NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Deviation Card with GPS
From: Richard B. Langley
Date: 2006 Jul 31, 07:46 -0500
This is discussed in the GPS World article I referred to earlier.
-- Richard Langley
On Sat, 29 Jul 2006, Gary J. LaPook wrote:
>
> From Gary LaPook:
>
>As long as we are discussing compass deviation and compass correction
>can anybody explain this one?
>
>My car has an electronic compass installed and the directions for
>correcting it for deviation (of course the car manual didn't use these
>technical terms) has you pushing a calibration button and then driving
>the car in a slow circle. How does the compass figure out the deviation
>from just the data it can capture while the car is driven in a circle?
>
>
>George Huxtable wrote:
>
>>Lars Bergman wrote-
>>
>>Some findings about Robert's deviation (having received his data off
>>list):
>>
>>Compass deviation could (approximately) be described by the equation
>>
>>dev = A+B*sin(cc)+C*cos(cc)+D*sin(2*cc)+E*cos(2*cc),
>>
>>where cc is the compass heading. Making a least square fit of Robert's
>>data (defining easterly deviation as positive) into this equation I
>>found the coefficients to be
>>
>>A=-1.9d
>>B=-18.5d
>>C=12.3d
>>D=1.4d
>>E=-1.5d
>>
>>Coefficient A represents a fixed offset, this could be due to e.g.
>>alignment error of lubber line or an errouneous variation value used.
>>Coefficients B and C represent permanent magnetic field components,
>>fore-and-aft and athwartship respectively. B and C are the main
>>contributors to the deviation in this case.
>>
>>The greatest deviation is found on a SE compass heading, amounting to
>>25d W. With such large values there are a few reasons to compensate
>>the
>>compass by external magnets, or finding a new location for the
>>compass,
>>or finding and removing the magnetic source creating the deviation.
>>
>>One reason is safety: If it for some reason is necessary to steer a
>>reversed course in a hurry, the easiest way is to make a 180d turn of
>>compass heading. With Robert's compass, worst case, the resulting
>>actual
>>change of ship's heading will be 43d off. This happens at cc=135 where
>>the deviation is 25W, thus magnetic=110. Turning to cc=315 the
>>deviation
>>is 18E, thus magnetic=333 instead of the desired 290d. I have never
>>seen
>>this phenomena described anywhere, and I don't think it is very well
>>known.
>>
>>Another reason is that the compass on certain headings will be "slow"
>>or
>>"fast", i.e. a certain change of actual ship's heading do not
>>correspond
>>to the change of compass heading. With Robert's compass we can look at
>>cc=205 with dev=5W making mag=200. If we make a ten degree on compass
>>turn to starboard we find the deviation of cc=215 to be 1W, thus
>>mag=214. Although the ship's head had turned 14 degs, the compass
>>shows
>>only ten. The compass is "slow". When changing course from cc=055 to
>>cc=065 we will find that the ship's actual change of heading is only 7
>>degrees; the compass is too "fast".
>>
>>=============end of Lars Bergman quote.
>>
>>Comment from George-
>>
>>As usual from him, there's good insight in Lars Bergman's analysis,
>>about Robert Eno's compass errors.
>>
>>The major problem that surfaces is that of the two major
>>contributions, B and C, which correspond to permanent magnetism,
>>perhaps originating from the engine.
>>
>>And it's the crosswise B component that's particularly worrying.
>>Presuming that the engine is mounted fore and aft, it implies a lot of
>>magnetisation, transverse to the engine block. To me, that's something
>>of a surprise. It prompts a number of questions for Robert to ask
>>himself.
>>
>>Is the compass (or perhaps even the engine) mounted a long way off the
>>line of symmetry of the boat?
>>
>>Does the compass share a panel with other instruments or components
>>close by that could be generating their own magnetic field? I'm
>>thinking particularly of a windsceen wiper motor, an inverter or
>>anything else with a transformer in it, loudhailer, analogue meters.
>>Is it mounted in a bulkhead, in which case, what's on the other side
>>of the bulkhead?
>>
>>Is the compass nearer to the engine than it really needs to be? For
>>example, can it be mounted close under the roof of the steering cabin,
>>just as high up as it can possibly go?
