Absolutely! I use the GPS in my finder, but a handheld works just as good. What takes from the accuracy of GPS is terrain and view of the sky. When you are on a lake, your horizon is almost always well above the GPS's lower limit (usually 15-20 degrees). You can typically get an accuracy of less than 10 feet on a lake, that translates into very accurate speeds. I have checked mine against the speedo in my car and it is spot on.
The problem with gps speed is it isn't very accurate at a very slow speed such as trolling. however they are very acurate at higher speeds. thats why when i get the gps/fishfinder i will also get the water speed sensor to have accuracy at both ends of the scale. my car gps works well on the boat for max speed and is pretty accurate for distances.
yeah if yours doesn't do . miles just whole miles it will be good for top speed. i figure when mine is bouncing between 21 and 22 i am doing about 21.5. i noticed for trolling it would bounce betwwen 0 and 1 or 1 and 2. when trying to clock trolling speed you want to be able to measure the tenths which a lot of gps don't show. also like i mentioned they aren't very acurate at these slow speeds where you are trying to clock tenths of a mile. do some google searches and check some fish finder/gps combos that have review or something. I spent weeks researching the few in my price range and learned a lot about the gps relaibility of speed so i will be buying the water wheel for trolling speed and overlaying both speeds on the screen.
guys all the gps gives will be the speed over land, and that has absolutely no bearing on your true trolling speed which is water speed. On a lake with no current the gps can work but not in rivers.
if your on a river your water speed can be several knots per hour and your land speed be almost nothing at all, or downstream trolling like for springers your speed over land can be cruizing and your water speed be almost nothing.. same concept for anything tidally influenced.
Hey thanks for pointing that out i forgot to mention all that. i knew there was some better points to mention like the fact that it is land speed. the wheel is a must for trolling speed. only thing is it doesn't do well at a high speed. thats were a gps comes in because it measures how fast the boat is travelling over the land beneath the boat. but like you mentioned you can be trolling with the tide or down the current in a river and it seems like you are going fast and you are but you are really only trying to go slightly faster than the water to make your gear work that were water speed versus land speed can really be 2 diferent numbers.