How come the published radials for way points differ to what you would read off the map. For example in the Pooleys guide EGAE is given as "BEL 117.2 314 39" which I interpret as saying that EGAE is 314 degrees magnetic from the Belfast VOR with a distance of 39miles. Yet if you were to plot that line on the map you would get 311 magnetic. This isn't just in Pooleys, if you look at waypoints on the CAA approach charts you'll find that there is a difference of a few degrees as well. Any ideas as to what I'm missing here
I get near as 314 degress as I can differentiate with the Jepp Low Alt Enroue chart and an IFR plotter. Dumb question because you already mentioned magnetic, but are you using the VOR's magnetic hook/pointer?
Yes, I've taken magnetic into account, i.e., I'm drawing a straight line, measuring the true heading and then adding the variation (local to the vor). I am, however, using the CAA VFR sectionals and I've tried it on both quarter and half mil.
OK, maybe I have got the wrong end of the stick here
Quote:
I'm drawing a straight line, measuring the true heading and then adding the variation (local to the vor).
but why would you want to do that? The compass rose surrounding a VOR on a VFR Map is already in degrees magnetic, not true. Is the BEL VOR located on an aerodrome? (Not got a chart handy at the mo) If so, it's probably not going to be right in the centre of the aerodrome. Draw a line from the centre of the VOR to the centre or EGAE. Where does the line intercept the VOR compass rose?
Also, how old is the chart/pooleys that you're using. Remeber the magnetic variation changes from time to time.
Edited again to say I dont mean to sound patronising
I don't have my instrument plates to hand, so I can't quote an exact example, but from one plate to another, the radial of a specific point from a VOR can vary by a couple of degrees, even on plates published by the same publisher, for airports in the same country, with the same date.
For example, if you look at a selection of Aerad plates for airports like Southampton, Alderney and Guernsey, as well as comparing these to the Aerad airways chart covering the same area, and find the radial from SAM to ORTAC, I think you will find that it can be either 206, 207 or 208 degrees.
Unless you have a glass cockpit, I wouldn't worry because you can't set the OBS or CDI that accurately! If you do have glass, though, then you're bu**ered!
AV8, the VOR compass rose on the chart increments in 10 deg units, so I'm simply using the true heading + var for accuracy. The charts that I'm using are current, as is Pooleys. I've no doubt that I can replicate this on any of the CAA sectionals so I must be missing something
FFF, I see what you are saying about the descrepancy. I first noticed this when I was first attempting to use a GPS and entering my own waypoints by using radial/distance (hence the accurate take off the map). Then after I had taken note of the radial/dis for a VRP I came across a ref to it in a CAA visual approach plate and noticed that mine was different. Since then I've used L/L as waypoints references but it still confuses me.
Drauk, that's a nice tool. My charts are up to date. Look at the current approach chart for EGAE http://www.ais.org.uk/aes/pubs/aip/p...s/32AE0807.PDF It shows Dungiven as being R312 from BEL. Again, if you measure it you'll find that it's 308M
G B Z, I'm not really sure what you're saying but I take it that it has something to do with a flat map of curved surface??? If this is the case I'm not sure that a distance of 38 miles would show up an error of 3 to 4 Deg. I'm phyically drawing a line from the centre of the VOR and extending it to the waypoint. Then with the protractor I measure the true heading and add the variation to it.
I know that VOR radials are shown as magnetic radials, that's why I'm adding the variance (that is the var' where the VOR is located) to the measured true track from the radial, and the reason that I'm not just putting a ruler over the vor and directly reading the magnetic heading is because the VOR compass rose has only 10 deg increments!
"This may sound like a daft suggestion but are you allowing for convergence relative to the projection of the VFR chart you are using?"
Heavy stuff, GBZ !
Isn't convergence only applicable to Great Circle tracks on a Lamberts.
Usual half and quarter mil CAA charts are Mercator...........
The tolerance (or allowable error) from a VOR is +/- 4 degrees The tolerance (or allowable error) on a OBS is +/- 1 degree
If you were 60nm from the VOR and 1 radial (degree) off track and both the VOR and your kit were exactly correct that equates to 1nm lateral displacement which, whilst IFR would be iffy*, VFR would let you see what you were supposedly "over"
* Which is why it would never be used for an IFR approach
Are you measuring your angle to the ARP or a runway?
Rustle, as you say, my issue is a bit academic but I was just concerned that my meaurment of a radial differed by up to 4 deg from that as is published in the "current" Pooley and CAA maps. Needless to say I haven't gone through every one but those that I looked at differed to what was published, so I was wondering if I was missing something, i.e., could I trust myself to measure a VOR radial in the air if I was trying to pinpoint my position. Look at the link that I posted earlier. The link is to the current CAA chart for Londonderry airport (EGAD). Dungiven is a VRP to the south east and it is also used for an instrument app. The radial is shown as R312 from BEL but if you measure it you'll find that it's actually 308. This isn't a "one off" as I've come across it in Pooleys where it gives you the position of an airfield referenced to a nearby VOR and again there's a difference of up to 4 deg for all the references, no matter what the airfield is.
What/why are you adding for your conversion from mag to true?
The correct way to read a VOR from a chart is with a chart plotter lined up to the magnetic flute on the top of the VOR rose. When I measure the examples you are giving I get with a degree (subject to my eyesight) of the published radials.
So I suspect you are not measuring correctly or have a misunderstanding of how to plot a VOR radial on a chart. You do not measure true and then add variation you measure mag and you plot mag. Thats what the flute on the VOR rose is for.
OK, I have just replicated what you are doing. You have a fundamental misunderstanding of how to plot a VOR radial!
If I draw a line from BEL to EGAE and measure true course, I get 305d add 6d from the variation on the chart and I get your 311d radial.
If I use the radio nav plotter from my Jepp IFR kit on the flute on the VOR radial I get 314d.
The VOR station is corrected for the variation at that point. The chart gives an average across the variation lines.
You should not plot VOR radials in true and convert you should take them straight from the flute as this is the information displayed on your instrument.
Thanks for all your posts. They have all been helpful. I agree, put the plotter in line with the compass rose and off you go. BTW the errror is more noticeable on the 1/4 mil chart. I did, however, assume that by lining the plotter that way, or adding the variation, or reading the radial of the chart or whatever, that they would all produce the "exact" same result (in theory). I'm obviously too pedantic
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