I just received my new Humminbird 858c HD Di and have never installed a GPS Module. The instructions are not to good, anyway I'm trying to figure out where to install it. My control head I'm going to install on the right side of the dash. I could install the GPS Module on a stand pipe made of PVC and extend it so it clears the top of the control head. Could I install it flat just under the control head? Don't know if it would be clear enough there. Any ideas would be appreciated.

You can simply flush mount the GPS antenna at the Bow, or next to the unit or at the rear of the boat. Where ever you want the information to read from.

I recommend installing close to the transducer to prevent adding any extra offset to the waypoints when you mark sonar data on the screen.

Watch this video it will show you why I recommend this: http://youtu.be/4btE-Vm4fmw

If you don't want to drill holes to mount you can mount with commercial double stick tape or silicone.

Exactly! Close as possible to the transducer.

OK, thanks! I'll flush mount it near the transducer. I see what you mean Doug. The video shows it pretty plain. Thanks all of you!!

I would like to understand why mounting the gps as close to the transducer makes a difference. What if your marking a waypoint 100 feet off to the right? I don't think it matters then where your transducer is. I can see if your marking something directly behind you or below your(If on your trolling motor) but not to the side.

Think you answered your own ?.

If you are using the imaging technology and create a waypoint that is not under the transducer or GPS antenna, the display is still using the transducer to create the image and GPS data for the curser position.
That technology is smarter than you.

Everytime the sonar data is displayed on the unit it remembers the GPS Location with that Data String. So when you move the cusor on the screen it knows the exact location and distance for Side Imaging away from the receiver where the cursor is and marks the exact location. It's one of several patents Humminbird has on the Technology and one of the greatest benefits of Side Imaging to be able to mark the exact location. The closer the relationship of the GPS Receiver to the Transducer where the data is being created and received the more accurate the waypoint data will be. Especially when you return from a different direction. Look at the image below and it will show the GPS to Transducer Releationship the Yellow Circle is my recommendation the outside of the circles are the GPS Receiver in the exact same spot and where the transducer could be and the GPS dead on the waypoint...farther the receiver to transducer the greater the distance from the transducer to the Receiver and the more offset you can add. Notice the Blue Circle is bigger the outside of the circle is where the transducer could be and the GPS says you are back on the same spot.