[OSM-talk] GPS Accuracy under Forest Canopy

[Apollinaris Schoell]

>    My Garmin eTrex HCx makes reasonable tracks under forest cover,
> although the tracks are certainly worse under forest than under a  
> clear
> sky.  It's not the cheapest GPS unit you can get,  but it's reasonably
> priced and it's a great navigator to enjoy both OSM and commercial  
> maps
> on foot or sitting in the passenger seat of a car.  The ability to see
> my own track has gotten me "unlost" more than once;  it seems that  
> once
> I've gotten into GPS mapping I've been more aggressive about going  
> into
> unfamilliar and confusing terrain,  so I've been getting lost more!
>

compared to a SiRF III powered the eTrex is pretty lame in accuracy.  
but it uses less power and runs twice as long on a set of batteries

>    I think of track accuracy from a practical viewpoint.  Having a
> trail off by 20 meters isn't so important so long as I get the  
> topology
> right.
>

+1, and only the rich guys with expensive tools will ever figure out  
how bad your track was.

>  I walked a segment of trail that followed a creek and always stayed  
> by
> one side:  when I looked at the tracks overlaid with Garmin's Topo
> 2008,  I saw the track crossing the creek.  I was often within 10  
> meters
> of the creek,  so this isn't 'crazy'  If I'm loading this into OSM and
> if the creek is there,  I certainly feel pressured to manually push  
> the
> trail across the creek so that the trail doesn't show false creek
> crossings:  that's an error that people when they're using the map and
> could even cause confusion.
>

this is very important. consistency and relative positions wins over  
accuracy of a single point.
traditional maps are always consistent but rarely accuract.

>    As for speed,  it's an issue that GPS errors have a "brown noise"
> characteristic:  they look worse on longer timescales.  If you're
> standing at one place and your GPS seems to be swirling around in lazy
> nested circles,  it looks real bad.  It's hard to average the
> coordinates to get a betting point position.  If you take a track or  
> go
> walking for 4 miles or drive 40 miles in your car,  that craziness is
> still there,  but it's made invisible by the scale of the map.
>
>
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