[OSM-talk] Garmin eTrex Vista Hcx

Randy rwtnospam-newsgp at yahoo.com
Sat Oct 31 21:45:39 GMT 2009


edodd at billiau.net wrote:

>>
>>Hi!
>>
>>Shalabh schrieb:
>>>Would just like to figure out if any of you have had the same issue with
>>>this model or any other Garmin GPS.
>>
>>I have a similar Issue with the Garmin Vista HCx. Occasionally I
>>observe, that the GPS position is way off the known road/path I am on.
>>The satellite accuracy is high +-5m.
>>
>>When I switch the device off and on again, it positions me right where I
>>am supposed to be. So it seems to accumulate some sort of error in its
>>internal calculations and needs the occational reset when it is "going
>>wrong with great confidence"
>>
>>
>>bye
>>	Nop
>>
>>
>
>
>All of the matters described are possible at all times with a GPS system
>of position finding.
>However the notes that some accumulate errors are significant, and again
>may refer to particular firmware so that it will be difficult to determine
>the actual cause.
>150m horizontal error, for example, across a time gap of a few hours, may
>well relate to changes in satellite position and reflection of signal
>occurring one one but not both times.
>Vertical errors are greater at all times.

Also,

a) if "road snap" if not defeated it may snap you to a different position, 
with only a small change in measured position, and

b) some of the more sensitive auto GPSrs will lock your position, when you 
aren't moving (or aren't moving very fast) until position changes up to 
100 meters, to avoid showing a lot of drift on the display. This could be 
the reason for better accuracy while moving. This is added to the design 
because receiver sensitivity has improved so much that the receiver will 
pick up sats at very low signal to noise ratio, and consequent larger 
timing (and therefore position) errors. I guess the theory is that it's 
better to display an inaccurate stationary position than a wandering one, 
not something I agree with.

These are more likely the reason for the issue Shalabh described. The 
position has been frozen at an inaccurate point, and when the GPS power is 
cycled, it comes back up with a good lock and resets the position based on 
the new measurement. I would doubt an "error buildup" just because GPS 
fixes are based on discrete fixes, rather than any kind of incremental 
measurements.

However, I believe one of the purposes of the different modes (auto, 
cycle, walking) in some Garmins is to adjust for these two situations, so 
you need to make sure you are in the correct mode for what you are doing.

-- 
Randy





More information about the talk mailing list