Magnetic North Lines
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Magnetic North Lines
I ran in a level C event this weekend on a map, which I noticed later had magnetic north lines dated 2003. This was despite map updates listed as 2007, 2010 and 2014. I may be old fashioned but when I update a map my first task is always to check MN bearings and amend if necessary before starting to survey. My question is in modern day orienteering how important is the correctness of MN and when is the point reached when the Controller should insist that in the interest of fairness the MN lines should be corrected.
- Muddy Boots
- off string
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Re: Magnetic North Lines
I can't comment on how good you are at holding a bearing through a forest
but if the Mag. North variation on the map is 4 degrees from the actual Mag north
then IF YOU ARE PERFECT your deviation at the end of 200 metres will be 14.4 metres.
If you are not perfect then 14.4 metres is pretty much neither here nor there.
but if the Mag. North variation on the map is 4 degrees from the actual Mag north
then IF YOU ARE PERFECT your deviation at the end of 200 metres will be 14.4 metres.
If you are not perfect then 14.4 metres is pretty much neither here nor there.
- MIE
- green
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Re: Magnetic North Lines
Try about -1.7 in this part of Dorset - so about 7m for us
You will be lucky to tell any difference at all - it is almost irrelevant at the moment
You will be lucky to tell any difference at all - it is almost irrelevant at the moment
- Barny
- yellow
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Re: Magnetic North Lines
http://www.geomag.bgs.ac.uk/data_servic ... _calc.html
I'd rather deal with my navigating error plus 0 deg error on the map than both, which could be the aggregate
I'd rather deal with my navigating error plus 0 deg error on the map than both, which could be the aggregate
- Gnitworp
- addict
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Re: Magnetic North Lines
It is no big deal to rotate a map for MN movement. I can't understand, if a map is being updated, why it would not be done. A concern of mine would be the accuracy of the mapping, even if a remap. The competitor may stray on a bearing but the mapper should get it right.
- ianandmonika
- red
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Re: Magnetic North Lines
ianandmonika wrote:It is no big deal to rotate a map for MN movement. I can't understand, if a map is being updated, why it would not be done. A concern of mine would be the accuracy of the mapping, even if a remap. The competitor may stray on a bearing but the mapper should get it right.
I think there's a difference between an update by a mapper and map updates by a planner just to make a map fit for use. In the case Muddy Boots quotes, it was the latter.
- mikey
- diehard
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Re: Magnetic North Lines
All major updates that I do these days involve a geo-referencing of the map, I would love to do this for all my club's maps, but life is too short.......
Just remember the 2000? World Cup final in Portugal where the north lines were at least 10 degrees out.... and no competitor noticed.
Just remember the 2000? World Cup final in Portugal where the north lines were at least 10 degrees out.... and no competitor noticed.
- Big Jon
- guru
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Re: Magnetic North Lines
Let's not give people the wrong idea, it's no big deal to update the magnetic north and that should be done after no update for some years. We should not give the impression that it does not matter. That leads to sloppy mapping in the end.
- ianandmonika
- red
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Re: Magnetic North Lines
Big Jon wrote:remember the 2000? World Cup final in Portugal where the north lines were at least 10 degrees ou
It wasn't until that training camp in Feb following year that Stan was first to notice... and only because the excersise was all about remember the terrain
Go orienteering in Lithuania......... best in the world:)
Real Name - Gross
http://www.scottishotours.info
Real Name - Gross
http://www.scottishotours.info
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Gross - god
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Re: Magnetic North Lines
For a georeferenced map, correcting for the change in MN is relatively easy.
For a non-georeferenced map, things can be a lot harder. How many maps are you aware of which quote the correction for MN applied when the map was produced / last updated? I try and do this on mine (off the printed area), but sometimes this can be missed. It is easy to find out what the correction should be now, but determining the difference between now and what was applied when the map was created / last updated is much harder. As far as I'm aware, there is no website or app that can provide historical declination values for a location.
For a non-georeferenced map, things can be a lot harder. How many maps are you aware of which quote the correction for MN applied when the map was produced / last updated? I try and do this on mine (off the printed area), but sometimes this can be missed. It is easy to find out what the correction should be now, but determining the difference between now and what was applied when the map was created / last updated is much harder. As far as I'm aware, there is no website or app that can provide historical declination values for a location.
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Wayward-O - light green
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Re: Magnetic North Lines
I always check it in the field with a few long bearings. Watch out for local attraction (metal pipes, tanks, fences, pylons etc etc) if you do this, and iron ore in the ground can have a considerable 'local' effect. I believe a compass is useless on the Cullin Ridge on Skye because of this.
- Gnitworp
- addict
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Re: Magnetic North Lines
There is an IPad app that shows current Magnetic Declination on a world map. You can enlarge the map as required so declination in Cannock, for example, shows as -1.99 degrees.
- ianandmonika
- red
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Re: Magnetic North Lines
Using the 2003 "difference and the 2014 difference gives an answer of 1.5 degrees
(approx) in the variation of North lines on the map and the real magnetic north. This
results in a difference over 200 metres, whilst keeping to a perfect bearing, of 10 metres.
Not significant, methinks.
I can get a bigger variation in accuracy of angles when using a sighting compass
to survey but viewing with my sub-ordinate eye instead of my dominant eye !!
And if, like some mappers, you use a standard (non-sighting) compass .........
- MIE
- green
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Re: Magnetic North Lines
2014 WTOC will be remembered by many competitors for:
* Day 1 PreO having tiny MN lines that were only 1 or 2cm long.
At the TL briefing we were promised by the SEA they wouldn't be that short the next day
* Day 2 PreO didn't have any MN lines at all! Some noticed straight away, some not at all
and some part way round (control 9 for me I think it was - probably since I was so
used to using the control description lines on Day 1 that I did the same for Day 2).
Hint to those rotating maps for MN changes - you also need to certain symbols too as many of them are N-S aligned - it is annoying to use OOB (or veg screen) hashing for bearings and then find that they aren't N-S
* Day 1 PreO having tiny MN lines that were only 1 or 2cm long.
At the TL briefing we were promised by the SEA they wouldn't be that short the next day
* Day 2 PreO didn't have any MN lines at all! Some noticed straight away, some not at all
and some part way round (control 9 for me I think it was - probably since I was so
used to using the control description lines on Day 1 that I did the same for Day 2).
Hint to those rotating maps for MN changes - you also need to certain symbols too as many of them are N-S aligned - it is annoying to use OOB (or veg screen) hashing for bearings and then find that they aren't N-S
JK
- JK
- diehard
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