[RE-wrenches] GPS units

Tom Elliot telliot at wagonmaker.com
Tue Jan 27 14:37:32 PST 2009


The elevation accuracy of commercial GPS units is horrible, I've seen them 
off by more than 100 feet and they are not reliable at all for the requested 
usage in this instance.  Surveyors use GPS units that are accurate in both 
horizontal and vertical axis measurements but they use radio frequency units 
to get down to accuracy levels within inches on either.

My suggestion is to contact a manufacturer of surveying equipment and asking 
them for technical information on how their units achieve high degrees of 
accuracy.  But be advised, they are very expensive.

Tom


  ----- Original Message ----- 
  From: Peter Parrish
  To: 'RE-wrenches'
  Sent: Tuesday, January 27, 2009 12:32 PM
  Subject: Re: [RE-wrenches] GPS units


  A caution to all. There are various types of GPS units, with varying 
accuracies. The most accurate are spec'd at around +/- 6 feet. I have a 
Garmin Etrex and under normal circumstances it delivers at that spec or 
better (+/- 3 feet). Near tall buildings or in a steep canyon, the accuracy 
goes to pot but for known reasons (multiple reflections of the radio 
signals). I have no manufacturer's info on elevation, but knowing how GPS 
units work, the resolution/accuracy in elevation should be roughly the same.



  - Peter

  Peter T. Parrish, President
  California Solar Engineering, Inc.
  820 Cynthia Ave., Los Angeles, CA 90065
  Ph 323-258-8883, Mobile 323-839-6108, Fax 323-258-8885
  CA Lic. 854779, NABCEP Cert. 031806-26
  peter.parrish at calsolareng.com


------------------------------------------------------------------------------

  From: re-wrenches-bounces at lists.re-wrenches.org 
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Jason 
Lombard
  Sent: Tuesday, January 27, 2009 2:02 PM
  To: RE-wrenches
  Subject: Re: [RE-wrenches] GPS units



  William,

  Topographic maps are great if your familiar with them. A laser if you can 
afford it or the best I have heard of is fill a tube or pipe with water from 
the pump to your tank and measure the psi at the base of it. The calculation 
is 2.31 feet of lift for every psi. Not everyone has their piping rolled out 
or on site yet so this could be a challenge and again is very accurate.

  BTW - GPS units are inaccurate to the extent that they are 40 feet off 
horizontally and 90 feet vertically. This means you could be up to 130 feet 
off. Big difference when your sizing for particular pump out put.

  Good luck,

  On Tue, Jan 27, 2009 at 2:34 PM, William Miller <wrmiller at charter.net> 
wrote:

  Does anyone have a foolproof tool to measure elevation?  I have researched 
GPS units recently.  They are notoriously inaccurate for elevation:

  http://wiki.motionbased.com/mb/GPS_Unit_Elevation

  Here are few more links with GPS fundamentals:

  http://www.adventurenetwork.com/cgi-bin/adventurenetwork/GPS_101.html
  http://www.gpsreview.net/electronic-compass/

  I have a Suunto Observer watch with an altimeter.  I think I can get 
relatively accurate measurements of DIFFERENCE in elevation if I measure the 
two elevations in quick succession.  I will be trying it.



  William

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  -- 
  Jason Lombard
  Open Hand Solar LLC.
  Certified Sustainable Building Adviser
  openhandsolar at gmail.com
  505 795 8646

  "I'd put my money on solar energy. I hope we don't have to wait 'til oil 
and coal run out before we tackle that."

  -Thomas Edison



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