The answer first: stand just inside the main door, face outward, hold the phone flat, take three readings and use the median. If the three readings differ by more than 8 degrees, steel or electrical interference has got you — move and measure again. Also: your phone reports magnetic north, and a traditional luopan compass uses magnetic north too. They agree. Do not convert against the true north on Google Maps — declination is about −4 degrees in Taiwan and about +11.8 degrees in Melbourne, and that gap alone is enough to push you into the neighbouring "mountain".
Facing is the foundation of the entire Flying Stars analysis. Get the foundation wrong and every sentence after it is wrong — wrong in the way that still reads as professional and well-organised. So it is worth ten minutes to do properly.
What "facing" actually means
Facing (向) = the direction you look when you stand just inside the main door and look out. Sitting (坐) = what is behind you, always the exact opposite (N↔S, NE↔SW, E↔W, SE↔NW).
When feng shui says "sitting north, facing south" (坐北朝南), that is what it means: seated in the north, looking south. Two common misunderstandings:
Why precision matters this much
Most people think there are eight directions: N, S, E, W and the four corners. On a luopan there are 24 mountains (24山), each spanning 15 degrees.
Take "south". It actually contains three mountains:
| Mountain | Degrees | Yuan-long (元龍) |
|---|---|---|
| Bing 丙 | 157.5°–172.5° | Earth (地元龍) |
| Wu 午 | 172.5°–187.5° | Heaven (天元龍) |
| Ding 丁 | 187.5°–202.5° | Human (人元龍) |
Here is the crux: the Heaven and Human mountains share the same chart, but the Earth mountain gets a completely different one. (The Earth mountain's yin/yang is the reverse of Heaven and Human, so the mountain star and the facing star both flip between forward and reverse flight at once.)
How different? In Period 8, Zi mountain facing Wu (子山午向) is "both stars to facing" (雙星到向); in Period 8, Ren mountain facing Bing (壬山丙向) is "both stars to sitting" (雙星到坐). The wealth positions in those two charts land in entirely different places — and on the compass they are 15 degrees apart. When you pick "South" from a dropdown, the system can only guess which one you are.
It gets finer still. Within each mountain:
So "roughly south" and "172.5 degrees" are answers carrying very different amounts of information.
How to measure properly
1. Stand in the right place
Stand just inside the main door, body squared to the outward direction. Do not stand outside and look back (the direction reverses), and do not estimate from the middle of the living room.
2. Hold the phone flat
Phone level in your palm, screen up. Tilting makes the magnetometer drift. Some people press the phone against the door itself — don't: doors usually have metal structure or magnetic catches.
3. Get away from metal
This is the largest source of error indoors. All of these interfere with the magnetometer:
If you can, step one or two paces outside the door and measure there while holding the same orientation. It is usually more accurate than measuring from inside.
4. Take three readings and check the spread
Take three readings in a row and write all three down.
Plenty of people skip this step, but it is the only way to know whether a reading can be trusted at all. Measure once and you will never know whether that number was accurate or dragged sideways by the fridge.
5. Calibrate the phone
Most compass apps prompt you to "wave the device in a figure eight" to calibrate. Do it once before measuring, especially if the phone has just come out of a pocket, a bag, or a car.
Can you actually trust a phone?
Plainly: good enough in most cases, not good enough in the cases that matter.
A phone magnetometer's real-world indoor error is often ±5 to ±10 degrees. Set that against the thresholds above — the xia-gua safe band is only ±3 degrees, and a void line is only 1.5 degrees wide — and one thing becomes clear:
The phone's own margin of error spans those boundaries.
So the sensible approach is to handle it in bands:
This is why our form stops and asks you to confirm manually when it detects a reading near a boundary, instead of quietly erecting a chart from it. A tool that tells you "your phone can't measure this angle reliably" is more useful than a tool that always hands you an answer.
Magnetic north vs true north: don't convert
This deserves its own section, because getting it wrong moves you a whole mountain.
The angle between the two is called magnetic declination: roughly −4 degrees in Taiwan, −3 in Hong Kong, about 0 in Singapore, and +11.8 in Melbourne.
Feng shui uses magnetic north, with no declination correction. If you check against a map's true north, or add and subtract declination yourself, you have moved the entire chart for no reason. Melbourne's 11.8 degrees alone is enough to take a reading from the centre of one mountain into the next one.
What you still don't have after measuring
With an accurate degree reading you can work out two things yourself: which of the 24 mountains your home sits on, and whether that is xia-gua or jian-xiang.
What is still missing:
Our approach: every chart is pre-computed and verified palace by palace against classical texts such as Shen's Study of Xuan Kong (《沈氏玄空學》). The AI only interprets; it never erects the chart. The reasoning is simple — erecting a chart is arithmetic, getting the arithmetic wrong makes the entire report wrong, and that is precisely the job you should not hand to a language model.
In one line
Stand inside the door, face out, phone flat, away from metal, three readings, take the median; a spread over 8 degrees means measure again. Remember it is magnetic north — don't convert. What you end up with is a number in degrees, and that number is the foundation of the whole analysis. Worth ten minutes.