Compass bearing corrections.

C

clarke belt

Guest
#1
Installation of motorised dishes has caused a lot of headaches and a lot of words have been written to help people overcome the problems and when you have succeded in doing it yourself its a great feeling so I thought I'd add a bit more to the endless subject.

As if the job isn't difficult enough for newbies they then discover that their trusty little compass is not pointing to true North and a number of degrees has to be either added or subtracted depending where they live.

This is because the compass points to Magnetic north and not true north the difference is called magnetic deviation and the points on the globe where there is no difference run down from eastern Scndanavia through the centre of Italy and the lucky installers there don't have do any extra calculations.

Its quite simple really you just remember the rule, if you live to the west of the aformentioned zero line as we do in Britain then just add the extra degrees to the compass reading for example Liverpool UK at (53.4degNorth, 3.1 deg West) the magnetic devation is plus 7 degs so to for instance to find 1deg west we add about 7deg to the compass reading and that gives us the azimuth which should only leave the dish tilt (elevation) to worry about, of course I'm just talking about one satellite location here and achieving the correct arc has already been covered extensively.

For the benefit of people living the other side of the zero line they just subtract the number of degrees, and there are charts in good satellite books showing contour lines with the correction values on the map of Europe, and I have one but its a bit too large to copy here.

The change is a linear one and to give an idea of the distance west or east for a given deviation here are the figures for UK. 'London and south east' add 5degs, 'Birmingham and midlands' add 6 degs,' North west england' and 'Scotland' add 7degs, 'Northern Ireland' add 9 degs.

Sighting up satellite positions with a compass is prone to slight inaccuracies and we all like to give that final tweak either on a signal meter or if possible a portable TV on a long lead to the sat receiver.

I hope this additional information is helpfull.

clarke belt.
 

Burnham Beech

Specialist Contributor
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#2
I'm curious to know where you got your information from. Are you sure it's 7 degree deviation at Liverpool in year 2001 ? Have you looked at a recent Ordnance Survey map of the area to check.

Burnham
 
S

sat__watcher

Guest
#3
If Liverpool is approximately N53.00.00 W003.00.00, then the Magnetic Variation (not Deviation, that's a hobby)today is 4.45 degrees west. No I'm not some smart alec know it all, I looked at

http://www.best.com/~williams/magvar.html

and fed in the co-ordinates. There is a small annual change to the amount of variation, so if you want to be really accurate, use the link above.

sat__watcher
 
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