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Testing GPS accuracy


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First, let me say that I'm not complaining about a hider's coordinates. I'm new to this game and still learning how things work.


Ok, with that said, is there a way to test the accuracy of a GPS receiver? I was thinking of maybe comparing a reading of a nearby benchmark.


I have a Magellan Companion for the Palm m515 and I tried to find my first cache today. My GPSr showed that I was within 5 feet of the original coordinates; however the landscape wasn't even close to the hint that was listed with the cache.


Before I try to find this cache again, I'd like to make sure that my GPSr is working ok. I understand that it's not perfect. I just want some peace of mind. icon_biggrin.gif

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I've been putting off doing this myself, but a simple test, or confidence builder, would be to measure the coords of a point in your front yard for instance, then go back and see if you get the same reading, or if the GPS will guide you precisely to that same point again and again on different days and conditions.


You might also record coords from a topo map or from MS Streets & Trips of a series of road intersections, load them as a route, then see what the GPS says on different days when you drive the route. I did this once. The GPS was right on for some and not for others. I don't know how good the map was of course. A topo map or aeriel photo ought to be perfect. I had used MSS&T.

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Measuring against a benchmark might not be the best way to check, as many of them have adjusted values - or can otherwise be several feet off from the given coordiates. I forget the reasons/explanation - but if you go benchmark hunting, you'll notice many of them aren't where the coordinates say they will be. Depends on what type of marker they are, or something.


I've never had reason to doubt my GPSr... if it says accuracy is within 20 feet, and I let it sit for a few minutes... it usually will settle in at once specific reading. And when I check my waypoints against their display in USA PhotoMaps, it is usually dead on.


Make sure your receiver is set to WGS84 data, too... that's one thing that might be wrong.



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if it says accuracy is within 20 feet, and I let it sit for a few minutes... it usually will settle in at once specific reading.


This is part of my problem. I've tried several different applications for the Palm and none of them report the GPSr's accuracy (within xx feet). icon_frown.gif

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Originally posted by Railfan:

This is part of my problem. I've tried several different applications for the Palm and none of them report the GPSr's accuracy (within xx feet). icon_frown.gif


For one a GPS really has little idea how accurate it is anyway because it simply doesn't know exactly where it is in the first place.


The accuracy that manufacturers estimate is just that an estimate anyway and varies depending on how the manufacturer implements the calcs. Not all manufacturers make this estimated accuracy available to secondary devices even if the software supports this type of function/output.


Comparing to a "known" point will give one an indication at that specific point in time but that difference doesn't necessarily equate to what accuracy is.


Cheers, Kerry.


I never get lost icon_smile.gif everybody keeps telling me where to go icon_wink.gif

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Here's a quick way to get some idea in less than five minutes.


Take a reading of your current position. Go to www.lostoutdoors.com and click on MapMaker. Enter your coordinates and look at the resulting aerial photo.


Did it put the marker on your location?


Thats a great way to see how close your GPSr is reading. Topozone works well too, allthough you will have to change your settings to decimal or UTM on your GPSr to do it right. Make sure the datum setting is correct also!


Another thing to look at is the existing logs from other finders on the cache page. Alot of times others will log how close, how far, or how much trouble they had finding the cache. These are a good indication too!

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Forget about using maps including street and topos from the GPS manufacturers. Maps from Delorme and other like them are also off 50-100 feet and beyond, depending on the mfr and the area of the map. They're all notoriously off from what I've seen. Even topos from USGS won't help since you really cannot discern distances down to the accuracy of the GPS which is 10-20 feet. The best way to check is with a known established spot like a benchmark.


I think you will find that your unit will be within the 10-20 feet at worst case mentioned above when you finally check as long as you have a clear view of the sky and leave the GPS on the benchmark so it will be steady. My experience is it'll be within 12 feet.



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Railfan -


For your purpose, there are 2 kinds of benchmark disks. To tell them apart, on the geocaching page for the benchmark, look at the line below the coordinates. Some say "location is scaled" and others say "location is adjusted". Don't use the kind that say "location is scaled", because their position is estimated by scaling from a topo map. Instead, use any benchmark that says "location is adjusted". Its location is known within an inch. The geocaching page won't give coordinates as precisely as that (within-1/1000-minute is about within-3-feet), but they'll be more precise than your GPS can manage (about within-13-feet).


When checking, put your GPS on the disk and walk away a few feet so you'll be less likely to be blocking its view of satellites.

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