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On 2013-04-10, at 3:51 PM, Brad Morris wrote:

> Hi Marcel
> 
> I just tried "GPS Averaging" by Destil.   When I tried to take the waterline 
vs my eye height, I found the report was inverted.  That is, it reported the 
water over my head.  nonsense.  I think that this is a function of the poor  
GPS receiver in my phone, because my GARMIN GPSMap 76 reported a fairly 
accurate 17 feet HoE.

Some receivers used to mix up geodetic height (obtained directly from GPS) and 
orthometric height (or height above sea level). At sea level, it is possible 
for the geodetic height to be a negative or a positive value. The difference 
between the two types of height is the geoidal undulation. Perhaps some 
software mixes up these heights, too.

> Richard Langley
> 
> Forgive me for not understanding the technical terms in your description of 
this HoE measurement by GPS.  Could you please explain that in layman's 
terms?  Is the HoE measurement by reported GPS altitude feasible with/without 
averaging?

Depends on the accuracy you are looking for. If you want 1 metre accuracy 
(1-sigma level; i.e. a value with an error of 1 metre or less 68% of the 
time), then using a handheld receiver (making pseudorange measurements) will 
require averaging. A single spot value will not be accurate enough. How much 
averaging? Will depend on local conditions. One way to do this would be to 
store the positions as they accumulate in the receiver, download them, and 
use Excel or something else to do the averaging, noting how the position 
uncertainty becomes better as more values are averaged.

After sending my previous message, I recalled we had an article on the GPS 
Standard Positioning Service accuracy in GPS World a few years ago. Here's a 
link to it:
http://gge.unb.ca/Resources/gpsworld.february03.pdf
The topic is probably worth re-visiting in my column -- 10-years on.

-- Richard Langley

> Brad
> 
> : http://fer3.com/arc/m2.aspx?i=123506
> 

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| Richard B. Langley                            E-mail: lang@unb.ca         |
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