If your device has a built-in GPS, your location is shown as a red circle with cross-hairs on the map. The center of the circle flashes to indicate when the position is updated, usually every second or two if there is good GPS reception. If the circle turns grey, GPS reception has been lost. The estimated accuracy of the GPS position is displayed on the Flip-side data page, accessed by touching the -> button on the toolbar.
At any time when the GPS has a valid position, you can touch the GPS Lock 
button
on the toolbar. This scrolls the map to show the position at the center
of the screen. As you move the position is locked and the map scrolls
automatically to keep the position in the same place on the screen (this
effect is sometimes called a "moving map").
You can drag the map to re-locate the GPS position to one side, to show more of the map ahead. Place the GPS icon anywhere you want on the screen, and as you move, the map will automatically scroll to keep the GPS icon in the same place on the screen.
As long as you don't drag the GPS position off the screen, it will remain locked at that screen location. If you drag the GPS location off the edge of the screen, GPS Lock mode is cancelled.
If the GPS location is locked (moving-map mode), when the location moves off the edge of a map, another map is automatically loaded.
If you are moving, you may also see a Velocity
Vector. This is an arrow that shows your direction and speed of
travel. The length of the vector is scaled to show where you will be if
you maintain the same speed and heading for 5 minutes. You can change
the scale by touching the Settings button 
on the Flip
side page, and selecting GPS settings.
If your device does not have an internal GPS (eg, iPod Touch or iPad with WiFi only) there are some 3rd party GPSs that connect to the device by bluetooth and deliver position information to the app through the location service. No special settings in the app are required.
You can also connect to a NMEA 0183 data stream via a WiFi gateway. These are available for marine instrumentation, and allow you to connect AIS, wind, depth and boat speed instruments. See Connecting to Marine Instrumentation.
Next: Compass Heading