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SRV-34

Magnetic declination

TN = MN + δ. True bearing from magnetic bearing and declination (east positive).

Reading speed
CompassMagnetic declination

Governing equation

TN=MN+δ\mathrm{TN}=\mathrm{MN}+\delta

where

MN
Magnetic bearing (°)
\delta
Declination E+ (°)
TN
True bearing (°)

Lecture brief

Historical brief

From chain and theodolite through Bowditch’s compass-rule (1802) to GPS PDOP, surveying is the propagation of small errors across a network. These sheets close traverses, reduce stadia and state map scale. This sheet (SRV-34 — Magnetic declination) is the form associated with Magnetic declination. Working symbols: MNMN, δ\delta →\rightarrow TNTN. Declination is the angle from true north to magnetic north. It wanders; IGRF models it year by year.

Purpose

Purpose: compute TNTN from MNMN, δ\delta in Surveying via TN=MN+δ\mathrm{TN}=\mathrm{MN}+\delta TN = MN + δ. True bearing from magnetic bearing and declination (east positive). Use it when a real surveying question must be answered in SI before a code check.

Live realistic example

In symbols

Live case. Given MN=42.000 ∘MN = 42.000\,\mathrm{^{\circ}}, δ=−2.400 ∘\delta = -2.400\,\mathrm{^{\circ}}, the governing relation TN=MN+δ\mathrm{TN}=\mathrm{MN}+\delta yields TN=39.60 ∘TN = 39.60\,\mathrm{^{\circ}}. A compass rose, two norths, a δ. Move a slider: the numbers are this situation, not a canned story.

Calculator

Inputs

Outputs

  • True bearing TN39.60 °
Reading speed

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SRV-34 · gauge
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Narration of this film

A compass rose, two norths, a δ.

Declination is the angle from true north to magnetic north. It wanders; IGRF models it year by year.

Reading speed

Watch on YouTube