[Alberg30] Fw: GPS Errors

[Bob Chambers]

RE: GPS ErrorsI also posted my GPS question to Garmin and thought I would share their response with you.  Not a technical response as I wanted, but at least they confirm it will not work.





Thank you for contacting GARMIN International, 

The GPS system is not accurate in describing elevation. You will constantly receive readings of +/- 150 feet given a fixed elevation or location. A couple of other significant points I should quickly make. If you are comparing one GPS receiver next to another you may be introducing electronic interference into one or both of the receivers. This will cause the units in some cases to not only read much differently but also incorrectly. Also, how a particular signal reacts to these atmospheric articles of degredation is likely varying from signal to signal and thus from unit to unit. Only having these GPS units located next to one another does not ensure that they will read the same lat/lon and even less likely is the idea they will display the same altitude. 

Best regards, 

Tom Buchman 
Product Support Specialist 
GARMIN International 
1-800-800-1020 
www.garmin.com 
Fax: 913-397-0836 

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-----Original Message----- 
From: Bob Chambers [mailto:BlueDolphin at rivnet.net] 
Sent: Friday, September 13, 2002 6:55 AM 
To: sales at garmin.com 
Subject: GPS Errors 



I have two Garmin GPS II Plus units and am quite happy with them.  I use 
them on our 42 foot sailboat.  This week, I needed to get the height of our 
land above the river and thought of using my two units getting the 
difference in their readings, one at sea level and one up on the cliff.  It 
is my understanding that DGPS compares the measured location versus a known 
location and broadcasts this error signal so that units so equipped can 
correct to an accurate location.  The errors removed are atmospheric errors 
and relatively constant over a large area.  Since I did not have DGPS, I 
assumed that the atmospheric error for both of my units side by side would 
also be constant and that they would both give me the same reading.  If 
there was an electronic calculation error in one or both of the units, 
perhaps they would be different but the difference would remain constant.  I 
put the units side by side and was shocked to see that they were not only 
different, sometimes by up to 100 feet in altitude, but that they both 
drifted around in different directions.  Sometimes they were the same and 
sometimes a hundred feet apart. 

Could you please explain where my thinking was flawed? 

Thanks 

Bob Chambers 

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