Take a look at the Contacts starter solution in FMP 12.
If you go here:
And you download this PDF:
FileMaker Pro 12 Functions Reference
You can read all about every possible function in Filemaker.
Two new functions in Filemaker Pro 12 are under the header "Mobile Functions" on pages 184 - 185.
The Functions are:
Returns the current latitude and longitude on a device such as an iPad or iPhone that is running
FileMaker Go. The location is obtained via GPS, cellular network, or WiFi.
Returns the current latitude, longitude, and altitude on a device running FileMaker Go. Returns the
horizontal and vertical accuracy of the values returned and the number of minutes since the values
were returned. The location is obtained via GPS, cellular network, or WiFi.
Now you can set these values using a script attached to a button with a scriptstep of "Set Field"