Some Garmin GPS receivers such as the G-III and G-III PLUS offer no method of initializing the longitude/latitude starting point for the simulator mode. Here is a workaround that will assist.
1) Turn the simulator mode ON.
2) Go to the SATELLITE screen. (The one with signal strength markers and circles.)
3) Press MENU.
4) Select INITIALIZE POSITION and press ENTER.
5) Move the pointer to wherever you want. (You can use ZOOM to precisely position your pointer.)
6) Press ENTER and your initial point will be moved.
This workaround gets the job done pretty well.