Magnetic Variation: Isogonic Lines, the Agonic Line, VOR Station Declination, and True vs. Magnetic
Updated: Sep 26
Every pilot learns early that headings and courses are based on magnetic north, not true north. But behind that simple rule lies a constantly changing phenomenon that affects navigation, charts, runways, and even instrument approaches: magnetic variation. Understanding variation goes deeper than "East is least, West is best" — it involves the isogonic and agonic lines on your charts, where variation fits in the true-to-compass conversion sequence, why an old VOR's radials can be subtly wrong (station declination), and how your GPS decides whether to show you true or magnetic. Understanding magnetic variation and how it's applied is critical for accurate navigation, situational awareness, and regulatory compliance.
This post covers magnetic variation in practical depth: what it is and why it exists, isogonic and agonic lines, where variation fits in the TVMDC sequence, VOR station declination (the deep one), how GPS and avionics apply variation, runway numbers and renumbering, IFR operations, and why it still matters in the GPS age.
Study this full length lesson (video, podcast, flashcards, and quiz) here: Full Length Lesson >
What Magnetic Variation Is
Magnetic variation (also called declination) is the angular difference between True North (toward the geographic North Pole) and Magnetic North (the direction a magnetic compass points).
Why it exists:
The Earth's magnetic field doesn't align perfectly with its rotational axis
The magnetic pole is geographically separated from the geographic pole
The angle between them (from your location) is the variation
A geographic phenomenon
How it's expressed:
East variation: magnetic north lies east of true north
West variation: magnetic north lies west of true north
Example: 10° East = magnetic north is 10° east of true north
Example: 7° West = magnetic north is 7° west of true north
The changing nature:
The Earth's magnetic field is generated by molten iron moving in the outer core
It's irregular and shifts over time
Magnetic north moves (tens of miles annually — recently accelerating toward Siberia)
Variation changes slowly but continuously
Charts, runways, and systems must be periodically updated
The magnitude:
Variation ranges from zero (on the agonic line) to large values (10°, 15°, 20°+ in places)
Greater at higher latitudes and certain locations
In the continental US, roughly 0° to 20° depending on location
Location-dependent
Isogonic and Agonic Lines
The chart depiction, in depth.
Isogonic lines:
Lines on aeronautical charts connecting points of EQUAL magnetic variation
"Iso" (equal) + "gonic" (angle)
Dashed magenta lines on sectionals
Labeled with the variation (degrees east or west)
Reading isogonic lines:
The label shows the variation along that line (e.g., "10°W")
Often includes the year and annual rate of change: "10°W (2025) Annual Change 0.2°E"
Interpolate between lines for your exact location
The current variation for planning
The annual change:
The label may show how fast variation is changing (e.g., "Annual Change 0.2°E")
This lets you estimate current variation between chart updates
Variation drifts over time
Accounts for the moving magnetic pole
The agonic line:
The special line where variation is ZERO
"A" (without) + "gonic" (angle) = no angle
Along the agonic line, true north and magnetic north are aligned (variation = 0°)
No correction needed there
Where the agonic line is:
It runs through the continental US (its position drifts over time)
Historically through the eastern/central US, moving westward
On the agonic line, magnetic and true headings are the same
East of it: generally west variation; west of it: generally east variation (in the US)
The east/west pattern (US):
West of the agonic line: EAST variation (magnetic north is east of true)
East of the agonic line: WEST variation (magnetic north is west of true)
The agonic line divides them
A useful mental model
Where Variation Fits: The TVMDC Sequence
A framework the basic treatment doesn't provide: variation is one step in converting true to compass.
The TVMDC sequence:
True → Variation → Magnetic → Deviation → Compass
The progression from true course to compass heading
Variation converts true to magnetic
Deviation converts magnetic to compass
The mnemonic:
"True Virgins Make Dull Companions" (TVMDC)
True, Variation, Magnetic, Deviation, Compass
(Some add "Add Whiskey" for the wind correction step, making it True→WCA→Magnetic...)
The order of conversions
The variation step (T to M):
Start with the true course/heading (from the chart)
Apply variation to get magnetic
"East is least (subtract), West is best (add)"
True ± variation = Magnetic
The full example:
True course: 090°
Variation: 10° West → ADD (West is best)
Magnetic course: 100°
(Then apply deviation to get compass — covered in the compass post)
Why the sequence matters:
It organizes the conversions
Variation is the true-to-magnetic step
Deviation is the magnetic-to-compass step
Understanding the sequence prevents confusion
The reverse (magnetic to true):
Going from magnetic to true, reverse the rule
"East is least, West is best" applies true→magnetic
Reverse it for magnetic→true (East add, West subtract)
Know the direction of conversion
"East is Least, West is Best"
The memory aid, clarified.
