Can anyone tell me how to calculate the magnetic deviation from OS grid north for a location in the Dales (West Kinsgdale)?

I have found this calculator which gives me deviations, but I am not sure what they are using for North (true North?).
http://www.ngdc.noaa.gov/seg/geomag/jsp/struts/calcDeclination

How do I use the numbers from here to get the correct deviation from the OS grid north at this location?
Latitude:    N 54.19156667 Longitude:    E 2.46271667

How do people go about actually measuring it in the field too? 

If you take a look at the OS map you are using (OL2 for kingsdale) the key gives you a host of numbers for working it out.

My OL2 map says:

At the centre of the S and W sheets true north is 0?04' and 0?20' east of grid north respectively.

Magnetic north is estimated at 3?33' and 3?28' west of grid north respectively for July 2004.

Annual change is approximately 13' east.

Hope that helps.

Magnetic deviation is that which attracts other cave surveyors to menacer.

I needed to know mag deviation a week or so back. I easily found a link to a calculator which gave me deviation from grid, and as I was drawing relative to True, I just referred to the paper map to get the grid to true variation. I think the link was to the BGS site (Keyworth).

Thanks, I assume it was this one:
http://www.geomag.bgs.ac.uk/gifs/gma_calc.html

That will enable me to verify my information from the NOAA site which lets me specify any historical date.

Not sure exactly when but at some point in the next few decades it will decrease to, and then pass through zero - for a brief period the two will be so close as to be essentially the same.

Presumably at any point in time there is a North/South line running round the globe where magnetic and true north are the same bearing. It is just that as the magnetic pole moves about this line also moves. So the question is when will this line cross over the UK? Even then, there will probably be a deviation from grid North for whatever grid you are using. So the places where grid North aligns with magnetic North are going to be more complicated to determine. There may be several or there may be none? It's all to do with projecting flat grids onto the curved surface of the Earth. So grid lines should be parallel to each other, where as lines aligned along magnetic North/South will converge towards the poles (which is why magnetic deviation differs across the width of a map). Wow, now my brain hurts 

footleg said:

Presumably at any point in time there is a North/South line running round the globe where magnetic and true north are the same bearing. It is just that as the magnetic pole moves about this line also moves. So the question is when will this line cross over the UK? Even then, there will probably be a deviation from grid North for whatever grid you are using. So the places where grid North aligns with magnetic North are going to be more complicated to determine. There may be several or there may be none? It's all to do with projecting flat grids onto the curved surface of the Earth. So grid lines should be parallel to each other, where as lines aligned along magnetic North/South will converge towards the poles (which is why magnetic deviation differs across the width of a map). Wow, now my brain hurts 

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Yes - any point on the great circle that passes through the poles and the magnetic N pole.

We are also lucky at this period in the Earth's history to have a clearly defined magnetic field whose poles are so close to the rotation poles. The Earth's magnetic field has been at a variety of strengths (including zero!), orientations and even polarity changing overperiods of 10s or 100s of thousands of years.

If the human race's eye-blink technological history had happened at a different time the field may have been so weak or badly oriented that no one would have noticed it or been able to take advantage of it.


Always worth taking a bearing on a distant fixed point on the horizon that will not change over time and working from that - then magnetic deviation ceases to be time sensitive as long as all survey measurements are reduced relative to this fixed check bearing.

Different sets of measurements made over several decades could be 3-4 degrees out otherwise.

When I learned about map-reading as a teenager in the late 1970s I recall that it was 6 degrees, now it's about 3 so it should pass through zero sometime in the 2030s

Dep said:

Always worth taking a bearing on a distant fixed point on the horizon that will not change over time and working from that - then magnetic deviation ceases to be time sensitive as long as all survey measurements are reduced relative to this fixed check bearing.

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Aye, if you always calibrate your compass to a known fixed bearing then magnetic and instrument errors can be taken care of in one hit.

Yes, we are calibrating the instruments before and after each trip against fixed targets near the cave. But I need to determine the magnetic deviation from grid North in order to correctly align the survey with the OS grid, as that is what my GPS entrance fixes are being recorded against.

Year after year, surveying more or less every week, we did this, and never saw any noticeable variation, until one evening it was about 1.5 degrees different, just for that one occasion. It may be rare, but when it does happen, you can be sure a solar storm will mess up the most vital surveying session of them all.

Footleg,

If the positions you are calibtrating the compass to/from are known (either by GPS or because you can see them on the map) then you have already produced your correction to Grid North, as long as the calibration sight is long enough to reduce the errors.

