NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Deviation Card with GPS
From: Lu Abel
Date: 2006 Jul 30, 11:05 -0500
Gary:
Your car's compass works exactly like an electronic marine compass: A
pair of sensors give the direction of magnetic north regardless of which
way the car is pointing. This reading is, of course, subject to deviation.
In our list's discussion of developing deviation tables we've spoken
about sailing the boat in one particular direction and comparing the
magnetic direction (gained from a GPS bearing on a distant object or by
running a course between two known points) with the ship's compass.
There's another possible way to determine deviation, and that's what
electronic compasses use: Turn the compass at a uniform rate and read
off magnetic headings at a fast, but uniform, rate. Let's assume you're
making your turn to the right, ie, the compass reading is increasing
from 0 to 360. If, as you turned, you were pointed in a direction where
your compass had increasingly easterly deviation, the compass's reading
would increase faster than your rate of turn. Likewise, if you were
pointed in a direction where the compass had increasingly westerly
deviation, the compass's reading would increase more slowly than your
rate of turn.
Of course we don't know the boat (or car's) rate of turn, but it isn't
particularly important. The compass just stores a series of readings
until it senses that it's been turned 720 degrees. Then it can figure
out the rate of turn, from that what the average change in heading
should have been per sample interval, and by comparing the actual
readings to what they should have been in a deviation-free compass and
thence develop a deviation table which is immediately applied to
readouts from the compass, no need for a manual deviation table.
The only thing this technique depends on is maintaining a uniform rate
of turn. Easy to do on a boat, with a car I'd suggest a large empty
parking lot so there's no hindrance to maintaining a constant rate of turn.
While all this may sound complex, in reality it isn't a particularly
difficult thing to implement within the guts of an electronic compass.
Lu Abel
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?
>
>
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From: Lu Abel
Date: 2006 Jul 30, 11:05 -0500
Gary:
Your car's compass works exactly like an electronic marine compass: A
pair of sensors give the direction of magnetic north regardless of which
way the car is pointing. This reading is, of course, subject to deviation.
In our list's discussion of developing deviation tables we've spoken
about sailing the boat in one particular direction and comparing the
magnetic direction (gained from a GPS bearing on a distant object or by
running a course between two known points) with the ship's compass.
There's another possible way to determine deviation, and that's what
electronic compasses use: Turn the compass at a uniform rate and read
off magnetic headings at a fast, but uniform, rate. Let's assume you're
making your turn to the right, ie, the compass reading is increasing
from 0 to 360. If, as you turned, you were pointed in a direction where
your compass had increasingly easterly deviation, the compass's reading
would increase faster than your rate of turn. Likewise, if you were
pointed in a direction where the compass had increasingly westerly
deviation, the compass's reading would increase more slowly than your
rate of turn.
Of course we don't know the boat (or car's) rate of turn, but it isn't
particularly important. The compass just stores a series of readings
until it senses that it's been turned 720 degrees. Then it can figure
out the rate of turn, from that what the average change in heading
should have been per sample interval, and by comparing the actual
readings to what they should have been in a deviation-free compass and
thence develop a deviation table which is immediately applied to
readouts from the compass, no need for a manual deviation table.
The only thing this technique depends on is maintaining a uniform rate
of turn. Easy to do on a boat, with a car I'd suggest a large empty
parking lot so there's no hindrance to maintaining a constant rate of turn.
While all this may sound complex, in reality it isn't a particularly
difficult thing to implement within the guts of an electronic compass.
Lu Abel
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?
>
>
--~--~---------~--~----~------------~-------~--~----~
To post to this group, send email to NavList@fer3.com
To , send email to NavList-@fer3.com
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