The rule (true to magnetic):
"East is least" — subtract EASTERLY variation
"West is best" — add WESTERLY variation
Converts TRUE to MAGNETIC
The standard direction
Applying it:
East variation: True − variation = Magnetic
West variation: True + variation = Magnetic
Example: True 090°, 10°E variation → 090 − 10 = 080° magnetic
Example: True 090°, 10°W variation → 090 + 10 = 100° magnetic
The direction caveat:
The rule as stated converts TRUE to MAGNETIC
If converting MAGNETIC to TRUE, reverse it (East add, West subtract)
Know which way you're converting
A common error source
Why it works this way:
With west variation, magnetic north is west of true, so a magnetic heading is a larger number than true
With east variation, magnetic is a smaller number
The rule reflects the geometry
Consistent
VOR Station Declination: The Deep One
A genuinely deep and tested topic the basic treatment touches on but doesn't fully explain: VOR radials and station declination.
What station declination is:
Each VOR is oriented to magnetic north AT THE STATION at the time it was last calibrated/aligned
This is the "station declination" (the magnetic variation used to align that VOR)
VOR radials are magnetic based on this declination
The VOR's own reference
Why it can be "off":
Magnetic variation drifts over time
A VOR aligned decades ago used the variation AT THAT TIME
The current actual variation may differ (the pole moved)
So an old VOR's radials may not match current magnetic north exactly
The discrepancy:
If a VOR hasn't been realigned recently, its radials reflect the OLD variation
Your current magnetic heading (from a modern compass) reflects CURRENT variation
There can be a few degrees of discrepancy
Especially at VORs not recently updated
Why this matters:
When navigating via a VOR, you fly the VOR's radials (its magnetic reference)
This may differ slightly from your compass magnetic heading
Usually small, but noticeable
The VOR defines its own courses
Station declination updates:
VORs are periodically realigned to current variation (station declination updated)
When updated, the radials shift to match current magnetic north
This is why some approaches/radials get slightly revised
Maintenance keeps them current
The practical takeaway:
VOR courses are magnetic per the station's declination (which may be dated)
Small discrepancies between VOR radials and your compass are normal (especially at older VORs)
Fly the published/VOR courses (don't "correct" them)
The VOR is the reference for its radials
The GPS-overlay nuance:
When flying a VOR approach with GPS overlay, the GPS may use current variation while the VOR uses its declination
Small differences can appear
Follow the primary navigation source for the approach
A modern wrinkle
How GPS and Avionics Apply Variation
A modern topic the basic treatment mentions but doesn't detail: how your avionics handle true vs. magnetic.
GPS computes in true:
GPS fundamentally computes position and track in TRUE north references
(It's satellite-based, geographic)
But it DISPLAYS magnetic (usually) for pilot compatibility
A conversion happens internally
The magnetic conversion:
The GPS applies a variation model (a stored magnetic variation database) to convert true to magnetic
It displays magnetic track, bearing, etc.
So your GPS shows magnetic (matching your compass/heading indicator)
Automatic conversion
The true/magnetic setting:
Many GPS/avionics have a setting for TRUE or MAGNETIC display
Default is usually magnetic (for compatibility with ATC, charts, compass)
Can be set to true (for certain operations, high latitudes)
Know your setting
Why the setting matters:
If your GPS is set to TRUE and everything else is magnetic, headings won't match (confusion)
Ensure consistency (usually all magnetic)
Check the setting
A source of confusion if mismatched
The variation database:
The GPS's variation model is periodically updated (with database updates)
Keeps the magnetic conversion accurate
Part of the database currency
Automatic with updates
The high-latitude issue:
Near the magnetic poles, variation is extreme and changes rapidly
Magnetic navigation becomes unreliable
Some operations use TRUE north (grid navigation — below)
The setting matters at high latitudes
Runway Numbers and Renumbering
The runway connection, expanded.
