Depends on who the someone is, which apps you have on your phone and how exact "exact" needs to be.
The cell phone service providers get your location on cell basis. That's all they need to know.
Some apps will (try to) get your location based on available (and already mapped) WiFi conections, or your GPS position, and forward this position to, well, that will depend on the app.
If you call emergency services, they will ask you to release your GPS position or, depending on the network you're currently booked in, retrieve your GPS position automatically.
Anyone with the technical knowledge can set up an IMSI catcher an, when you're in range, get your phone position pretty accurately. Illegal (and easy to catch) if you don't have a judge's permission.
Since a cellphone is a transmitter, anyone with a directional antenna and some basic RF knowledge can get your position (and that of any other cellphone) while you're within range. Pretty much useless, though, as it's very difficult (and, again, possibly illegal) to filter out one specific phone from the crowd.
Oh, and there's the "silent SMS", a technique formerly employed by state security (at least over here) where they send an SMS to your phone that won't show up, and the phone would respond, thereby telling the provider its current cell (which the provider would then have to forward to said security under judicial order).