Hi Alain!
Thank you and Happy holiday's! The only way to work with positioning would be limited to the surface with the use of GPS. The catch is that once you are underwater, GPS is useless. http://www.eagletreesystems.com/inde...product_id=131 shows their boat speed sensor. As for a formula I think maybe you meant the formula referenced in #2? Again my math skills and understand of how the math works is seriously lacking especially when trying to even begin to write the code for a PIC.

Best, Ed