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# This cache?

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http://www.geocaching.com/seek/cache_details.asp?ID=15026

I was looking at this cache to find one day when I noticed the location has more the 3 decimals points after it. Is this right?

My GPS only goes to 3 places so would rounding off make my way point out and by how much?

quote:

My GPS only goes to 3 places so would rounding off make my way point out and by how much?

According to my calculations (which could be wrong), rounding to three decimals in minutes in the latitude moves it by 0.6m, and in the longitude moves it by 1mm.

At that latitude, a change in the third decimal means less than two metres of difference.

(Mea culpa note: I programmed these calculations myself, so if anybody gets a different answer, I'd be interested in hearing.)

Rob.

0.001 minutes of arc equals 1.85m. At 45 degrees latitude, that same 1/1000 of a minute equals 1.3m of longitude.

Your consumer grade GPSr can only determine a location to the best accuracy of the system itself, which is 13m RMS/SIS @ 95%. That figure is SIS (Signal-In-Space) and doesn't account for users who want to hide under trees etc.

You can add as many digits as you want after the decimal. It does absolutely nothing, as the best accuracy that can be obtained is 0.007 minutes (1852/13). This is why a GPSr that only displays 2 decimal places (.01) is yields almost the same accuracy as one that displays 3 (.001 +/- .007)

Here's what I see:

The cache hider says that he took a couple of readings and averaged them. If that's the case, it's very easy to end up with a bunch of extra digits.

What's the average of 4.5, 6.8, and 3.2? It's 4.8333.... mathematically, but scientifically, you can only say that it's 4.8.

The owner simply did not round to the correct number of significant digits.

Jamie

If I download my waypoints from my Etrex Vista into some programs they come in with 4 decimal places, the others round to 3, as does the display on the Vista. Seems they are stored internally as 4 places but displayed as 3.

I've never actually got lost just using 3.

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