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#1
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Great circle formulae, True cource and actual heading
Hi,
I am trying to write a formula to compute flight headings. If i use 2 lat/lon i can get a "true course" but due to magnetic variations the true course is not really all that useful, (knowing that my true course is 118deg will not tell me that i should fly 93 deg). What i mean is if i go from A to B i can calculate the True course, ('tc') but my actual heading varies a lot depending on my position along the course. Is there a reliable formula to calculate my actual heading at a certain point along the course? Many thanks in advance. Sims |
#2
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"Sims" wrote in
: Hi, I am trying to write a formula to compute flight headings. If i use 2 lat/lon i can get a "true course" but due to magnetic variations the true course is not really all that useful, (knowing that my true course is 118deg will not tell me that i should fly 93 deg). What i mean is if i go from A to B i can calculate the True course, ('tc') but my actual heading varies a lot depending on my position along the course. Is there a reliable formula to calculate my actual heading at a certain point along the course? Many thanks in advance. Sims At least in sci.geo.satellite-nav this sort of question about formulae has been asked numerous times, including quite recent threads, so a search via Google Groups would likely have already found you your answer. Take a look at Ed Williams page about Aviation Formulary: http://williams.best.vwh.net/avform.htm -- Dave Patton Canadian Coordinator, the Degree Confluence Project http://www.confluence.org dpatton at confluence dot org My website: http://members.shaw.ca/davepatton/ Vancouver/Whistler - host of the 2010 Winter Olympics |
#3
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Sims wrote: Hi, I am trying to write a formula to compute flight headings. If i use 2 lat/lon i can get a "true course" but due to magnetic variations the true course is not really all that useful, (knowing that my true course is 118deg will not tell me that i should fly 93 deg). What i mean is if i go from A to B i can calculate the True course, ('tc') but my actual heading varies a lot depending on my position along the course. Is there a reliable formula to calculate my actual heading at a certain point along the course? Many thanks in advance. Sims You mean your magnetic heading. You have your actual heading. You need a table of magnetic variations and interprolate from the table based on your location as there is no mathmatical model that will provide this information reliably. Dale -- _ _ Dale DePriest /`) _ // http://users.cwnet.com/dalede o/_/ (_(_X_(` For GPS and GPS/PDAs |
#4
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"Dale DePriest" wrote in message You mean your magnetic heading. You have your actual heading. You need a table of magnetic variations and interprolate from the table based on your location as there is no mathmatical model that will provide this information reliably. I found the post a little confusing because it seemed to be mixing the problem of the course heading changing due to the great circle route (mentioned in the subject) with heading changes due to changing declination mentioned in the body. Two very different issues. Steve |
#5
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Dale DePriest wrote:
You need a table of magnetic variations and interprolate from the table based on your location as there is no mathmatical model that will provide this information reliably. Given a table of magnetic variations at known locations, there are statistical methods to derive a useful mathematical model. The web site cited in a previous article provides such a model as a polynomial. http://williams.best.vwh.net/avform.htm#Var |
#6
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red rover wrote: "Dale DePriest" wrote in message You mean your magnetic heading. You have your actual heading. You need a table of magnetic variations and interprolate from the table based on your location as there is no mathmatical model that will provide this information reliably. I found the post a little confusing because it seemed to be mixing the problem of the course heading changing due to the great circle route (mentioned in the subject) with heading changes due to changing declination mentioned in the body. Two very different issues. Steve Yes, it was my mistake, I misread the original post by not comparing the text to the subject line. I tried to cancel but too late, Sorry Dale -- _ _ Dale DePriest /`) _ // http://users.cwnet.com/dalede o/_/ (_(_X_(` For GPS and GPS/PDAs |
#7
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"Dale DePriest" wrote in message
... Yes, it was my mistake, I misread the original post by not comparing the text to the subject line. I tried to cancel but too late, Sorry Dale Well I actually meant the original post was confusing, not your reply. |
#8
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Sims wrote: flight headings. If i use 2 lat/lon i can get a "true course" but due to magnetic variations ... actual heading varies a lot depending on my position along the course. ...formula to calculate my actual heading at a certain point along the course? As Steve has noted there are two things happening: (1) The bearing to point B along a great circle/ellipsoid path from Point A to B varies along the path. (A computer/autopilot can be set to follow the ever varying bearing.) (2) The Earth's magnetic variation (a.k.a. declination) that affects a magnetic compass heading varies with latitude and longitude and also with time. This magnetic field offsets the true great circle bearing at a given point along the path. If one follows a rhumb line course instead of a great circle path, the rhumb line bearing is constant. This is easier for a human to follow at the expense of having a longer path. One can find this on a Mercator projection map. Again the magnetic variation offsets the rhumb line bearing at a given point. As Dave notes, Ed Williams' site discusses these issues. http://williams.best.vwh.net/avform.htm The World Magnetic Model (WMM) maintained by the U.S. DoD and others http://www.ngdc.noaa.gov/seg/WMM/DoDWMM.shtml predicts the components of the Earth's steady state magnetic field for a given latitude and longitude for a given date. The horizontal component direction corresponds to the magnetic variation. The target accuracy is one degree over a 5-year period. My freeware DOS command line program GCGC* calculates the great circle bearings between two points (WGS-84 default) and then uses the WMM to also calculate the magnetic bearings at the end points. If one needs the true and magnetic bearings along the flight path (certainly a reasonable thing to need in flying), one would recalculate the great circle path to point B from the position at the moment. One can use the included Direct function (Lat1/Long1, Bearing 1-2, and distance 1-2 to get Lat2/Long2) to calculate positions along the great circle path by splitting it into pieces. I also have a program (gcb12) that does rhumb lines at my web site, but I haven't gotten around to adding the magnetic bearings. Cheers, 73, Ron McConnell w2iol N 40º 46' 57.9" W 74º 41' 21.9" Magnetic Variation = 13.0º W in October 2003 FN20ps77GU46 [FN20ps77GV75] * GCGC executable and source at http://home.earthlink.net/~rcmcc |
#9
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As Steve has noted there are two things happening: (1) The bearing to point B along a great circle/ellipsoid path from Point A to B varies along the path. (A computer/autopilot can be set to follow the ever varying bearing.) Thanks I was looking for a formula that does the same, so that i can tell my heading at a certain point on the course. I will look at the links given. Again many thanks all for the help. Sims |
#10
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"red rover" wrote in message .. . "Dale DePriest" wrote in message You mean your magnetic heading. You have your actual heading. You need a table of magnetic variations and interprolate from the table based on your location as there is no mathmatical model that will provide this information reliably. (Sorry to reply here your post was cancelled). Yes i do mean the magnetic heading. Sorry I was not clear. Where can i get a table to compute my magnetic heading? Thanks Sims. |
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