Sliding into a fix


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Sliding into a fix

On my tips page I recommended waiting a couple of minutes to save a waypoint after powering up your GPS receiver and getting your initial position lock. I've found that current consumer GPS units are often quite inaccurate when initially getting a lock. They tend to sort themselves out fairly quickly however. The errors can range from just a few feet, to literally miles sometimes, and are far more likely if you have changed position, and more importantly altitude significantly since your last position lock. While I haven't spent a lot of time trying to measure the slide distances, my impression is that my Magellan units initially lock on farther away from my real position than my Garmins do, but not by a whole lot. Be it the GIII+, Legend, or Sportrak, they all do it. You should expect position slide and give your GPS a few minutes to settle in if you want to prevent possible problems in returning to your trailhead or other waypoint.

The above photo shows the slide in my tracks experienced by both of my sportraks having been fired up at a trailhead a considerable distance in both altitude and lat/lon from where they where last used. As you can see, intially they had considerable error measured in hundreds to thousands of feet, which would have thrown me way off had I saved the waypoint too soon. 004 is where I actually was, and the combined track at the bottom of the map is me heading off down the trail.

As another example, we have a trailhead where I drove up a couple thousand feet in elevation prior to firing up my GIII+. As you can see, it initially locked on a considerable distance from the vehicle, then slid into place at the trailhead. Once again, saving a waypoint prior to the unit stabilizing at the real location would have made returning to my truck difficult.