Can anyone suggest a compass which can overcome magnetic interference
from, say, steels, electric cables in building? Thankyou. 

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Request for Question Clarification by iaint-ga on 15 May 2002 08:54 PDT

Do you specifically require a compass that can be used in indoor areas
with high magnetic fields, or would you simply require a method to use
a standard compass in such an area?

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Clarification of Question by budgie-ga on 15 May 2002 10:03 PDT

My problem is when I use a traditional magnetic compass in indoor
areas, the readings vary with my position of measurement, due to
different magnetic influence sources. Since I require accuracy in
terms of degrees, I don't know how to achieve this. The working
environment is assumed to have general magnetic influences, no
extraordinary high magnetic fields. Thankyou.

Hello,

Magnetic compasses work by aligning the needle to the magnetized field
of Earth.  The two poles, North and South, position the slightly
magnetized needle to a north-south alignment.  However, two problems
with this form of directional navigation have arisen:  one is that
instead of pointing "true north," magnetic compasses point to magnetic
north creating an "angle of magnetic declination":
http://academic.brooklyn.cuny.edu/geology/leveson/core/linksa/magnetic.html
http://www.ngdc.noaa.gov/cgi-bin/seg/gmag/fldsnth1.pl

This point will become very important in the latter part of my answer.

The second problem with magnetic compasses is interference from large
metallic structures such as buildings and ships, or even large
magnetic fields.  This problem was especially evident in iron and
steel ships in the 19th century with scientists such as Sir G.B. Airy
attempted to neutralize the ship's attraction by creating manmade
fields around the compass:
http://www.nmm.ac.uk/education/fact_files/fact_compass.html

The invention of the gyroscopic compass in 1908 sidestepped the
magnetic interference problem by using a gyroscope to keep the compass
needle pointing to true north.  To accomplish this, the gyroscopic
compass (or gyrocompass) is calibrated on land by pointing the needle
to magnetic north.  Once the magnetic north variation (the "angle of
magnetic declination" discussed above) is taken into consideration,
the compass is then set via the gyroscope to maintain that alignment
no matter which axis or angle the compass is turned to.  The
gyroscopic compass is critical for navigation in large metallic
transports such as airplanes or boats due to the varying axis put upon
the compass; the needle will invariably rest back upon its preset
plane.
http://www.howstuffworks.com/compass1.htm
http://www.geocities.com/SouthBeach/Marina/3026/Compass.htm
http://www.canadiangeographic.ca/Magazine/ND01/findingourway.html

I am assuming by your question that you require a semi-portable device
capable of giving multiple readings at various areas of a room. 
Therefore, gyroscopic compasses are not ideal for you.  They are
manufactured almost exclusively for commercial enterprises such as
aircrafts, ships and large navigational computers.  The smallest
consumer-sized version I was able to locate was a small boat engine
with a gyrocompass installed to aid in autopilot navigation.

Another technology used to circumvent magnetic interference is
fluxgate technology.  This is the type of compass found in most
consumer cars and boats.  Fluxgate compensates for interference by
“cancelling out” magnetic fields that are not the Earth’s.  Usually
accomplished by turning the unit completely around a few times, these
compasses use a number of sensors to judge differences in magnetic
strength as the unit turns.  By computing these differences, the unit
produces a fairly accurate directional bearing (if you own a car with
such a compass installed, you may read in the manual that it is
required to drive in a tight circle at least three times in an empty
parking lot to calibrate the compass).
Google cache of KVH Inc.’s FAQ:
http://216.239.33.100/search?q=cache:iX2KkUii1yUC:www.kvh.com/support/faqs/sailcomp_faq.html+&hl=en&start=1

Devices such as the Precision Navigation V6000 will give you a bearing
down to one degree and can be compensated for magnetic interference. 
In addition, the unit will also warn you when a reading may be
influenced and therefore inaccurate.
http://www.precisionnav.com/compass-products-and-digital-compasses/v6000-digital-vehicle-compass.html#Digital_Vehicle_Compass
http://www.precisionnav.com/technical-information/technical-information.html#what-is-magnetic-distortion

