GPS Unit Off or Cordinates Off

[+proudcdn]

Hello All,

 

I have a Garmin GPSMap 60Cx that I have owned for less than 6 months (it was brand new when I got it). How do I know if it is accurate or not? Is there a way to know for sure? And if it is not accurate is there a way to calibrate it?

 

Here is why I ask ...

 

1) I set out a GC of my own and took an averaged reading of over 150 reads. The area was not terribly tree covered and the signal was strong. Most people that have gone to find this cache have told me that my coordinates are off.

 

2) I find that when I am caching that the area that my unit leads me to is often not where the cache is. It is out by 30ft or more. This is the case for about 80% of the caches I have visited.

 

I figure one of three things is happening:

 

1) The satellite signal on the days I have been using the unit is not overly strong.

 

2) The people that originally placed their geocaches were using units with older technology and were perhaps not as accurate as my unit.

 

3) My unit is off.

 

Any suggestions?

 

Thanks in advance.

 

Proudcdn.

[+PeoriaBill]

Just in case, have you checked to see which datum you are using? WGS84 should be selected as most of the cachers that find your hide will have that setting. Also, turn off the compass and check to see if that makes a difference.

[+dfx]

try benchmarks with well-known coordinates. check Waymarking.com for nearby benchmarks and see if they have official coordinates, either in WGS84 or NAD83 datum (the difference between them is about 1.5 meters max.)

[+kunarion]

How do I know if it is accurate or not? Is there a way to know for sure? And if it is not accurate is there a way to calibrate it?

There's supposedly some kind of internal hoodoo that does a sort of "calibration" over time. That's about it for accuracy calibration. From your post, I'm guessing your GPSr is working just fine. You might try checking for any difference with "WAAS" turned on and off. 30 feet is a reasonable error for finding a Geocache.

 

I've placed three caches, and didn't average any readings. I just saved a boatload of waypoints, over several days, coming from several directions, and picked the waypoint that was consistently the closest.

Edited June 13, 2010 by kunarion

[+DonB]

Hello All,

 

I have a Garmin GPSMap 60Cx that I have owned for less than 6 months (it was brand new when I got it). How do I know if it is accurate or not? Is there a way to know for sure? And if it is not accurate is there a way to calibrate it?

 

Here is why I ask ...

 

1) I set out a GC of my own and took an averaged reading of over 150 reads. The area was not terribly tree covered and the signal was strong. Most people that have gone to find this cache have told me that my coordinates are off.

 

2) I find that when I am caching that the area that my unit leads me to is often not where the cache is. It is out by 30ft or more. This is the case for about 80% of the caches I have visited.

 

I figure one of three things is happening:

 

1) The satellite signal on the days I have been using the unit is not overly strong.

 

2) The people that originally placed their geocaches were using units with older technology and were perhaps not as accurate as my unit.

 

3) My unit is off.

 

Any suggestions?

 

Thanks in advance.

 

Proudcdn.

30 feet is no big deal. Figure if the CO's error was even 10 or 15 feet and your gps was off by ten or 15 feet in a different direction you now have an error of 20 to 30 feet. When my gps says I'm within 20 or 25 feet I usually put it away and start relying on my eyes.

[knowschad]

You have arguably the best handheld GPS there is. No, there is no way to calibrate it in the sense that I think you are asking about. You can (and should) calibrate the compass if you are using it, but that won't affect the coordinates when you mark a spot.

 

Garmin's engineers have stated in a blog post on Garmin's website that they feel that averaging coordinates as we do it is useless, at best. They say that for averaging to really work, you'd have to come back over a period of days and at different times during the day.

 

I have taken recently to doing both. I will let my GPS "settle down" by setting it down, or holding still. Then I'll take a snapshot without averaging. I'll also take an average, generally 150-250 samples. Next, I'll go to the "find nearest" listing and see which waypoint is first on the list. For now, I will assume that is the best, but I will still test it by walking well away, selecting that waypoint, and letting it take me to GZ. I will also test the other one in the same way.

 

There may be times when your best option is simply to try another day, hoping for a better satellite constellation.

[+proudcdn]

Ok this is all starting to make sense

 

I have enabled the WAAS and it was already set to WGS 84. This might make a difference.

 

Good to know I am "normal."

 

Cheers,

 

Proudcdn

[knowschad]

Good to know I am "normal."

Wait just one second... who gave you that idea?

[+Avernar]

Good to know I am "normal."

Wait just one second... who gave you that idea?

He put normal in quotes. Us Canadians are definitely not normal compared to the rest of the world.

[7rxc]

Good to know I am "normal."

Wait just one second... who gave you that idea?

He put normal in quotes. Us Canadians are definitely not normal compared to the rest of the world.

 

Not quite right! The rest of the world is not quite normal compared to us Canadians...

 

Doug 7rxc

[7rxc]

Ok this is all starting to make sense

 

Glad to hear that!... I just picked up one of those on a Wally World sale... same model.

I usually cache using a mid 90's 45xl... recently I've been comparing them in the field.

Along with a 'new' Legend. I find the map60cx is by far the better way to find GZ. That sensitive

receiver is a big help, sometimes. Good nav practices can almost compensate for not having one though.

The Legend has a faster processor, but is not high sensitivity, it does know WAAS... another help. But I still find caches with the old rig and a compass, most of the time. I've recently had a few that were all but impossible with out high sensitivity... but it wasn't all that helpful in other cases... one was actually worse...

 

One thing to watch on the CO vs Cacher comparison that was mentioned... distances like that are estimates first off. They also represent a radius... and thus an area. Your radius might be 5 m and the same for the CO... but unless someone is specific about direction, one circle has to move around the perimeter of the other in worst case... so it represents a circle with radius a plus radius b... a really large increase in search area... not just distance. That is one reason I still like triangulation with a compass from good spots when resolution is needed. Speaking of which... no triangulation is exact... the references to a 'cocked hat' area as a result is well deserved. The better the data the smaller the area of that... but it is why the error exists.

Sats move and the calculations change second by second... but the area defined is still an area not a point... with our gear... worse with a compass of course. Also why no two GPS will give the same result exactly... unless it's a fluke...

 

didn't find it exactly, wait a few minutes and try again... new GZ sometimes... different search to do.

 

Doug 7rxc