Here is something strange I have noticed about my USB GPS. When I am not moving I notice that the GPS thinks I am moving (showing speed). My USP GPS is an Ambicom USBGPS. This happens regardless of what I am connect to. Been torn between Seaclear and OpenCPN, but starting to use OpenCPN more. My poor boat is jumping all over the place (if zoomed in really close).
Anyone else using a USB GPS and have this issue?
If you think about it, if your GPS is reporting your location within 50 feet, you really have to consider just how miraculous that is! Considering how GPS works, triangulating from multiple satellites, and measuring the difference in arrival times from the various satellites which are 22,500 miles away, more or less, and it says the accuracy is plus or minus 12 feet, we really can't complain about accuracy. As that accuracy fluctuates due to whatever factors influence those various signals from those several satellites, the GPS has to take that data and give you a position. So, it will vary by a few feet at at time, even when you are not moving. It gets lost in the noise when you are moving.
By comparison, consider what is involved in doing sun sights with a sextant, and then reducing the sights to a position. If you are able to fix your position to within 5 miles you are doing pretty well. At that distance, you should be able to find an island in the middle of the ocean. So, I wouldn't be too concerned about the GPS showing my position varying by 25 feet one way or the other. As others have pointed out, you really can't count on charts using the WGS-84 chart datum being that consistently accurate anyway.
So, snug up the lines and try not to get seasick due to all of that motion!
