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AAAA 04-29-2011 12:12 PM

Garmin 430N question
 
I was parked by the Hudson River on Wednesday. I expected the altitude to show 50 to 100 feet of elevation at the most. It showed -16 feet! I doubt I was below sea level! What is the elevation error factor? How do you reset it?

ternst 04-29-2011 03:28 PM

Re: Garmin 430N question
 
I was parked in a bank's parking lot and my 430N told me the nearest one was 16 miles away; it pointed the wrong direction and city for the nearest hospital; and said there was no gas for 17 miles while there were three stations open within a couple blocks of my location. These things are not really very accurate in a lot of ways...

JoeSchmoe007 04-29-2011 03:31 PM

Re: Garmin 430N question
 
Quote:

Originally Posted by ternst (Post 406279)
I was parked in a bank's parking lot and my 430N told me the nearest one was 16 miles away; it pointed the wrong direction and city for the nearest hospital; and said there was no gas for 17 miles while there were three stations open within a couple blocks of my location. These things are not really very accurate in a lot of ways...

You are talking about POI database being incomplete/incorrect.

OP is talking about GPS hardware precision.

JoeSchmoe007 04-29-2011 03:35 PM

Re: Garmin 430N question
 
Quote:

Originally Posted by AAAA (Post 406126)
I was parked by the Hudson River on Wednesday. I expected the altitude to show 50 to 100 feet of elevation at the most. It showed -16 feet! I doubt I was below sea level! What is the elevation error factor? How do you reset it?

I am fairly positive you can't reset it and most likely this error is within allowable limits. From what I understand horizontal location also has the same error but software in your navigation unit "snaps" you to the road if you are detected to be within X feet near the road, so you may think that horizontal precision is better but it isn't.

Cherodude 04-29-2011 05:15 PM

Re: Garmin 430N question
 
If you are in the Garmin NAV screen and press the signal strength bars in the upper left corner for a few seconds, the screen will show you the number of satellites and signal strengths it is currently using for position calculations. One of the lines displays the estimated position accuracy - mine usually says 3 metres. Presumably this could be in any direction, so in this case even the elevation could be off by 10 feet.

Sounds like the OP had a larger error than usual. Try this display switch next time to see what accuracy the unit is claiming at the time.

pwguy 04-29-2011 05:28 PM

Re: Garmin 430N question
 
Quote:

Originally Posted by Cherodude (Post 406349)
...One of the lines displays the estimated position accuracy - mine usually says 3 metres. Presumably this could be in any direction, so in this case even the elevation could be off by 10 feet.

Sounds like the OP had a larger error than usual. Try this display switch next time to see what accuracy the unit is claiming at the time.

Because of the geometry of the satellite orbits, elevation errors tend to be larger than horizontal position errors.

granto3k 04-29-2011 06:01 PM

Re: Garmin 430N question
 
Mine usually seems to be accurate within 10 or so feet of elevation.

JoeSchmoe007 04-29-2011 06:13 PM

Re: Garmin 430N question
 
Quote:

Originally Posted by Cherodude (Post 406349)
If you are in the Garmin NAV screen and press the signal strength bars in the upper left corner for a few seconds, the screen will show you the number of satellites and signal strengths it is currently using for position calculations. One of the lines displays the estimated position accuracy - mine usually says 3 metres. Presumably this could be in any direction, so in this case even the elevation could be off by 10 feet.

Sounds like the OP had a larger error than usual. Try this display switch next time to see what accuracy the unit is claiming at the time.

I can't find numbers now but AFAIK 3 meters precision is not possible in civilian GPS hardware. I think it's 30-50 meters.

Cherodude 04-29-2011 11:52 PM

Re: Garmin 430N question
 
1 Attachment(s)
Quote:

Originally Posted by JoeSchmoe007 (Post 406387)
I can't find numbers now but AFAIK 3 meters precision is not possible in civilian GPS hardware. I think it's 30-50 meters.

All I know is what I read off the screen:

DurangoZJ 04-30-2011 12:45 AM

Quote:

Originally Posted by JoeSchmoe007 (Post 406387)
I can't find numbers now but AFAIK 3 meters precision is not possible in civilian GPS hardware. I think it's 30-50 meters.

This all changed within the past 5ish years. Civilian GPS is down to .5-1M accuracy. Shoot my iPad gets me within 1m.

Edit: Was May 2000 that the gov turned off SA. Allowing 2-5m accuracy.

AAAA 04-30-2011 08:13 AM

Re: Garmin 430N question
 
Thanks all for your comments. I was viewing the screen that is thumbnailed at that time by the river. It said accuracy was 16 feet. (Usually it says 10 feet.) My assumption was that it referred to horizontal feet as on a 2D map rather than vertical feet too. I guess I was wrong. None the less, I was 125 miles up stream from the ocean, so I know I was not at sea level either.
The reason I really asked the question was to determine the altitude accuracy as I wanted to reset my barometer at my home. Well, on to other things on a beautiful Saturday.


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