Runway numbering:
Runway numbers are based on the MAGNETIC heading of the runway, rounded to the nearest 10 degrees (dropping the last digit)
A runway aligned to 183° magnetic → "18" (rounded to 180)
The reciprocal runway → "36" (180° opposite)
Magnetic-based
The rounding:
Take the magnetic heading, round to the nearest 10°, drop the last zero
183° → 180° → Runway 18
267° → 270° → Runway 27
The numbering convention
Why runways get renumbered:
As magnetic north drifts, the runway's MAGNETIC heading changes (the physical runway doesn't move, but magnetic north does)
Eventually the magnetic heading crosses a rounding boundary
The runway number must change to match
A consequence of variation drift
The renumbering example:
A runway at 185° magnetic is "Runway 18" (rounds to 180)
Over years, variation drift changes it to 186°, 187°... still 18
Eventually it reaches 185°+ toward 190° → becomes "Runway 19"
Renumbering required
What renumbering requires:
New runway signage and markings (repaint the numbers)
Updated charts
Revised approach procedures (the approach course magnetic reference)
Notification to pilots
A significant undertaking
The real-world occurrence:
Many airports have renumbered runways as variation drifted
(Notably, some airports renumbered as the magnetic pole accelerated)
It's a periodic reality
Driven by variation
Magnetic Variation in IFR Operations
The IFR applications, expanded.
Instrument approaches:
Published courses are MAGNETIC
Final approach courses aligned to magnetic north
Missed approach headings are magnetic
Fly the published magnetic courses
VOR navigation:
VOR radials oriented to magnetic north (per station declination)
Station declination periodically updated
Older VORs may have slight discrepancies (as discussed)
Fly the radials
ATC instructions:
Headings assigned by ATC are MAGNETIC
Radar displays referenced to magnetic north
Pilots should NOT "correct" ATC headings for variation (they're already magnetic)
Fly the assigned magnetic heading directly
The "don't correct ATC headings" point:
ATC gives you a magnetic heading
You fly it directly (on your magnetic heading indicator)
Don't apply variation to it (it's already magnetic)
A common misconception to avoid
The consistency principle:
Charts (courses): magnetic (for navigation)
Heading indicator: magnetic
ATC headings: magnetic
GPS display: magnetic (usually)
Everything aligns in magnetic (that's the point)
Grid Navigation and True North Operations
An edge case the basic treatment doesn't cover: polar operations.
The high-latitude problem:
Near the magnetic poles, variation is extreme and changes rapidly over short distances
The magnetic compass becomes unreliable (it points nearly straight down, or swings wildly)
Magnetic navigation doesn't work well
A real issue for polar flights
Grid navigation:
For polar operations, aircraft use GRID navigation (referenced to TRUE north or a grid)
The GPS/INS provides true-north reference
Avoids the unreliable magnetic compass
Used by airliners on polar routes
The true-north setting:
In polar/high-latitude ops, avionics may be set to TRUE north
Grid heading references
The magnetic compass is unreliable there
A specialized operation
Why most pilots don't deal with this:
Grid navigation is for polar/high-latitude flights
Most GA flying is at mid-latitudes (magnetic works fine)
But understanding why it exists is useful
The limits of magnetic navigation
Variation vs. Deviation
The distinction, brief (the compass post covers deviation in depth).
Magnetic Variation:
Difference between true north and magnetic north
Depends on geographic LOCATION
Shown on charts (isogonic lines)
The Earth's field
Magnetic Deviation:
Error caused by the AIRCRAFT's own magnetic interference
Depends on the aircraft's electrical systems and structure
Corrected using the compass deviation card
The aircraft's field
The key difference:
Variation: the Earth's (geographic, location-based)
Deviation: the aircraft's (onboard, aircraft-specific)
Both must be applied for an accurate compass heading (the TVMDC sequence)
(For deviation and the in-flight compass errors — ANDS, UNOS — see our magnetic compass post)
Why Pilots Still Learn Variation in the GPS Age
The enduring relevance, expanded.
Modern avionics automate conversions, but pilots must understand variation because:
Charts are still TRUE-referenced (the grid) even though courses are magnetic
ATC operates magnetically
Partial-panel or equipment failures require raw navigation skills (dead reckoning with variation)
Checkrides and knowledge tests require it
Situational awareness improves with the understanding
The instruments-disagree scenario:
When instruments disagree, understanding variation helps you reason it out
(Is the discrepancy variation-related? Station declination? A setting?)
Deeper knowledge prevents confusion
Troubleshooting
The dead-reckoning backup:
If GPS fails, you navigate by dead reckoning (true course → magnetic heading via variation)
Variation is essential for the conversion
A backup skill
Why it's taught
The professional standard:
Understanding variation is part of being a knowledgeable pilot
Not just following the magenta line
Deeper competence
The mark of proficiency
Real-World Example
The multi-reference scenario, expanded.
