[arachne] Re: Web standards {and OT GPS}

Arachne at FreeLists---The Arachne Fan Club!

On Fri, 3 Dec 2004, Greg Mayman wrote:

> On Thu, 2 Dec 2004 00:27:13 -0500 (EST), Steve <steve@xxxxxxxxxxxxxxx> wrote:
> 
> > Probably.  The GPS (Garmin etrex) claims that under ideal
> > conditions, it can attain 1 m accuracy, and velocity
> > accuracy to within .1 knot.
> 
> That's incredible!

  Yeah, I sure thought so.  When I first looked at GPS 
receivers about a decade ago, such capability didn't exist 
(or I should say it wasn't allowed outside the military), 
and even with the limited abilities extant at the time (no 
better than 50ft, IIRC... or was it 50m?), the units started 
in the $400 range.
 
> The problem with altitude is that it is expressed relative to some
> hyperthetical "sea level". But since the earth is an oblate spheroid,
> not a true sphere, the calculations for the height of "sea level" would
> be extramely complex.

  Yeah, I've been reading up on USGS benchmarks, and
their elevations are "updated" from time to time 
depending on whatever method was/is used to calculate
them.  Orthometric height, geoid height, dynamic height...
I guess now they use an "observed gravity" model.

> > sitting at the computer desk, the accuracy is remaining
> > fairly constant betwen 14 ft. (>4 m) and 17 ft. (<6m).
> 
> Do you get land slips in your part of the world? Maybe the desk is
> moving....

  I'm sure you know the satellites aren't geosynchronous, so 
the signal locations are dynamic.  Number of satellites and 
their relative positions in the sky determine accuracy. 
Conditions are rarely (if ever) "ideal" so I've never seen 
the theoretically possible one meter accuracy.  Best 
accuracy readout I've seen was 8 feet, though I think that 
may have briefly flashed to 6 and back again.

> > Why own a GPS?
> > Geocaching.com
> >  Fun, and exercise.
> 
> Why not own one? Only because I don't have enough spare money...

  The model I own goes for $89 in places.  Since GPS's are 
basically just multi-channel receivers with little computer 
chips, their prices have fallen right along with the rest of 
semiconductor gadgets.

  Give 'em another 10 years, and we'll probably be able to 
get a survey quality GPS for $20.

  It's almost good enough now for a lot of site work.  For 
instance, take a measured fence line.  Walking north to 
south, the GPS tells me I've gone 8.3 feet MORE than the 
tape measured distance.  I reset the trip meter, turn around 
and follow the same fence to the start point, and I've 
supposedly travelled 9.1 feet LESS than the taped distance.

  Round trip error is -0.8 feet (based on precision of 
readout)... or we can just round it off and say "within 10 
inches or so."  :-)

-- 
Steve Ackman
http://twoloonscoffee.com       (Need beans?)
http://twovoyagers.com          (glass, linux & other stuff)


Arachne at FreeLists
-- Arachne, The Web Browser/Suite for DOS and Linux --

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