>>
>>Is there any vertical mast or other component close by, that's made of
>>steel?
>>
>>Robert has to be aware that he is especially vulnerable to the effects
>>of magnetised metal on board, because of his high magnetic latitude
>>and the resulting weakness of the horizontal component. And
>>particularly vulnerable to any nearby piece of vertical(ish) metal,
>>that might come to an end near to the level of the compass
>>
>>Lars puts an interesting slant in his analysis, that I haven't come
>>across before, in pointing out zones where the compass was "fast"
>>(turning faster than the boat does) and conversely slow at others. I
>>take it that Robert's course, in Frobisher Bay, when his wildly
>>varying compass showed up, was roughly Northwest. I might have
>>expected such instability to result from being in a particuarly fast
>>zone, but my understanding of Lars' analysis indicates a slow zone
>>around that direction.
>>
>>I agree completely with Lars, that this compass installation indicates
>>a particularly dangerous state of affairs. It needs some sort of
>>urgent remedy. I would think hard about installing some sort of
>>remote-reading compass up a stick, if it was my boat.
>>
>>I don't know how much change of latitude comes into Robert's voyaging,
>>but he should be aware that any deviation table or magnetic correction
>>will be local to his high latitude, and may need redoing if he
>>journeys far South (or North).
>>
>>George
>>
>>contact George Huxtable at george@huxtable.u-net.com
>>or at +44 1865 820222 (from UK, 01865 820222)
>>or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
>>
>>
>>
>>
>>
>>
>>
>>
>>
>>>
>>
>>
>>
>>
>
>
>>
>
===============================================================================
Richard B. Langley E-mail: lang@unb.ca
Geodetic Research Laboratory Web: http://www.unb.ca/GGE/
Dept. of Geodesy and Geomatics Engineering Phone: +1 506 453-5142
University of New Brunswick Fax: +1 506 453-4943
Fredericton, N.B., Canada E3B 5A3
Fredericton? Where's that? See: http://www.city.fredericton.nb.ca/
===============================================================================
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To , send email to NavList-@fer3.com
-~----------~----~----~----~------~----~------~--~---
From: Richard B. Langley
Date: 2006 Jul 31, 07:46 -0500
This is discussed in the GPS World article I referred to earlier.
-- Richard Langley
On Sat, 29 Jul 2006, Gary J. LaPook wrote:
>
> From Gary LaPook:
>
>As long as we are discussing compass deviation and compass correction
>can anybody explain this one?
>
>My car has an electronic compass installed and the directions for
>correcting it for deviation (of course the car manual didn't use these
>technical terms) has you pushing a calibration button and then driving
>the car in a slow circle. How does the compass figure out the deviation
>from just the data it can capture while the car is driven in a circle?
>
>
>George Huxtable wrote:
>
>>Lars Bergman wrote-
>>
>>Some findings about Robert's deviation (having received his data off
>>list):
>>
>>Compass deviation could (approximately) be described by the equation
>>
>>dev = A+B*sin(cc)+C*cos(cc)+D*sin(2*cc)+E*cos(2*cc),
>>
>>where cc is the compass heading. Making a least square fit of Robert's
>>data (defining easterly deviation as positive) into this equation I
>>found the coefficients to be
>>
>>A=-1.9d
>>B=-18.5d
>>C=12.3d
>>D=1.4d
>>E=-1.5d
>>
>>Coefficient A represents a fixed offset, this could be due to e.g.
>>alignment error of lubber line or an errouneous variation value used.
>>Coefficients B and C represent permanent magnetic field components,
>>fore-and-aft and athwartship respectively. B and C are the main
>>contributors to the deviation in this case.
>>
>>The greatest deviation is found on a SE compass heading, amounting to
>>25d W. With such large values there are a few reasons to compensate
>>the
>>compass by external magnets, or finding a new location for the
>>compass,
>>or finding and removing the magnetic source creating the deviation.
>>
>>One reason is safety: If it for some reason is necessary to steer a
>>reversed course in a hurry, the easiest way is to make a 180d turn of
>>compass heading. With Robert's compass, worst case, the resulting
>>actual
>>change of ship's heading will be 43d off. This happens at cc=135 where
>>the deviation is 25W, thus magnetic=110. Turning to cc=315 the
>>deviation
>>is 18E, thus magnetic=333 instead of the desired 290d. I have never
>>seen
>>this phenomena described anywhere, and I don't think it is very well
>>known.