On an IFR cross-country:
GPS computes a TRUE track (displays magnetic)
Chart course is TRUE (the grid), but navigation courses are magnetic
Heading indicator is MAGNETIC
ATC assigns a MAGNETIC heading
Approach plate shows MAGNETIC courses
Knowing which reference applies:
Most operational references are magnetic (headings, courses, ATC)
The chart grid is true (but you navigate magnetic)
GPS displays magnetic (matching everything)
Understanding this prevents errors
The consistency:
In practice, you operate in magnetic (headings, courses)
Everything aligns
Variation is applied behind the scenes (by the GPS, in the published courses)
You fly magnetic
Final Thoughts
Magnetic variation is an invisible but ever-present factor in aviation. While technology has reduced the mental workload of applying it, understanding it deeply — the isogonic and agonic lines, where variation fits in the TVMDC sequence, why VOR radials reflect station declination (and can drift), how GPS converts true to magnetic, and why runways get renumbered — makes you a more capable pilot. Aviation is built on consistency and shared references, and the magnetic reference (with variation applied) is one of the quiet systems that makes that consistency possible.
Master it, and you'll be a more confident, accurate, and professional pilot — whether flying VFR, IFR, steam gauges, or glass, and whether the GPS is working or you're back on raw dead reckoning.
On the Written Test and Checkride
Magnetic variation appears on tests and checkride orals. The most commonly tested topics:
What variation is (true vs. magnetic north)
"East is least, West is best" (true to magnetic)
Isogonic and agonic lines
The TVMDC sequence (variation's place)
VOR station declination
Why runways get renumbered
Variation vs. deviation
Quick Reference
Magnetic Variation:
Angular difference between true north and magnetic north
Depends on geographic LOCATION
East or West variation
Changes over time (pole drifts)
Isogonic Lines:
Connect points of EQUAL variation
Dashed magenta on charts
Labeled (e.g., "10°W (2025) Annual Change 0.2°E")
Agonic Line:
Where variation is ZERO (true = magnetic)
Runs through the continental US (drifts)
West of it: East variation (US); East of it: West variation (US)
TVMDC Sequence:
True → Variation → Magnetic → Deviation → Compass
"True Virgins Make Dull Companions"
Variation: true → magnetic; Deviation: magnetic → compass
East is Least, West is Best (true → magnetic):
East variation: SUBTRACT
West variation: ADD
Example: True 090°, 10°W → 100° magnetic
Reverse for magnetic → true
VOR Station Declination:
VOR radials are magnetic per the station's declination (when last aligned)
May reflect OLD variation (pole drifted since)
Small discrepancies with current compass are normal
Fly the VOR's radials
GPS and Variation:
GPS computes TRUE, displays MAGNETIC (usually)
Applies a variation model to convert
True/magnetic setting available (default magnetic)
Ensure consistency (all magnetic)
Runway Numbers:
Based on MAGNETIC heading, rounded to nearest 10° (drop last digit)
183° magnetic → Runway 18
Renumbered as variation drifts (magnetic heading crosses a boundary)
Requires new signage, charts, approaches
IFR Operations:
Approach courses: magnetic
VOR radials: magnetic (station declination)
ATC headings: magnetic (don't "correct" them)
Everything aligns in magnetic
Grid Navigation (polar):
High latitudes: variation extreme, compass unreliable
Use TRUE north / grid navigation
Airliners on polar routes
Specialized
Variation vs. Deviation:
Variation: Earth's (geographic, location) — charts
Deviation: aircraft's (onboard interference) — deviation card
Key Principle:
Magnetic variation is the location-based angle between true and magnetic north, shown by isogonic lines (and zero on the agonic line). It's the true-to-magnetic step in the TVMDC sequence ("East is least, West is best"). VOR radials reflect the station's declination (which can drift from current variation), GPS computes true but displays magnetic, and runways are numbered by magnetic heading — requiring renumbering as the pole drifts. Everything operational aligns in magnetic.
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Author: Nathan Hodell
CFI, CFII, MEI, ATP, Creator and CEO
Nathan is an aviation enthusiast with thousands of hours of flying and dual instruction over the past 15+ years. Through his aviation career he has been able to earn his ATP, fly as an airline pilot, own/operate flight schools, and create and host wifiCFI.