True North vs. Magnetic North: Declination in Japan and Which to Use on a Floor Plan
The north arrow on a floor plan doesn’t always match the north your phone’s compass shows. The usual cause is the difference (declination) between true north, the north of maps, and magnetic north, where a compass points. This article explains the difference, approximate declinations by region in Japan, which way and how far to rotate, and which north to use when reading directions for Japanese kaso (house divination) or feng shui.
True north is the north of maps; magnetic north is the north of compasses
True north is the direction of the North Pole, the northern end of Earth’s axis of rotation. Maps are basically drawn with this direction up, and the north arrow on architectural drawings normally points to it too.
Magnetic north is the direction the N pole of a compass points. Earth’s magnetic field doesn’t line up exactly with the North Pole, so a compass points slightly away from true north. This angle of difference is called declination.
| True north | Magnetic north | |
|---|---|---|
| Reference | North Pole (Earth’s rotation axis) | Direction of Earth’s magnetic field |
| Typical uses | Maps, north arrows on drawings, aerial photos | Compasses, phone compasses (depending on settings) |
| Relationship in Japan | — | About 5–10° west of true north |
| Change over time | Doesn’t change | Shifts slowly, year by year |
Approximate declinations in Japan
In Japan, magnetic north lies west of true north (westerly declination). The difference grows as you go north and shrinks as you go south. These rough figures are based on the Geospatial Information Authority of Japan’s magnetic charts (2020.0 values).
| Region (representative city) | Approximate declination (west) |
|---|---|
| Hokkaido (Sapporo) | About 9.5° |
| Tohoku (Sendai) | About 8.5° |
| Hokuriku, Koshinetsu (Niigata) | About 8° |
| Kanto (Tokyo) | About 7.5° |
| Tokai (Nagoya), Kinki (Osaka), Chugoku (Hiroshima) | About 7.5° |
| Shikoku (Takamatsu), northern Kyushu (Fukuoka) | About 7° |
| Southern Kyushu (Kagoshima) | About 6.5° |
| Okinawa (Naha) | About 5° |
Even within a region, values differ by about 0.5–1° from place to place, and they shift slowly over the years. When you need an exact value, look up the geomagnetic value (declination) for your location on the Geospatial Information Authority of Japan’s website. For reading a home’s directions, the regional figure is rarely a problem for rooms well away from a boundary.
Which way and how far to rotate between true and magnetic north
The direction of rotation is the easiest thing to get wrong. Just remember that in Japan, magnetic north is on the west side of true north (the left side on a map with north up).
| North you have | North you want | How to rotate (Tokyo example) |
|---|---|---|
| True north (north arrow on a drawing) | Magnetic north | Counterclockwise by the declination (about 7.5°) |
| Magnetic north (measured by compass) | True north | Clockwise by the declination (about 7.5°) |
In Tokyo, for example, when a drawing’s north arrow (true north) points straight up on the image, magnetic north is about 7.5° counterclockwise from the top, pointing slightly up and to the left. If you’re drawing direction lines by magnetic north, rotate all the lines to match.
Which north should you use for a floor plan?
Neither is simply right. It depends on why you’re reading the directions.
- Sunlight and daylighting: true north, because the sun’s movement is based on it
- Comparing with drawings and maps: true north, because north arrows on drawings normally indicate it
- Kaso: many approaches use magnetic north measured by compass, though some schools use true north
- Feng shui: magnetic north is often used, since directions are measured with a compass (luopan)
If you’re consulting a practitioner or company, check first which north they use. Land survey maps may also show “grid north”, aligned with the survey’s coordinate axes, which differs slightly from true north. If you can’t tell which one a floor plan’s north arrow indicates, the surest way is to ask the person responsible for the design.
Example: how changing north changes a room’s direction
Take a home in Tokyo (declination about 7.5°) and convert angles measured from true north to angles from magnetic north. Magnetic north lies counterclockwise from true north, so measuring the same room from magnetic north increases the angle by the declination (20° from true north is 27.5° from magnetic north).
| Room | Angle from true north | Angle from magnetic north | 8 directions (45°) | Kaso (30° and 60°) |
|---|---|---|---|---|
| Toilet | 10° | 17.5° | N → N | N → NE |
| Entrance | 17° | 24.5° | N → NE | NE → NE |
| Kitchen | 100° | 107.5° | E → E | E → SE |
| Bathroom | 220° | 227.5° | SW → SW | SW → SW |
The toilet doesn’t change with 45° sectors, but with kaso’s sectors it moves from north into northeast (kimon). The entrance is the opposite, changing only with 45° sectors. The bathroom stays southwest, but its distance from the kimon line (225°) shrinks from 5° to 2.5°. Rooms near boundaries or lines can change with the combination of north and sector scheme, so always record which you used.
How to check your home’s orientation yourself
- Aerial view in a map app: maps have true north up, so the angle between an outer wall and the vertical shows the orientation relative to true north
- Phone compass: on iPhone, “Use True North” can be toggled under Compass in the Settings app. When it’s off, it shows magnetic north
- Magnetic compass: points to magnetic north. Steel frames, appliances, and speakers throw the needle off, so measure in an open place outside the house
In the Floor Plan Direction Lines tool, use “② Align north” to choose whether the north arrow indicates true or magnetic north, then choose the north for drawing lines, and it automatically rotates the lines using the regional declination. You can also enter a declination in 0.1° steps. For finding the center, see How to Find the Center of a House, and for reading kimon, see How to Find the Kimon and Ura-Kimon.
FAQ
- How far apart are true north and magnetic north?
- In Japan, magnetic north lies about 5–10° west of true north: roughly 9–10° in Hokkaido, 7–8° in most of Honshu, and 5° in Okinawa.
- Which way do I rotate to get magnetic north from a drawing’s north?
- In Japan, magnetic north is on the west side of true north, so rotate counterclockwise from the drawing’s north arrow (true north) by the declination. To get true north from magnetic north measured by compass, rotate clockwise.
- Does a floor plan’s north arrow show true or magnetic north?
- North arrows on architectural drawings normally show true north. Floor plans on sales flyers and the like may be only approximately oriented, though, so for accuracy, ask the person responsible for the design or check the home’s orientation on an aerial photo in a map app.
- Which north do kaso and feng shui use?
- Many approaches use magnetic north measured by compass, though some schools use true north. If you’re consulting a practitioner or company, follow their standard.
- Does a phone compass show true or magnetic north?
- It depends on the app and settings. The iPhone Compass shows true north when “Use True North” under Compass in the Settings app is on, and magnetic north when it’s off.
- Does declination stay the same over the years?
- No. Earth’s magnetic field changes slowly, so declination also shifts little by little, year by year. Regional figures are enough for reading a home’s directions, but for exact values, check the latest geomagnetic values from the Geospatial Information Authority of Japan.
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