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Garmin III+ Calibration?

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Hey all!


I have noticed that my Garmin GPS III+ appears to be out based on the map I am using. When I am standing at the corner of a major intersection, the GPS thinks I am about 30 - 40 meters out.


I have checked the date and time to make sure I am synced with UTC, and I am using WGS84, so I am wondering if it is just that the map that I have loaded in there is a little out?


Also, can a GPSr be calibrated if it is showing the incorrect coordinates? I have been to a number of caches where, when I am sitting at a known location (like a landmark given in a clue) my GPSr tells me I am 10 or so metres away from my destination.


Any ideas?


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The maps are the most likely cause of the positional errors you're seeing on the map page. Depending on which maps you have loaded, errors of as much as a quarter of a mile aren't unheard of. 200-300 feet is actually quite common with tiger map based products like the US R&R, which is the mapping product designed for use with the GIII+.


As for an error of 10 meters or so when trying to find a cache, that's pretty good really. To start with, there is going to be the error the cache hider experienced when placing the cache. Even with clears skys and WAAS, being within 3 meters or so is about the best that can be expected. Then you have the error from your receiver that gets involved in the equation. Being a Non WAAS unit, your error circle in good conditions can easily be somewhere in the neighborhood of 10 meters or more. Under the right (Or should we call it wrong) circumstances, the errors can be in opposite directions resulting the cache being quite a ways away from where your unit tells you it is.


For a look at some of the map errors you can expect, check out the mapping section of my webpage:



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NGS Adjusted Benchmarks


If you want to have confidence in the accuracy of your GPSr units, go to one of the "adjusted" benchmarks. http://www.ngs.noaa.gov/datasheet.html


These are listed to the one-hundred thousandth of a second. When you go to the site, choose GPS marks only. In urban areas, they can be found every few miles.


The accuracy of my Legend, in a couple of tests was very good; under 4'and 6'in 2 tests. One test on my Meridian was about 10'.


Convert the benchmark's coords to deg.ddddd and set your GPSr to the same, and make it a waypoint. Because of rounding the benchmark's coords, your waypoint may be a few feet off, but this is easy to adjust.

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