How do you write geographic coordinates to a tag?
A geo record holds a latitude and a longitude and asks the phone to open them in a map. Support has always been uneven across platforms and applications, so for anything a stranger will tap, a link to a mapping service is more dependable even though it needs a network.
The geo record
A record holding a coordinate pair, written as a URI with the geo scheme. A writing tool asks for latitude and longitude, or lets you pick a point on a map.
Where it is supported, tapping opens a mapping application at that point. It works with no network for the record itself, though the map that opens will usually want one to draw anything.
Why support is uneven
The geo scheme is handled by whichever application has registered for it, and that varies by platform, by which mapping applications are installed, and by user defaults.
The result is a record that works beautifully on the device you tested and does nothing on the next one.
That unpredictability is the whole problem. A tag on a trailhead post that opens a map for some walkers and not others is worse than a tag that behaves the same way for everyone.
The link alternative
A URL to a mapping service with the coordinates in it. It is longer, it needs a network, and it works on essentially every phone, in the browser if no application claims it.
It also gains everything a URL gains: you can change the destination, add a name for the place rather than a bare coordinate, and point at a page with context rather than a pin.
Choosing between them
For an internal deployment on devices you control, the geo record is fine and slightly more elegant. You can test the behaviour once and rely on it.
For anything the public taps, use a link. Predictability beats elegance when you cannot see the phone.
And consider whether coordinates are the right payload at all. A tag pointing at a page about a place can carry the coordinates, the opening hours, a photograph and a route, which is usually what the person actually wanted when they tapped a marker.
Frequently asked questions
Does a geo tag work offline?
The record is read offline, and the mapping application that opens will generally need a network to show anything useful unless maps are downloaded. Treat it as needing coverage in practice.
Which mapping app opens?
Whichever has registered for the scheme and been chosen as the default, which differs between devices. This unpredictability is the main argument against the record type.
Can I include a place name as well as coordinates?
The scheme allows a label and support for it varies. A link to a mapping service handles names reliably, which is another point in its favour for public deployments.
Is a coordinate accurate enough for a doorway?
Coordinates are precise enough, and consumer positioning is not, so a person navigating to one may arrive several metres out. A short description of the entrance helps more than extra decimal places.