NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Deviation Card with GPS
From: Lars Bergman
Date: 2006 Jul 26, 07:14 -0500
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".
Lars
59N 18E
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From: Lars Bergman
Date: 2006 Jul 26, 07:14 -0500
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".
Lars
59N 18E
--~--~---------~--~----~------------~-------~--~----~
To post to this group, send email to NavList@fer3.com
To , send email to NavList-@fer3.com
-~----------~----~----~----~------~----~------~--~---
