How accurate is your GPS. I have a new Raymarine A67 chart plotter which I love. When mounted on deck without an external gps sensor has an accuracy of about 4-10 feet. When mounted below deck it has an accuracy between 8-24 feet without an external gps. I plan to mount it below deck without a external gps sensor. With an accuracy of +/- 24 feet on a 40 foot is it worth the additional money to get another few feet of accuracy?