HB GPS speed bounce

I have a humminbird 788ci finder and the speed fluctuates quite a bit. If I take an average and call that my speed, it is fluctuating +/-.5 mph from the average and sometimes more. Is this normal?

In that range, I think it's normal and probably accurate. Say you're on plane and going 45 mph as you hit waves and the hull contacts more water, it actually slows down a little due to the additional friction of the water. As you clear a wave, you actually speed up until the next one. The GPS calculates the speed every X seconds or fraction there of, so with your speed varying, the posted speed will jump around depending on when it was calculated.

Now if you're spider rigging with a constant speed on the TM in slick conditions, it should remain relatively steady....at least mine does.

Wannabe...

refresh rate. I noticed that my 997 refresh rates abit slow. That'll cause the maps to load slowly but it will also cause you're speed to fluctuate. One other thing is the satallites might be in a low altitude and their not getting enough signal to the antenna to keep it current. This time of year, they're lower in the sky and if you get around trees, you're not getting a strong enough signal to be exact on your location which will in turn cause your speed to be off a tad bit. They'll calculate your position based on the Ellipsiod of the Earth and adjust accordingly

This is when I am spider rigging, the 788ci is mounted on my trolling motor. I was by some trees with a levee on the other side of the trees. Would an external puck improve on this?

getting in the way. I'm a licensed surveyor and I have a $50,000 unit with a huge base station and they still get in the way, especially this time of year. We get precise measurements with ours, down to 100th of an inch though and right now after about 1 PM, we can't hardly use ours, it won;t lock in on our points

Thanks guys.