[BC] GPS

[Cowboy]

On Friday 30 January 2009 10:41 am, Mike McCarthy wrote:

>  This is especially applicable on the walk-in's and in areas which there are 
>  no easily defined physical references (such as a fire plug, sewer cover, 
>  residential DW etc).

>  You could easily be several degrees off and not know it. 

 ESPECIALLY on close-ins, where a few feet can be a significant
 difference. ( did you turn left, or right, when reading the FIM ? )
 This is one area where practiced and correct use of a GPS is
 critical at worst.

>  Also, one other thing to keep in mind.  Unless you have WAAS or DGPS, your 
>  accuracy will never be better than 20 meters. It's a function of computing 
>  power and receivers.

 16 meters seems to be the norm.

>   And forget doing walk-in's unless 
>  you have the 2 mile point precisely defined by another more precise GPS.

 As Alan points out ( and I do ) merely carrying the line in from a more
 distant point will negate this error, and is why I use an eliptical earth
 model from 32 miles.
 One can plot a radial more accurately than one can walk it.

>  CHEAP GPS units will not likely have WAAS.  And they certainly don't have 
>  DGPS (which requires a 2nd RX. and terrestrial source.)  DGPS gets you down 
>  to a few feet.  WAAS down to maybe 5-10M.  Which is good enough for even 
>  the two mile first point and MP's.

 In practice, 12 channel WAAS will typically get within 4 feet.

>  So, while the mapping software is nice tool, the GPS receiver and it's 
>  accuracy is more critical to insure the points being defined are in fact on 
>  the radial. Which is what really matters.

 THANK YOU !

>  As Mike Golchert and Robert  
>  commented, the mapping programs don't handle big hills well.  Thus, don't 
>  go cheap on the RX or your errors will be compunded.  You might live to 
>  regret it down the road if any questions arise as to the methods and 
>  equipment employed.

 Typically, the first sign of the error, and quality of the receiver, shows
 as elevation errors. Usually a changing elevation when one is stationary.
 ( or showing you 30 feet ASL when the water is lapping the GPS at low tide )

>  >  I'll plot one free radial as a demo for anyone who asks.
>  >  If someone wishes plots of all radials in a full proof, send a check.
>  >  If I'm doing the proof, it's a free "value added" service.
>  >  Too blatant ?  ;)
>  >
>  >--
>  >Cowboy

-- 
Cowboy