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Surveyors Compass

Surveyors Compass. Box with center point Magnetized Needle Lift needle Blade Sights Level tubes Base Jacob’s staff Tripod. Magnetic Compasses. Magnetized Needle Points North Dips down Wire wrap Compass Rose is backwards Local Attraction. Magnetic Declination.

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Surveyors Compass

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  1. Surveyors Compass • Box with center point • Magnetized Needle • Lift needle • Blade Sights • Level tubes • Base • Jacob’s staff • Tripod

  2. Magnetic Compasses • Magnetized Needle • Points North • Dips down • Wire wrap • Compass Rose is backwards • Local Attraction

  3. Magnetic Declination • Longitude – true North • Magnetic North not true • Angular difference – magnetic declination • Varies by place – E or W • Varies with time

  4. Agonic Line East Declination West Declination Annual Drift Isogonal Map Map Date

  5. Declination in Moorhead • In 1980 • 7° E • Drifting 10’ W/year • Now • Assume E = (+) • 7° + (-10’/yr)(24 yr)= 3°00’ • Use most current map available

  6. N M Declination Calc’s • Old survey in Moorhead sets line AB • Decl then = 11°15’ E • Mag Bearing AB = S38°22’E • Find True direction now • Convert to Azimuth Mag Az = 141°38’ • True = Mag + Decl True Az = 141°38’ + 11°15’ = 152°53’

  7. M N M Problem 9-28 • 1890 Survey • Decl = 7°00’ E (+) • Mag Az = 134°30’ • True = Mag + Decl • True Az = 134°30’ + 7°00’ = 141°30’ • Mag Az Now • Decl Now = 3°30’ W (-) • Mag = True – Decl • Mag Az = 141°30’ – (-3°30’) = 145°00’

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