, I'm about to plan a VFR flight from my home airport, KRLD (Richland, WA) to L06 (Furnace Creek, CA). I thought it might be interesting to plan it with a couple tools, like: SkyVector, Sectionals, GPSMAP 196, GNS 430, etc., tabulate & compare the results. I could send you the spreadsheet if I had an email I could attach it to. Thanks, Mike


Why can't the sectional magnetic deviations be used? They are a lot more up-to-date?

Its actually 2 different questions. The first question is: Should we ever use a predicted variation over a surveyed variation? Our opinion is no. A surveyed value, even from 20 years ago, is going to be a better value than the predictive model. The model is very low resolution, and represents a good “average” value for the region. A locally surveyed value will be a measurement of the effect of the local geological structures at the exact location. We trust that the FAA will re-survey the variation if the value becomes too far out of date. The second question is: Are we using the most up to date values? The date for the Isogonic lines on the sectionals is labeled separately in the Legend, and is currently “2010” for most of the sectionals. Those lines are computed using the World Magnetic Model. For named fixes, we use the values in the Coded Instrument Flight Procedures (CIFP, formerly NFD), which do change every 28 days. For example, the variation of LACIL is 18.4 East in the 1202 cycle of the CIFP, but in the previous 1201 cycle it was 18.8 East. We use the CIFP value even though we could compute a more accurate mid-cycle value. We only use the predictive model for GPS waypoints, and then we use the current time. So its not surprising that in 2012 you could have SkyVector give you a variation that doesn't agree with the Sectional's isogonic lines, even if we didn't compute it ourselves. The last thing to remember is that with VORs, it's not variation at all but installation declination. Declination is set at installation and very infrequently realigned. When they do that, all of the airways using the navaid get new radials, so it's a pretty big deal. If you use SkyVector to flight plan from a VOR, you'll get the VOR's installation declination as the variation for that waypoint. Otherwise the heading wouldn't match the radial. In areas with rapid change, you can sometimes see a dramatic heading change in your flight plan when tracking a radial through a fix, even though there was no turn. The first leg's heading was adjusted to the VOR's declination, and your heading for the second leg was adjusted to the CIFP variation for that fix. Don't forget to correct according to your airplane's compass card and leave your iPad smart cover on the ground.