An Annoying GPS position error in Street Atlas
Dave and Helen Damouth www.damouth.com
(minor update 10/24/00)
Does your GPS have a large and consistent location error on your computer mapping program? If so, read on:
For those of you using a hand-held GPS with Street Atlas, Map'n'Go, or other mapping programs, I'll describe an annoying problem which had been bothering us for a month or so before I found the solution.
The symptom: Street Atlas consistently placed our location about a city block east of our actual position. This caused us significant navigation problems while in big cities.
The cause: While in Canada this summer, I used the GPS for backcountry hiking, and reset the map grid in the GPS to NAD27-Canada, since many older Canadian topographic maps use this grid. This option is hidden in one of the seldom-used setup menus in my Garmin 12XL. It didn't occur to me at the time that this change would affect the way that the GPS interacts with Street Atlas.
The fix: Just change the grid. I've heard from a DeLorme employee that DeLorme products all use NAD83. I changed the Garmin to NAD83 (also used by many U.S. government topo maps), and Street Atlas now seems to locate my position quite accurately (often within 20 feet now that the random "Strategic Availability" error introduced by the military has been discontinued).
Since doing this, I've discovered that the default grid in the Garmin 12XL is WGS84, so that's probably what I had been using before entering Canada. I've also received data showing that WGS84 and NAD83 are essentially identical. So if you've never heard of all this, and have never changed your Garmin from the way it was when you bought it, you can relax - in this case, ignorance really is bliss.