>>
>>Another reason is that the compass on certain headings will be "slow"
>>or
>>"fast", i.e. a certain change of actual ship's heading do not
>>correspond
>>to the change of compass heading. With Robert's compass we can look at
>>cc=205 with dev=5W making mag=200. If we make a ten degree on compass
>>turn to starboard we find the deviation of cc=215 to be 1W, thus
>>mag=214. Although the ship's head had turned 14 degs, the compass
>>shows
>>only ten. The compass is "slow". When changing course from cc=055 to
>>cc=065 we will find that the ship's actual change of heading is only 7
>>degrees; the compass is too "fast".
>>
>>=============end of Lars Bergman quote.
>>
>>Comment from George-
>>
>>As usual from him, there's good insight in Lars Bergman's analysis,
>>about Robert Eno's compass errors.
>>
>>The major problem that surfaces is that of the two major
>>contributions, B and C, which correspond to permanent magnetism,
>>perhaps originating from the engine.
>>
>>And it's the crosswise B component that's particularly worrying.
>>Presuming that the engine is mounted fore and aft, it implies a lot of
>>magnetisation, transverse to the engine block. To me, that's something
>>of a surprise. It prompts a number of questions for Robert to ask
>>himself.
>>
>>Is the compass (or perhaps even the engine) mounted a long way off the
>>line of symmetry of the boat?
>>
>>Does the compass share a panel with other instruments or components
>>close by that could be generating their own magnetic field? I'm
>>thinking particularly of a windsceen wiper motor, an inverter or
>>anything else with a transformer in it, loudhailer, analogue meters.
>>Is it mounted in a bulkhead, in which case, what's on the other side
>>of the bulkhead?
>>
>>Is the compass nearer to the engine than it really needs to be? For
>>example, can it be mounted close under the roof of the steering cabin,
>>just as high up as it can possibly go?
>>
>>Is there any vertical mast or other component close by, that's made of
>>steel?
>>
>>Robert has to be aware that he is especially vulnerable to the effects
>>of magnetised metal on board, because of his high magnetic latitude
>>and the resulting weakness of the horizontal component. And
>>particularly vulnerable to any nearby piece of vertical(ish) metal,
>>that might come to an end near to the level of the compass
>>
>>Lars puts an interesting slant in his analysis, that I haven't come
>>across before, in pointing out zones where the compass was "fast"
>>(turning faster than the boat does) and conversely slow at others. I
>>take it that Robert's course, in Frobisher Bay, when his wildly
>>varying compass showed up, was roughly Northwest. I might have
>>expected such instability to result from being in a particuarly fast
>>zone, but my understanding of Lars' analysis indicates a slow zone
>>around that direction.
>>
>>I agree completely with Lars, that this compass installation indicates
>>a particularly dangerous state of affairs. It needs some sort of
>>urgent remedy. I would think hard about installing some sort of
>>remote-reading compass up a stick, if it was my boat.
>>
>>I don't know how much change of latitude comes into Robert's voyaging,
>>but he should be aware that any deviation table or magnetic correction
>>will be local to his high latitude, and may need redoing if he
>>journeys far South (or North).
>>
>>George
>>
>>contact George Huxtable at george@huxtable.u-net.com
>>or at +44 1865 820222 (from UK, 01865 820222)
>>or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
>>
>>
>>
>>
>>
>>
>>
>>
>>
>>>
>>
>>
>>
>>
>
>
>>
>
===============================================================================
Richard B. Langley E-mail: lang@unb.ca
Geodetic Research Laboratory Web: http://www.unb.ca/GGE/
Dept. of Geodesy and Geomatics Engineering Phone: +1 506 453-5142
University of New Brunswick Fax: +1 506 453-4943
Fredericton, N.B., Canada E3B 5A3
Fredericton? Where's that? See: http://www.city.fredericton.nb.ca/
===============================================================================
--~--~---------~--~----~------------~-------~--~----~
To post to this group, send email to NavList@fer3.com
To , send email to NavList-@fer3.com
-~----------~----~----~----~------~----~------~--~---
