[pct-l] GPS, distance and speed.

[msaenz at mve-architects.com (Mike Saenz)]

Hi Adrian!

Whoa! Information overload!!!!
;)

I haven't really studied GPS technology. Just learned enough to try and make a good decision one which one to buy, since I was looking at spending $100 to $200 on one. Dropping that kind of change tends to make one hesitate and think about what you're getting for it!

Though since I bought it, any info I come across immediately becomes interesting.
The most fascinating tidbit is the idea that the government restricts the accuracy of the retail GPS units.
I'd be interested in seeing what the US soldiers are carrying in Iraq and Afghanistan.
I thought that the electronics inside the GPS was what was "limited". The idea that the government embeds "jitter" in the signal emitted from GPS satellites is intriguing!

All said and done, researched, discussed and experienced, I've come to the belief that my GPS is reporting more accurate data than a wheeled odometer. As an architect, having a little knowledge of how Civil Engineers survey topography, I do know that surveyors have all now abandoned their old plumb line transits for GPS and GIS mapping technologies. As you know, just about ALL the current USGS topo maps still are based on old survey data, only recently being updated with GIS technology, map by map. Just about each and every benchmark I come across while hiking disagrees with my GPS elevation report. I'll tend to believe that human error would be a more plausible cause for the discrepancy and trust the GPS data.

Has anyone brought a GPS to the top of Whitney to see how accurate the old benchmark elevation is? Is it REALLY 14,494'?
Though, I suppose peaks (especially high ones with many, many distant line of sight triangulation points) would be easier to survey and therefor more likely to be accurate.

Though I'm by no stretch of the imagination a "technophile", I can admit human fallibility. I make more than my share of mistakes each and every day! I know some folks will swear by the guidebook "bibles" and the talented hand of someone wheeling off miles. We all have our (humble?) opinions. But if my GPS comes within a 1/10 of a mile over a 10 mile day of hiking, then that's accurate enough for my purposes!

Horse beaten. Onto new subjects!

No wait! One more item: My buddy Bill uses a weight watchers pedometer when he hikes. And THAT came within 1/4 mile of what my GPS reported as 21.8 miles!!!!!

M i c h a e l   S a e n z
McLarand Vasquez Emsiek & Partners, Inc.
A r c h i t e c t u r e    P l a n n i n g    I n t e r i o r s
w  w  w  .  m  v  e  -  a  r  c  h  i  t  e  c  t  s  .  c  o  m

 -----Original Message-----
From: 	adrian.borner@ch.abb.com [mailto:adrian.borner@ch.abb.com] 
Sent:	Wednesday, April 14, 2004 12:58 AM
To:	Mike Saenz
Subject:	RE: [pct-l] GPS, distance and speed.


Mike,
I see you looked into the matter quite carefully. I cannot remember the
real figures, so my numbers may be a bit off.
The speed of the radio signal has nothing to do with your speed moving
around. The radio signal basically moves with the speed of light and is
more or less independent of the frequency.
The errors introduced are basically as follow:
- inaccuracy of the satellite clock (shift between different satellites)
- introduced jitter by the US government (I do not know how big it is now)
- inhomogenity of the propagation path (changes due to weather, air
temperature, etc)
- angle and numbers of satellites and signal strength (the calculation
process is geometry, the smaller the angle the greater the error, same as
when you draw it on paper)
- inaccuracy of the GPS unit
As you mentioned, when standing still, your reading will vary. I saw ones a
plot, where somebody took readings over several hours. They are basically
random distributed.
If you want an accurate reading, you would have to employ differential GPS.
This basically uses two GPS, one at a fixed point with known reference and
a second in the field. As long as they are not too far apart, they would
basically encounter the same conditions and only the last two errors would
basically have an effect. With this and with additional integration one can
go down to the centimeter range absolute and millimeter range when looking
at movements (e.g. earth movements!
When I said that your readings when moving by car seem to be more accurate,
I meant, that when you move several 10 or 100m between readings, an error
of a couple of meters is small. But when you only move a couple of meters
the error is suddenly in the range of your movement.
Hope I made myself understood
Good luck in caching, I have not gotten into this hobby
Adrian