Simply measure the bearing between your cailbration positions (from high scale map, or calculation from grid positions) and the difference between the angle measured by the compass during calibration will provide the correction to grid for that particular session - i.e you roll up the calibration and correction to grid north in one go, bypassing the need to know the difference between magnetic and grid north.

Cheers,

Ed

True, but I was told to calibrate instruments on targets which are typical long survey leg distances apart, as people site differently onto distant objects. And the GPS positional error is too great over such short distances.

Still I suppose picking points at either end of the Kingsdale road and GPS fixing them is as good as sighting on distantly spaced natural features. Then having determined the calibration for that day, I can measure my closer spaced calibration targets and get a grid bearing for them.

Anyone got any better suggestions?

Peter Burgess said:

Year after year, surveying more or less every week, we did this, and never saw any noticeable variation, until one evening it was about 1.5 degrees different, just for that one occasion. It may be rare, but when it does happen, you can be sure a solar storm will mess up the most vital surveying session of them all.

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Unless that error just happens to give a better loop closure than the true reading 
As long as the data gives a good loop closure then no-one will ever know 

footleg said:

...as people site differently onto distant objects. ...

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I don't understand - a bearing is a bearing. If someone is sighting differently then they are doing it wrong!

Dep said:

footleg said:

...as people site differently onto distant objects. ...

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I don't understand - a bearing is a bearing. If someone is sighting differently then they are doing it wrong!

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It is not as simple as that.

You sight through a compass using both eyes, one through the viewing lens onto the compas reading and one onto the object you are sighting on. This gives an optical illusion of the sighting line in the viewer being superimposed on the point you are sighting to.

As a result of needing both eyes there are substantive differences if eyes are further apart or if you have different vision characteristics (short sighted, long sighted, lazy eye, etc) These will amount to different readings by different people and even different readings if you swap eyes. Calibration is the way to correct these errors, which if carried out correctly will result in accurate data.

Dep said:

If the human race's eye-blink technological history had happened at a different time the field may have been so weak or badly oriented that no one would have noticed it or been able to take advantage of it.

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But it can be argued (perhaps I can call it the anthropic principle) that a prerequisite for technological development was trade, and trade was driven by shipping, which was dependent on the compass. That is, if the magnetic field was weak or badly orientated, the technological development wouldn't have happened.

And of course, if the technological development hadn't happened, everybody would still be living in caves, which would be right pain for the cavers.

Chris

Les W said:

...
You sight through a compass using both eyes, one through the viewing lens onto the compas reading and one onto the object you are sighting on. This gives an optical illusion of the sighting line in the viewer being superimposed on the point you are sighting to.
...

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I don't! I use one eye to avoid such parallax errors.

To be truthful I have less experience of using this type of compass (I guess we're talking Suunto here) and more of using a theodolite - I was a surveyor for 5 years.

Using a handheld compass for map reading/navigation I sight across it with both eyes but then I'm not trying to read it that precisely.

But if I was trying to get an accurate reading for a critical bearing such as a distant object RO  then I would still try to lay a compass on using one eye (with the compass sat on something so it does not move) and then read the scale separately with both eyes looking straight down on the scale.

I have a Suunto in my possesion at present - I will go and fiddle to see if I can get what you mean...

The point about the way in which we use Suuntos is that they are hand held thus the scale must be read at the same time as the instrument is properly positioned; that is why both eyes must be used simultaneously - one on the far point and one on the scale. A tripod mounted instrument can be set up, properly positioned and then left while you have a cup of tea before the relevant figure need be read off the scale. It is this use of both eyes that means that an allowance needs be made for the user as well as for the instrument and for magnetic variation. All these factors can be rolled up into one global "correction" provided that this calibration is done on every trip and that on that trip the instrument is then used in the same way by the same person to take all the readings.

I recommend that you read 'The effect of binocular vision disorders on cave surveying accuracy' by Mark Dougherty, Cave and Karst Science Vol 33 No 2 2006. Using two eyes is great provided they both point in the same direction. The aforesaid binocular disorders may mean they do not, and from the paper it seems that could apply to about 20% of the population - and perhaps more on a morning after a night before! To achieve a grade 5 survey the surveyer should check that they don't suffer before taking any readings. If they do, or suspect they might, then using one eye is perfectly possible and eliminates the possibility of error. I find that two eyes can be okay provided I stay focused on the far station. However if my attention switches to attempting to read the Suunto's scale through the film of mud inside it, the reading can go haywire. To avoid a possible source of eror I always use just one eye for the compass switching my attention back and forward between the far station and the scale. Another way of eliminating binocular and parallax errors when using two eyes is to turn your head on its side so one eye is vertically above the other! I think that suggestion in teh paper might even have been serious?