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Waypoints created are different to the track recorded which is correct?

5759823
2 days ago

I was using a GPSMAP 67i on a boat to record the track (for the whole journey) and waypoints at certain points. When I loaded the data into basecamp, I noticed that the waypoints recorded (with the same device) were not very close to the recorded track. The median difference was about 30 m, but in some places the waypoint was completely wrong (by over 100km). To help troubleshoot I was able to download the vessel plotted data (a Lowrance system) which I recorded for most the trip. The Lowrance track agreed with the GPSMAP 67i track on average 3-4 m and looks a good fit when plotted on top of each other. So, I am inclined to believe the recorded track, but want to understand why the waypoint data is so different? I don't think it is a format or datum issue as the difference was variable. Has any one else experienced something similar? Below is screen shot from Basecamp, notice the Waypoints 51-54 are quite a bit adrift from the track. N.B. Ignore the GPSMAP 64 data it is not shown in this screen shot. Any help or insight appreciated!

  • 0 2 days ago

    This MIGHT be related to the way iR devices (like the 67i) try to minimize battery usage. In my experience (none of it with the 67i. BTW), iR devices sometimes settle for a fix with a large estimated error - because the device does not want to leave the GPS receiver on long enough to acquire a fix with better estimated error. If so, then pressing MARK at a random time might result in a waypoint with poor location accuracy.

    That said, it would seem that tracking at the same time would mitigate those effects. Do you remember how you were recording the track points (fixed interval, auto aka whenever the device feels like it, or whatever)? Point is that tracking also uses the GPS receiver, so it might interact with the waypoint marking. Simultaneous tracking would probably make the waypoints better, not worse.

  • 0 2 days ago in reply to twolpert

    Thanks for your reply. I set the track to record once a second (i.e. 1 Hz), as vessels can be quite dynamic in nature. Maybe with such a high sample rate, the device struggles to output the position for a Waypoint as well?

  • 0 1 day ago in reply to 5759823

    Pretty sure that logging a track at 1/sec is enough to keep the GPS radio on continuously. That would argue that battery conservation is not an issue with waypoint accuracy. You might want to open a support ticket.