While there is some indication that the Global Positioning System may
provide useful information my research has shown that GPS receivers
will not accurately provide bearing information.  Most receivers are
equipped with either magnetic compasses, or a general bearing is given
from calculations rather than a precise magnetic north reading.  Also,
GPS receivers tend not to work very well under a sheltered ceiling
such as indoors, or under a canopy of trees rendering the unit useless
if you are using it for indoor measurements.  Some GPS receivers such
as the SILVA Multi-Navigator have a built-in electric compass, but I
have read many, many conflicting remarks on whether this compass will
work indoors.
http://www.silva.se/outdoor/products/mns_start.htm

Surveying equipment manufacturers will be your best bet when it comes
to cheaper devices that will accurately measure bearings down to one
degree.  Units such as the MapStar line use lasers to accurately
measure angles and take into consideration high magnetic interference.
 These systems will invariably use a combination of fluxgate
compasses, gyroscopic devices, or angle/prism compasses.
http://www.butlersurvey.com/products/measuringsystems/laserrangefinders/mapstar.htm

Industry suppliers such as TOKIMEC also produce gyrocompasses for
surveying and measuring, though these units will likely be too
large/expensive for your purposes.
http://www.tokimec.co.jp/marine/e/gyro.htm

Some companies will also rent out surveying equipment, though from the
requirements listed in your question, it does not sound like you will
require such large-scale procedures.  For info, you can visit such
companies as Ashstead Technology Rentals.  A quick search for
“gyrocompass” on their search page (accessible by clicking on “Search”
from their main page) revealed many promising leads.

http://www.ashtead-technology.com/rental/Default.htm


http://www.ashtead-technology.com/rental/Survey/Surface%20Positioning/Gyro%20Surface/TSS%20Meridian%20Surveyor%20Gyro.htm
http://www.ashtead-technology.com/rental/Survey/Surface%20Positioning/Gyro%20Surface/TSS%20Meridian%20Surveyor%20Gyro.htm
 

I hope this information has been of use to you.  Good luck in whatever
venture you will be using this technology for!

carwfloc-ga

Good Day, Budgie-ga.    
    
     Sounds like you need a gyroscopic compass such as used by
builders and tradesmen as it is not affected by micro magnetic fields
in houses and buildings. Invented by Sperry around 1908, it is also
used in planes and ships.

gyrocompass (or gyroscopic compass)   
A compass that relies on the properties of a gyroscope for its action.
It was invented independently (c. 1908) by two men, Hermann
Anschutz-Kaempfe in Germany and Sperry in the USA, after a principle
first demonstrated by the French physicist J.-B.-L. Foucault in 1852.
In a gyrocompass, a gyroscope is aligned on a north-south axis, and
maintains this alignment as the ship or aircraft carrying it changes
direction through gyroscopic inertia. The gyrocompass is unaffected by
the metal components of a vehicle, and points to true rather than
magnetic north. A 'master' compass can be used to operate several
repeater compasses, and can provide information to an automatic pilot 
or helmsman. The gyrocompass has replaced the magnetic compass for
navigation  in most ships, and is standard equipment in virtually all
aircraft.

http://www.xrefer.com/entry/216555
http://www.canadiangeographic.ca/Magazine/ND01/findingourway.html

While the majority of sophisticated Surveying levels have a gyroscopic
compass built into them, there has been the need to supply a modestly-
priced product suitable for the general builder and tradesman. This
demand has resulted in the production of a level with a simple
in-built compass which is linked to the bubble fences in order to
automatically correct the error when used in the North/South
orientation. Whilst this invention has attracted BSI approval, it has
only been on the open market for the last five months. On the strength
of this, the level was considered for inclusion as one of the key
products for the Millennium Dome but it failed to reach the final
round of the selection process. Thus, the self-correcting level only
found its way inside the Dome in the tool boxes of the workmen working
therein. Perhaps the situation will become more marked in another
thousand years and the level will be accepted for the next Dome!
Article Date: March 16, 2000
http://www.orc.soton.ac.uk/view/405/405.html

As this old article mentions, the level with gyroscopic compass should
be reasonably priced for tradesmen.  I haven't checked places like
Home Depot, but I imagine many of these places would have such a
compass, or be able to direct you to the seller of them.

Best,
Brad-ga

I would like to thank Carwfloc (and Brad) for their professinal knowledge!