How many decimal places you actually need
A degree of latitude is about 111 km, and each decimal place divides that by ten. Two places lands within roughly a kilometre, three within a hundred metres, four within eleven metres, five within about a metre, six within ten centimetres. Longitude behaves the same way at the equator and tightens as you move toward the poles.
Four decimal places is the sweet spot for getting a person to a building or a gate. Beyond six you are encoding precision that consumer GPS cannot deliver anyway — a phone in the open is typically accurate to a few metres, and considerably worse between tall buildings.
Getting the coordinates right
Latitude comes first and runs from -90 to 90; longitude second, from -180 to 180. Southern latitudes and western longitudes are negative. Transposing the two is the classic error and produces a pin that is confidently wrong rather than obviously broken, so it is worth scanning your own code once before printing a thousand flyers.
In Google Maps, right-click the exact spot and the coordinates appear at the top of the context menu; clicking copies them. On a phone, dropping a pin and opening the detail sheet shows the same pair. Decimal degrees are what this format expects — if you have degrees, minutes and seconds from a survey document, convert first.
What happens on the scanning phone
The geo: scheme is deliberately app-neutral, which is its strength and its weakness. Android hands it to whichever maps application the person has set as default and drops the pin there. On iOS, behaviour depends on the app doing the scanning: the coordinates resolve to a map, but the route from scan to pin is less consistent than it is for a URL.
If your audience is mostly iPhone users and the destination has a proper listing, a plain link to that listing in Apple or Google Maps is more predictable than a raw geo: code. Use coordinates where the place has no address to link to, which is exactly when they are worth the most.
Where a pin beats an address
Anywhere the postal address does not describe where a person should physically stand. Large sites where the delivery entrance is not the front door, festival gates, remote holiday lets, construction plots, boat moorings, car park levels, and meeting points in parks all fall into this category.
Print the human directions next to the code rather than relying on it alone. A scan gets someone to the right coordinates; a line of text telling them it is the blue gate behind the depot is what stops them phoning you when they arrive.