Compass Basics and Compass Basics and TypesTypes

Doug “Due North” Millar Doug “Due North” Millar

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TypesTypes

Boy Scout type “Lensatic”Boy Scout type “Lensatic”

Sighting- has a mirror and/or prismSighting- has a mirror and/or prism

Transit- for surveyorsTransit- for surveyors

Solar compass- for moving vehiclesSolar compass- for moving vehicles

GPS- Fluxgate Compass- need frequent GPS- Fluxgate Compass- need frequent calibration.calibration.

Compass BasicsCompass Basics

There are a number of types of There are a number of types of compasses, but for our needs a “sighting” compasses, but for our needs a “sighting” or lensatic type is best. or lensatic type is best.

Look forLook for Fluid filled, jeweled movementFluid filled, jeweled movement Declination adjustmentDeclination adjustment Good mechanics and stability. Good mechanics and stability.

WWI Military Lensatic TypeWWI Military Lensatic Type

Calibration ProblemsCalibration Problems

To find the local isogonic variation look on To find the local isogonic variation look on Aeronautical charts or airport diagrams.Aeronautical charts or airport diagrams.

The variation here changes by .1 degree/year.The variation here changes by .1 degree/year. Long Beach is 13.2 W and Camarillo is Long Beach is 13.2 W and Camarillo is

14.714.7 Setting the variation has to be done accurately.Setting the variation has to be done accurately. Matching the compass with true North is a Matching the compass with true North is a

mechanical problem.mechanical problem. But local man-made and natural magnetic But local man-made and natural magnetic

anomalies are the greatest error.anomalies are the greatest error.

Airport Diagram Airport Diagram Has isogonic variation per year and dated Has isogonic variation per year and dated

measurement. measurement.

Sighting CompassSighting Compass Worse than1deg. Worse than1deg.

Sunto $50Sunto $50

Below ½ DegreeBelow ½ Degree

Below this level of accuracy there are all Below this level of accuracy there are all kinds of variables that come into play and kinds of variables that come into play and an accurate heading is hard to know. Try an accurate heading is hard to know. Try for 1deg accuracy.for 1deg accuracy.

One idea is to use a Go To astronomical One idea is to use a Go To astronomical mount and align it with known objects. mount and align it with known objects.

Another is to use digital setting circles. Another is to use digital setting circles.

The Bad, Good and StrangeThe Bad, Good and Strange

Most inexpensive lensatic types are Most inexpensive lensatic types are useless. They don’t settle well and useless. They don’t settle well and calibration moves.calibration moves.Sighting compasses with a mirror are very Sighting compasses with a mirror are very good if the fluid dampens the movement good if the fluid dampens the movement and they are settable. Good ones are $$$and they are settable. Good ones are $$$The Morin is excellent but does not have The Morin is excellent but does not have dec. adjustment. Also $$$$dec. adjustment. Also $$$$The solar compass- great till it’s darkThe solar compass- great till it’s dark

A cheap Lensatic typeRead Wrong Guessometer

Typical Inexpensive Compass with fewExtras. Accuracy worse than 1deg.

Excellent sighting typeA Brunton 8099 <1deg $70 plus

Morin sighting typeBetter than 1deg

about $100

Solar Compass with Gnomon Used for vehicles in motion.

Best Bang for the BuckChinese Military lensatic type available for

About $20 on Ebay

The Chinese Military lensatic just shown has a number of

Important features.

•Magnified viewer with diopter adjustment•Fluid dampened card•Excellent sighting and mirror mechanism•Better than 1 deg resolution•Built in level•Built in photo tripod mount •Case•Smooth setting bezel and isogonic offset.

ConclusionConclusion

A good enough sighting compass should cost A good enough sighting compass should cost less than $50. less than $50.

Don’t put up with a hard to use read-wrong Don’t put up with a hard to use read-wrong guessometer. guessometer.

Don’t expect more than your calibration and Don’t expect more than your calibration and information can deliver. If you are within a information can deliver. If you are within a degree that may be about as good as you can degree that may be about as good as you can do. Any further and results go up in a linear do. Any further and results go up in a linear fashion and money goes up in an exponential fashion and money goes up in an exponential fashion. fashion.

I brought one of each type to try I brought one of each type to try out and to look at. out and to look at.

ReferencesReferences

Accuracy considerations: Accuracy considerations: http://www.geocities.com/magnetic_declination/#COMPEhttp://www.geocities.com/magnetic_declination/#COMPENSATENSATE

Compass use from a boater’s point of view. Compass use from a boater’s point of view. http://books.google.com/books?id=nfWSxRr8VP4C&pg=PA43&lpg=PA43&dq=isogonic+variation&source=web&ots=z8ZKEZS2TI&sig=vP9TLCKMHCPoEWwZkmj8o1OYDl8&hl=en&sa=X&oi=book_result&resnum=10&ct=result#PPA44,M1

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“Due North Doug”“Due North Doug”