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jcboliveira

Magnetic data updating

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Hi All. I have a question about magnetic data in FS04. It is rather old now and in most locations the magnetic variation does not match current terminal procedure chart values and runway magnetic headings. I would also believe that some navaids are now not accurate in their magnetic settings.As far as I know there is the magdec.bgl and there is some more magnetic data in the airport sections of the scenery bgl files. My questions deal with how do they interact and who uses what and when. 1. What function does the magdec.bgl file provide? 2. What function does the magnetic data provide in the scenery files? 3. Does one override the other? 4. If one updated the magdec.bgl file to current values would that be a good thing to do? 5. Would the mag data in the scenery files also need updating?6. Is there a site that explains all this and or provides tools to accomplish magnetic updates?There probably are other questions to ask but I am not up to speed enough to know what they are so please feel free to expand on my starter questions.Thanks for your thoughtsTerry

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Hi Terry,In FS9, LOCAL magvar data apears in APxxxxx.BGL files. These files define the airport data facility, the AI paths and (starting with FS9) visual parts of the airports (runway, taxiawys, approns, etc). If you decompile such a file with BGLXML you will see that elements as waypoints, navaids and airports have a maddev parameter. For example, if you decompile AP945180.BGL you will find, that the magnetic deviation for LPPT (Lisbon International Airport is 5.00 degrees).The referred to LOCAL value does not agree with the BASE value of 6.50 that you find in the file magdev.BGL (whose values are very old - 1993). This magdev.BGL files has a grid of values that apply for a whole 1 degree x 1 degree square. The value of 6.50 for Lisbon is the one corresponding to N39 W009 and should be used by FS in the square from N38 30' to N39 30' and from W09 30' to W08 30'.Your question could be rephrased - how FS uses these 2 values? My guess is that is uses the LOCAL value except when there is none available. In that case it uses the BASE value. In order to verify this, I could go to Lisbon, Runway 3. The true heading of the runway is (taken from the same AP945180.BGL file) is 22.67. If the compass uses the LOCAL magnetic deviation of 5.00, it should read 27.67. If the BASE value is used it should read 29.27. If I press SHIFT-Z to see the magnetic heading I see 29 degrees! I would expect 28 degrees! I am confused! I would like to have a more precise reading of the compass! Is it possible?From the point of view of a scenery designer I would enter the updated value in the LOCAL file. I will wait for some clarification on this issue before starting to use a new corrected magdev.BGL. Another question is - do 3rd part add ons use magnetic deviation in the same way as FS? Regards, Luis

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Ok, time to clear up some confusion...> in most locations the magnetic variation does not match current > terminal procedure chart values and runway magnetic headings.That's rather harsh. I think it's ok in most places, only wrong in a few...> 1. What function does the magdec.bgl file provide?Makes your compass work. Your compass needs to know what the magnetic deviation is, otherwise it would show your true heading.> 2. What function does the magnetic data provide in the scenery files?reference only. I think it comes up in the reference box, but it's also useful for the airport designer as "true heading" + "mag var" = "runway id". If there is a mismatch, either your data is wrong, you made a mistake somewhere else, or (in very rare cases) the runway hasn't been updated to reflect a recent mag var change.The airports in FS2004 that report wrong mag var are most likely mistakes in the reference mag var value rather than anything else. > 3. Does one override the other?Nope. I hope you know now why.> 4. If one updated the magdec.bgl file to current values would that be a good thing to do?Yes and no. Updating the data should be ok, but there will be little use in most cases. Ok, the data may be 10 years old, but the magnetic field isn't changing all that fast. I hacked into the magvar data a while ago and found that updating it for New Zealand would make barely any difference. However, there may be areas on the earth where an update would have an actual effect on the compass (especially close to the poles).Something to be aware of is that some airports may use an old mag var value and would have to be updated including runway ids (although one could argue that a correct compass value is preferrable to correct airport runway ids). In real life, airports do indeed change the runway ids to keep up with the magnetic variation, but this doesn't happen all that often as change is slow. Some runways close to the North Pole have a runway id of eg 03T ,to indicate it's 30 deg true heading (T is not an option in FS2004). 5. Would the mag data in the scenery files also need updating?Well, only if they are wrong or old. They should only reflect what's reported in real life diagrams to be accurate. Also, some runway ids may need updating if they are wrong (again, they should reflect real life). Some of the values in FS2004 are wrong as is. For example, I found that NZNV has a wrong reported mag var, and since MS gets true heading from runway id - magvar, the true heading was wrong also. However, if your runway ids and true heading are correct, an incorrect mag var has no effect what so ever (apart from reading wrong in the dialog box and maybe AI aircraft use them, although I doubt that).6. Is there a site that explains all this and or provides tools to accomplish magnetic updates?No site I'm aware of. However, if you want to create your own magvar.bgl it's actually not all that hard, I have uploaded a manual here at avsim. Search the file section. I've never done one, since it's not worth the hassle for the regions I fly in.I hope this makes sense, and, Luis, sorry for contradicting what you said, but I hope you understand now why you get 29 deg for Lisbon.Cheers, Christian

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>I hope this makes sense, and, Luis, sorry for contradicting>what you said, but I hope you understand now why you get 29>deg for Lisbon.Hi Christian,Thanks for discussing this. If I correctly understand you, the data found in magdev.BGL is the one used for the compass operation. The data found in the local BGL is not used. Therefore changing magdev.BGL to reflect today values would make the "FS compass" to read 28 degree in Lisbon as a "real compass" does today.Using your document, Jos

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LuisI went a litle deeper in this problem and this is what I understood. The Magvar in real life is updated each 5 years, the next update will be 12/2004. The airport charts will be updated in January (I hope) that should also be done in rwy designations. This data will became fixed in the charts for the next 5 years. So I believe that in January for Lisbon the 28

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